WO2013144420A1

WO2013144420A1 - A biodegradable circuit board

Info

Publication number: WO2013144420A1
Application number: PCT/FI2012/050316
Authority: WO
Inventors: Kari Luukko; Hanna KÄHÄRI; Stefan Fors
Original assignee: Upm-Kymmene Corporation
Priority date: 2012-03-29
Filing date: 2012-03-29
Publication date: 2013-10-03

Abstract

A circuit board (100) comprising a substrate (110), a first at least partly electrically conductive layer (120), and a second at least partly electrically conductive layer (130), wherein the second at least partly electrically conductive layer (130) is arranged a distance apart from the first at least partly electrically conductive layer (120). The substrate (110) comprises polylactide and lignin. An electronic device comprising the circuit board. A substrate material granule (510) for manufacturing a substrate for a circuit board. A method for manufacturing the substrate material granules. A method for manufacturing the circuit board. Use of the substrate material granule (510) for manufacturing a circuit board (100) or an electronic device.

Description

A biodegradable circuit board

Field of the Invention The invention relates to circuit boards, in particular to biodegradable circuit boards comprising lignin. A circuit board is a board for electrically and mechanically connecting electronic components to each other using the circuit board. Background of the Invention

Electronic devices have become smaller in size and the trend seems to continue. Smaller size is achieved by increasing the degree of integration and reducing the size of component packages. The components are commonly joined on to a circuit board. A large portion of the area of the circuit board may be utilized using surface mount joining techniques, such as the flip-chip technique, to join the components to the circuit boards.

Summary of the Invention

It has been discovered, that polylactide (PLA) can be mixed with lignin to produce a substrate for a multilayer printed circuit board. The substrate is biodegradable, and therefore the amount of electronics related mixed waste can be reduced. Furthermore, as lignin can be used as part of the substrate material, a more efficient use of lignin has been noticed. A large portion of the area of the circuit board may be utilized for electronics by using multiple conducting layers in the circuit board. For example both sides of a planar layer can be utilized for electronic purposes. The circuit board can be used as a conventional circuit boards, e.g. direct attach joining techniques, such as the flip-chip technique, can be used to join the components to the circuit board, to further increase the area of the circuit board utilized for electronics.

The technical features of the circuit board are disclosed in the independent claim 1 . Additional features of the circuit board are disclosed in the independent claims 2 to 13.

An electronic device comprising a circuit board according to an embodiment of the invention is disclosed in claim 14.

Additional features of the electronic device are disclosed in the independent claims 15 and 16. A substrate for the circuit board may be manufactured using substrate material granules. The technical features of a substrate material granule for the circuit board are disclosed in claim 17.

Additional features of the substrate material granules are disclosed in claims 18 to 25.

The technical features of the method for manufacturing substrate material granules are disclosed in claim 26. Additional features of the method for manufacturing substrate material granules are disclosed in claims 27 to 40.

The technical features of the method for manufacturing the circuit board are disclosed in claim 41 .

The circuit board and the substrate material for the circuit board can be made according to any of the claims 42 to 56. Alternatively, the substrate can be made from substrate material granules, as disclosed in claim 57. Additional technical features for the method for manufacturing the circuit board are disclosed in claims 58 to 61 .

Use of a substrate material granule for manufacturing a circuit board is disclosed in claim 62. Use of a substrate material granule for manufacturing an electronic device is disclosed in claim 63.

Description of the Drawings

The embodiments will be described with reference to figures, in which shows, in a perspective view, an embodiment of the circuit board, wherein the circuit board comprises a first at least partly electrically conductive layer on a first surface of the circuit board (shown in black colour) and a second at least partly electrically conductive layer on a second surface of the circuit board (shown in grey colour),

shows, as viewed from top, the circuit board of Fig. 1 a, shows, as viewed from bottom, the circuit board of Fig. 1 a, shows, in an enlarged perspective view, the circuit board of Fig. 1 a,

shows, in an enlarged perspective view, alternative or additional wirings between the partly electrically conductive layers, shows a process for producing a partly electrically conductive layer by first producing a fully electrically conductive layer and by etching the fully electrically conductive layer,

shows a process for producing a partly electrically conductive layer onto a substrate,

shows an electronic device comprising a two-sided circuit board and electronic components mounted on both sides of the circuit board,

shows an electronic device comprising a two-sided circuit board and electronic components mounted on one side of the circuit board,

shows an electronic device comprising a multilayer circuit board comprising three partly electrically conductive layers and two substrates,

shows an electronic device comprising two circuit boards according to embodiments the invention, Fig. 4 shows a process for manufacturing a circuit board according to an embodiment of the invention, and

Fig. 5 shows a process for manufacturing substrate material granules for a circuit board substrate and a process for manufacturing a circuit board using the substrate material granules.

Detailed Description of the Invention Electronic devices have become smaller in size and the trend seems to continue. Smaller size is achieved by increasing the degree of integration and reducing the size of component packages. The components are commonly joined on to a circuit board. A large portion of the area of the circuit board may be utilized using surface mount joining techniques, such as the flip-chip technique, to join the components to the circuit boards. However, the increased degree of integration has led to devices that are not easily repaired, and sometimes are hard to recycle. To reduce the amount of mixed waste, biodegradable materials have been introduced also to electronic devices. Such materials include polymers.

One type of biodegradable polymers are polymers comprised in wood and other plants. Wood comprises cellulose, hemicelluloses, and lignin. Lignin, as the biobased polymer, has many advantages, as lignin is produced in pulp and paper industry in large amounts. Typically lignin is used to produce heat (burned) in a recovery boiler. More efficient use of lignin in other application could be environmentally beneficial. Furthermore, as lignin is produced in pulp and paper industry in large amounts, the price of lignin is much cheaper than the price of other biodegradable polymers. However, the mechanical and electrical properties of these polymers as such are not sufficient for electrical devices. To improve these properties, polymers originating from wood may be mixed with other polymers. For example, an epoxy resin cross-linking agent may be used with lignin to produce a material to be used in electronic devices. In an embodiment of the invention, a composite comprising thermoplastic polymer and lignin is used as a substrate material for a circuit board. Thermoplastic materials can be reshaped in elevated temperatures. In particular, a thermoplastic material may be used to form granules of the thermoplastic material. The granules can later be reshaped and combined (e.g. melted) to form substrate material for circuit board and the substrate. A circuit board can be manufactured using these granules.

An embodiment of a biodegradable circuit board comprises polylactide (PLA) and lignin. The term "circuit board" refers to a board for electrically and mechanically connecting electronic components to each other using the board. The term "board" may refer to a planar object. However, the term circuit board in this description refers generally to a body for electrically and mechanically connecting electronic components to each other. The shape of the circuit board is not necessarily planar, but may form e.g. a part of the covers of an electronic device. Moreover, the cover of a device may be used as the circuit board. The term "biodegradable circuit board" refers to a circuit board comprising a part (e.g. a substrate) that is biodegradable. Some other parts (e.g. layers comprising metal) are not necessarily biodegradable. However, when recycling an electronic product, the electronics components and the metal layers may be removed from a biodegradable circuit board substrate. The substrate may be disposed with other biodegradable waste, while other parts may be recycled with electronics waste. The term "biodegradable" in general refers to a property of an object, such that the object may be degraded by living organisms.

Polylactic acid (PLA), also called as polylactide, is a thermoplastic aliphatic polyester derived from renewable resources, such as corn starch, tapioca products (roots, chips or starch) or sugarcanes. PLA is a biodegradable polymer. PLA can biodegrade under certain conditions, such as the presence of oxygen. Polylactides belong to the group of poly (cc-hydroxyacids).

When producing PLA, bacterial fermentation is commonly used to produce lactic acid from corn starch or cane sugar. Lactides may have a D or a L chirality. PLA may be formed by polymerization of at least one of D-lactide and L-lactide. Polymerization of a racemic mixture of L- and D-lactides usually leads to the synthesis of poly-DL-lactide (PDLLA) which is amorphous. Use of stereospecific catalysts can lead to heterotactic PLA which has been found to show crystallinity. The degree of crystallinity, and hence many important properties, is largely controlled by the ratio of D to L enantiomers used, and to a lesser extent on the type of catalyst used.

Due to the chiral nature of lactic acid, several distinct forms of polylactide exist. Poly-L-lactide (PLLA) is the product resulting from polymerization of L-lactide. Typically PLLA has a crystallinity of around 37%, a glass transition temperature between 60 - 65 °C, a melting temperature between 173 - 178 °C and a tensile modulus between 2.7 - 16 GPa. Poly-D-lactide (PDLA) is the product resulting from polymerization of D-lactide. Poly-DL-lactide (PDLLA) product resulting from copolymerization of L and D -lactides. Polylactic acid can be processed like most thermoplastics into fiber (for example using conventional melt spinning processes) and film. The melting temperature of PLLA can be increased by 40 - 50 °C by physically blending the polymer with PDLA (poly-D-lactide). The temperature, at which a polymer transits from a solid state to a viscous state, is generally referred to as the heat deflection temperature (HDT). The heat deflection temperature is the temperature at which a polymer or plastic sample deforms under a specified load. In addition to HDT, the melting point of the polymer can be used to characterize the polymer. Two aspects limit the thermo mechanical properties of the PLA used for the biodegradable circuit board

- the melting point should be high enough to enable common electronics joining techniques for the circuit boards such as soldering or adhesive joining. Common solders melt at a temperature from about 180 °C (183 °C for SnPb) to about 220 °C (217 °C for SnAgCu); however, the temperature of the circuit board may be somewhat lower than the temperature of the solder. Adhesives may be cured using e.g. heat or ultraviolet light. The temperature for adhesive curing is typically lower than for solder joining.

- the melting point should be low enough to enable mixing with lignin.

Lignin starts to degrade at a temperature of around 200 °C. As soldering may require a relatively high melting temperature, PLA comprising both PDLA and PLLA may be used as the PLA of the circuit boards. There high-heat polylactides have a relatively high HDT and melting point, and they crystallize faster than a pure PLLA polymer, since the PDLA enhances the crystallization of the PLA. The heat deflection temperature and the melting temperature of a PLA polymer may be increased also using at least one of an inorganic crystallization substance, such as talcum, and annealing. The melting or processing temperature of high-heat polylactides may be decreased using platicizers, such as polyethylene glycol (PEG) or polypropylene glycol (PPG), thereby facilitating mixing with lignin.

As PDLA can increase the melting temperature of PLLA, a PLA consisting essentially of PLLA may be used as the PLA of the circuit board. However, small amounts of PDLA does not increase the melting temperature a lot. Therefore, a PLA consisting essentially of PLLA may comprise at most 5 % of PDLA. Preferably, the PLA consisting essentially of PLLA may comprise at most 2 % of PDLA. In addition, the PLA consisting essentially of PLLA, may further comprise at least one of additives and impurities.

The high-heat polylactides may be block polymers of PDLA/PLLA. The high- heat polylactides may be stereocomplexes of PDLA and PLLA (sc-PLA). The melting point of these high-heat polylactides depend on the ratio of the PDLA and PLLA polymers. In case the PLA is synthesized from a racemic mixture of D- and L-lactide the resulting PDLLA is amorphous (i.e. not crystalline), and the heat deflection temperature remains low.

Wood comprises cellulose, hemicelluloses and lignin. Lignin is separated from wood with known techniques. Lignin is an integral part of the secondary cell walls of plants and some algae. It is one of the most abundant organic polymers on Earth, employing 30% of non-fossil organic carbon, and constituting from a quarter to a third of the dry mass of wood. Its most commonly noted function is the support through strengthening of wood in trees. The type of lignin is classified according to the chemical process that produces the lignin. Commonly lignin can be obtained from three pulping processes: the sulphate process, i.e. the Kraft process; the sulphite process; and the soda process (alkali process). The lignin obtained from these processes are referred to as Kraft lignin (or sulphate lignin); sulphite lignin (or lignosulphonate); and soda lignin (or alkali lignin), respectively. Lignin produced by the Kraft process (sulfate pulping) is usually burned for its fuel value, providing energy to run the mill and its associated processes. The chemical properties of different lignins are somewhat different. E.g. the alkalinity or acidity (i.e. the pH-value) of the lignin depends on the extraction process. Soda lignin is typically alkaline, while Kraft lignin and sulphite lignin are neutral or acid. The pH-value of the lignin can be measured e.g. from a water-lignin solution, wherein the mass ratio of dry lignin and water is 1 :1 , i.e. the dry matter content of the solution is about 50 m-%. The pH-value of soda lignin is typically 13 or more (as measured from the solution). The pH-value of Kraft lignin may be acid or neutral, e.g. from 5 to 8 (as measured from the solution). The pH-value of sulphite lignin is typically acid, e.g. in the range from 4 to 6 (as measured from the solution). The pH-value is affected also by the dry matter content of the lignin solution; the above values apply to the dry matter content as specified above.

The properties of lignin may vary also because of the feedstock used to produce lignin. Lignin can be separated e.g. from wood, corn, rice, cotton, and other biological feedstock.

The lignin used for the circuit board material may comprise lignin/lignins of one or more types (soda lignin, Kraft lignin, and sulphite lignin) and/or lignin from various sources (wood, corn, rice, cotton, and other biological feedstock). However, for reasons to be discussed later, the pH-value of the lignin used for the circuit board material, as measured from the solution as discussed above, is preferably in the range from 4 to 9, and more preferably in the range from 5 to 8. A circuit board is a body for electrically and mechanically connecting electronic components to each other using the board. The term "electronic component" refers to components used in electronic devices. An electronic component may be selected from the group comprising sensors, transducers, actuators, resistors, thermistors, capacitors, inductors, switches, relays, displays, conductors, and semiconductors such as diodes, varactors, light emitting diodes, transistors, lasers, and integrated circuits. Some of these components may also be embedded into the circuit board. The circuit board is not necessarily planar. The shape of the circuit board is not necessarily planar, but may form e.g. a part of the covers of an electronic device. Moreover, The cover may be used as the circuit board.

An embodiment of a circuit board is shown in Figs. 1 a-1 d. The circuit board 100 of Fig. 1 a comprises a substrate 1 10. The substrate 1 10 comprises polylactide and lignin. The composition of the substrate 1 10 and the manufacturing of the substrate will be discussed in more detail below.

The circuit board 100 of Fig. 1 a further comprises a first at least partly electrically conductive layer 120 (as shown in black colour in Fig. 1 a). The first at least partly electrically conductive layer 120 is shown in more detail in Fig. 1 b. The first at least partly electrically conductive layer 120 comprises electrically conducting wires 122 and pads 124. The wires 122 are used to conduct electrical signals and/or energy along the first at least partly electrically conductive layer 120. In addition, some of the wires 122 may be used as antennas for communication. The pads 124 are used for bonding or joining the electrical components to the circuit board 100. The first at least partly electrically conductive layer 120 may alternatively be a fully electrically conductive layer, wherein the layer is an essentially uniform electrically conducting layer, as will be discussed later. The wires 122 and pads 124 comprise metal for conducting electrical signals and/or electrical power. The wires 122 and pads 124 may comprise metal for conducting electrical signals from one electronic component to another electronic component. The wires 122 and pads 124 may consist of at least one metal. Alternatively, conductive adhesives, pastes, or inks may be used as the wires 122 and pads 124. These materials typically comprise a matrix material and particles comprising metal. The circuit board 100 of Fig. 1 a further comprises a second at least partly electrically conductive layer 130 (as shown in grey colour in Fig. 1 a). The second at least partly electrically conductive layer 130 is shown in more detail in Fig. 1 c. The second at least partly electrically conductive layer 130 may comprise wires 132 and pads 134, or the second at least partly electrically conductive layer 130 may be a fully electrically conductive layer, as discussed above for the first at least partly electrically conductive layer 120. The wires and pads or the layer itself may comprise metal or consists of metal, as discussed above for the first layer 120.

The second at least partly electrically conductive layer 130 is arranged a distance apart from the first at least partly electrically conductive layer 120. Referring to Fig. 1 a, in the figure the substrate 1 10 is a planar body having a first surface and a second surface, wherein the second surface is opposite to the first surface. The first at least partly electrically conductive layer 120 is arranged on the first surface of the substrate, and the second at least partly electrically conductive layer 130 is arranged on the second surface of the substrate. Thereby, the second at least partly electrically conductive layer 130 is arranged a distance apart from the first at least partly electrically conductive layer 120, wherein the distance corresponds to the thickness of the substrate, T_s (Fig. 1 a). The thickness of the substrate may be in the range from 10 μιτι to 10 mm. The preferable thickness T_s of the substrate 1 10 depends on the application. A thick substrate (T_s > 1 .5 mm) can be considered rigid, while a thin (T_s < 500 μιτι) substrate can be considered flexible. Therefore, a substrate, of which thickness is less than 1 .5 mm is non-rigid. In an embodiment, the thickness of the substrate is less than 1 .5 mm. The lignin content of the substrate may affect the rigidity of the substrate, as will be discussed below. In order to have reasonable strength, the thickness of the substrate 1 10, T_s, should be at least 10 μιτι, preferably at least 50 μιτι, and most preferably at least 100 μιτι.

For consumer electronics products, the substrate is preferably thin, because the products are miniaturized and integrated. For consumer electronics products, the thickness of the substrate may be in the range from 50 μιτι to 500 μιη. Also when the circuit board 100 comprises multiple substrates (Fig. 3c), these thickness values are applicable. However, the flexibility is affected by the total thickness of the circuit board, not only the thickness of one substrate.

Figure 1 c shows the other measures of the substrate 1 10; the length of the substrate, L_s, and the width of the substrate, W_s. The length and width are not critical, and depend on the application. Typically the length, L_s, may be from 1 cm to 1 m. Typically the width, W_s, may be from 1 cm to 1 m.

Fig. 1 d shows the circuit board 100 of Fig. 1 a in a perspective view, wherein the thickness of the substrate 1 10 is exaggerated. In this enlarged perspective view, the vias 1 12 between the layers are shown. The vias (the via 1 12 and the other vias, not indicated by reference numbers) are arranged to conduct electrical signals and/or electrical energy from the first at least partly electrically conductive layer 120 to the second at least partly electrically conductive layer 130. The via 1 12 may conduct electrical signals and/or energy from a pad or a wire to another pad or a wire.

The vias 1 12 may be formed by drilling (e.g. by laser drilling) a hole through the substrate 1 10. The hole may thereafter be filled with a conductor material. As the conductor material, a metal may be used. The metal may be the same metal as used for the wires of the circuit board. In addition to pure metals, conductive adhesives (isotropically conductive adhesives, ICAs, in particular) may be used. Typically conductive adhesives comprise a matrix material and conductive particles. The matrix material is more or less standard adhesive, and the conductive particles form a conductive path within the adhesive. The conductive particles typically comprise metal. The conductive particles may be e.g. metal coated polymer particles, e.g. spheres. The matrix material may comprise lignin. In an embodiment, at least one via is formed using a conductive metal. In an embodiment, at least one via is formed using conductive adhesive. In an embodiment, the conductive adhesive comprises lignin.

Referring to Fig. 1 e, the electrical connection between the first at least partly electrically conductive layer 120 and the second at least partly electrically conductive layer 130 can be made with a conductor 1 12b that is arranged on the edge of the circuit board 100. Moreover, the electrical connection between the first at least partly electrically conductive layer 120 and the second at least partly electrically conductive layer 130 can be made with a conducting wire 1 12c that is bonded to a pad on the first at least partly electrically conductive layer 120 and to another pad on the second at least partly electrically conductive layer 130.

Even if Figs. 1 a, 1 d and 1 e show the substrate as transparent, the substrate may be made of opaque material. Figs. 2a and 2b illustrate two ways of forming the wires and pads of the at least partly electrically conductive layer (first 120 or second 130). The wires 122, 132 and pads 124, 134 may be formed in at least two ways:

1 ) metalizing the whole surface of the substrate 1 10 by known means of metallization (Fig. 2a, "Uniform metallization"), patterning an image of the wires on pads onto the metalized surface (Fig. 2a, "Patterning"), and etching away at least some of the metallization (Fig. 2a, "Etching, Washing"), or

2) directly patterning the wires 122 and pads 124 to the substrate 1 10 by known means of metallization (Fig. 2b, "Selective metallization").

In Fig. 2a, a partly electrically conductive layer is formed by

- Arranging available a lignin-PLA substrate 1 10.

- Metalizing uniformly a surface of the substrate 1 10 using known means of metallization. In this way a fully electrically conductive layer 120 is formed.

- Patterning the wires and pads to the fully electrically conductive layer 120. In this step, a film is patterned on to the fully electrically conductive layer 120, wherein the film is resistant to an etching liquid (typically acid). The film may be sensitive to light such that part of the film may be washed away.

- Etching the fully electrically conductive layer 120 to form the partly electrically conductive layer 120. Finally, the etching liquid may be washed off the circuit board. In Fig. 2b, the partly electrically conductive layer is formed by

- Arranging available a lignin-PLA substrate 1 10. - Metalizing selectively the partly electrically conductive layer 120 onto a surface of the substrate 1 10 using known means of metallization. Selective metallization can be performed e.g. using a mask between a metal source and the substrate 1 10, or by using printing techniques, as will be discussed.

A circuit board may be a board, wherein the patterning and etching (Fig. 2a) have not been performed. The at least partly electrically conductive layer 120, 130 may therefore be:

(1 ) a fully electrically conductive layer, wherein the fully electrically conductive layer is a uniformly metalized surface, from which a part of metallization may be removed by etching, or

(2) a partly electrically conductive layer, wherein the partly electrically conductive layer comprises wires 122, pads 124, and/or antennas.

The "known means of metallization" include methods for producing a layer consisting of at least one metal and methods for producing a layer comprising at least one metal. The methods for producing a layer consisting of at least one metal include sputtering, evaporation, spraying (e.g. thermal spraying), vapour deposition (e.g. chemical vapour deposition (CVD), atomic layer deposition (ALD)), electrochemical growing, and epitaxy (e.g. atomic layer epitaxy (ALE)). Metals that are typically used as conductor for circuit boards include copper and silver. In addition, aluminium and gold are sometimes used.

With these methods copper is preferred because of its good thermal and electrical conductivity. Silver and gold have also good thermal and electrical properties, but they are more expensive, gold in particular. Aluminium has not so good properties and it tends to oxidize easily. Sputtering, evaporation, and spraying are relatively simple processes and do not need as expensive process equipment as CVD, ALD or ALE.

The methods for producing a layer comprising at least one metal include several printing methods, such as ink-jet printing, gravure printing, flexo printing, and stencil printing (stencilling). Electrically conductive inks, adhesives, or pastes may be used to metalize a surface of the substrate. These inks, adhesives, and pastes typically comprise electrically conductive particles in a matrix. The matrix may be hardened or (at least partly) evaporated during the process. The conductive particles typically comprise some metal. The conductive particles may be e.g. nanoscale metal particles, arranged to sinter together in a high temperature. The conductive particles may be e.g. metal coated polymer particles, arranged to form an electrically conductive path, when contacted with each other. In conductive inks, both silver and copper nanoparticles are used. In some studies, inks with silver nanoparticles show lower sintering temperature than inks with copper nanoparticles.

The printing methods, as discussed above, are developed for only partially metalizing the substrate. Therefore, the printing methods are particularly suitable for selective metallization, the selective metallization being described in Fig. 2b.

The wires, i.e. the areas of the partly conductive electrical layer that comprise metal and that are not to be used for joining electronic components, may be covered with a solder-resist mask. The solder-resist mask prevents solder from wetting the wires and thus also decreases the risk of forming short circuits in soldering process. In addition, the solder-resist mask may prevent the wires from oxidizing with air.

Figure 3a shows, in a side view, an electronic device, wherein the electronic components 222, 224, and 226 have been bonded to the first at least partly electrically conductive layer 120 of the circuit board 100. The electronic components 232 and 234 have been bonded to the second at least partly electrically conductive layer 130 of the circuit board 100. The components on the first layer 120 communicate with the components on the second layer 130 through the vias 1 12 or through a conductor 1 12b or through a wire 1 12c (cf. Figs. 1 d and 1 e, not shown in Fig. 3a). The electronic components 222, 224, 226, 232, and 234 are mounted to the circuit board 100 using surface mount technology (SMT). In SMT, the electronic components are mounted only onto the surface of the circuit board 100. Surface mounting can be done with conductive adhesives or with solders. Adhesives may be isotropically conductive adhesives (ICA) or anisotropically conductive adhesives (ACA). The adhesives may be supplied in the form of film or paste. In addition to adhesives, soldering may be used to mount the components to the circuit board. Common solders include the tin-silver-copper solders and the tin-lead solders with various metal compositions. In addition to surface mount technology, the through hole technology may be used to join the components to the circuit board. In the through hole technology (not shown), holes for the leads of the components are formed in the circuit board. The leads of the components are led through the holes, thereby leaving the component on a first side of the circuit boards, and the lead of the component onto a second side of the circuit board. The leads are joined, e.g. soldered, to the circuit board on the second side of the circuit board. Nowadays the surface mount technology is more common, in particular in consumer electronics products, than the through hole technology.

Figure 3b shows, in a side view, an electronic device, wherein the electronic components 222, 224, and 226 have been bonded to the first at least partly electrically conductive layer 120 of the circuit board 100. Some of the components may be used for radio frequency (RF) communication. In addition, part of the wires 122 (c.f. Fig. 1 b) of the first layer 120 may form an antenna or several antennas for RF communication. In this case, the second at least partly electrically conductive layer 130 may be used as an electromagnetic (EM) shield for the RF components. The shield 130 may be used to protect the rest of the device from RF signals generated on the layer 120. For EM shielding purposes, the layer 130 may be a fully electrically conductive layer. The layer 130 may alternatively be a partly electrically conductive layer.

Figure 3c shows, in a side view, an electronic device comprising components 222, 224, 226, 232, and 234, and a circuit board 100. The circuit board 100 of Fig. 3c comprises a first substrate 1 10a and a second substrate 1 10b. The substrates are manufactured from similar materials as the substrate of Figs 1 a-1 d.

The circuit board 100 of Fig. 3c comprises a first at least partly electrically conductive layer 120. The first at least partly electrically conductive layer 120 comprises wires 122 and pads 124 for bonding or joining the electrical components to the circuit board 100. The first at least partly electrically conductive layer 120 is arranged on a first side of the first substrate 1 10a. The circuit board 100 of Fig. 3c comprises a second at least partly electrically conductive layer 130. The second at least partly electrically conductive layer 130 is arranged on the second side of the first substrate 1 10a. In addition, the second at least partly electrically conductive layer 130 is arranged on a first side of the second substrate 1 10b. The second at least partly electrically conductive layer 130 is arranged a distance apart from the first at least partly electrically conductive layer 120.

The circuit board 100 of Fig. 3c further comprises a third at least partly electrically conductive layer 140. The third at least partly electrically conductive layer 140 is arranged on the second side of the second substrate 1 10b. The third at least partly electrically conductive layer 140 comprises wires and pads for bonding or joining the electrical components to the circuit board 100. The third at least partly electrically conductive layer 140 is arranged a first distance apart from the first at least partly electrically conductive layer 120 and a second distance apart from the second at least partly electrically conductive layer 130.

The second at least partly electrically conductive layer 130 can be used for to at least one of

1 ) Rerouting signals from the first layer 120 to the third layer 140.

Rerouting may be beneficial when the wires of the first layer 120 or the third layer 140 are closely spaced. When the second layer 130 is used for rerouting, the second layer 130 is partly electrically conductive. 2) Electromagnetic shielding. The first layer 120 and/or the third layer 140 may comprise radio frequency (RF) components and/or antennas.

To prevent interference between these layers, an electromagnetic shielding layer 130 may be provided in the circuit board 100. When the second layer 130 is used for shielding, the second layer 130 may be partly electrically conductive or fully electrically conductive. It is evident that the circuit board 100 may comprise a third substrate and fourth at least partly electrically conductive layer. It is also evident that the circuit board 100 may comprise a plurality (N) of substrates and another plurality (N+1 ) of at least partly electrically conductive layers, wherein N is an integer at least one.

Referring to Figs. 3c and 1 d, the circuit board may comprise a via 1 12. The via may be arranged to penetrate one substrate, as depicted in Fig. 1 d. When the circuit board comprises two substrates, a via may penetrate only one substrate, wherein the via is arranged to conduct electricity from an at least partly conductive layer to an adjacent at least partly conductive layer. However, when the circuit board comprises two substrates, a via may penetrate also both the substrates, wherein the via is arranged to conduct electricity from a surface of the circuit board 100 to another surface of the circuit board 100. In a corresponding manner, when the circuit board comprises a multiple (N) of substrates, a via may penetrate any number of substrates, wherein the any number of substrates is from one to the multiple (N) of substrates.

Referring to Fig. 3d, an electronic device may comprise several circuit boards 100. The circuit boards 100 may be stacked onto each other. Fig. 3d shows a device comprising a first circuit board 100a and a second circuit board 100b. The circuit boards 100a and 100b comprise the substrates 1 10a and 1 10b, and the at least partly electrically conductive layers 120a, 130a, 120b, and 130b. Furthermore, the device comprises an interposer 310 arranged to separate the first circuit board 100a from the second circuit board 100b. The interposer 310 comprises an inter-board via 312 arranged to conduct electrical signals and/or energy from the first circuit board 100a to the second circuit board 100b. The interposer 310 may be made of the same materials as the substrates 1 10a, 1 10b. The interposer 310 may be made using the same methods as used for making a substrate 1 10 or a circuit board 100. A hole may be made, e.g. cut or sawn, in the central part of the interposer, to make the space required for the components in between the circuit boards 100a and 100b. The inter-board via 312 can be made to the interposer using same methods and/or same materials as are used for the vias 1 12 in the substrates 1 10 (Figs. 1 d and 1 e).

The electronic device, as described in Figs. 3a-3d, may be a part of another electronic device. The electronic device or the another electronic device may be e.g. a portable electronics product such as a mobile phone, an MP3 player, a laptop computer, an electronic key (such as a car key), or a remote controller. In addition, the electronic device or the another electronic may be a sensor device. The sensor device may be arranged to measure some physical parameters, e.g. values on environment. The values may be stored in the device or sent from the device. As the device may be at least partly biodegradable, it may be possible to leave the device to the place where it measures, and not collect the devices after service life.

The substrate 1 10 of the printed circuit board 100 comprises polylactide (PLA) and lignin. The circuit board 100 may be manufactured by

- mixing PLA with lignin in an elevated temperature to form substrate material,

- forming a substrate 1 10 from the substrate material, and

- at least partly metalizing a first surface and a second surface of the substrate 1 10 by metallization means. Alternatively the circuit board 100 may be manufactured by

- forming a substrate 1 10 from substrate material and

- at least partly metalizing a first surface and a second surface of the substrate 1 10 by metallization means, wherein

- the substrate material is produced from substrate material granules.

In both cases substrate material is produced by mixing PLA with lignin in an elevated temperature.

The step "at least partly metalizing a first surface and a second surface of the substrate 1 10 by metallization means" has been described above. In an embodiment, the substrate material is manufactured by mixing polylactide (PLA) and lignin. Thus, the substrate comprises PLA and lignin. In some embodiments, the substrate consists of PLA and lignin. In some embodiments, additives are used in addition to PLA and lignin to improve the properties of the composite. Such additives may comprise at least one of

- crystallization substances for facilitating crystallization of the PLA-lignin composite,

- fire resistant additives; the circuit board substrate should comply with a plastics flammability standard, e.g. the standard U94 (released by Underwriters Laboratories; with reference to the latest version of the standard available on March 19^th, 2012) , preferably class V0; or another applicable standard for flame retardancy,

- impact modifiers,

- mold-release additives for releasing the circuit board substrate from the mold,

- slip agent for improving extrusion properties, and

- chain extender to prevent hydrolysis in the process; wherein the chain extenders are known to a person skilled in the art. Thus, the substrate may comprise PLA, lignin, and at least one additive. The substrate may consist of PLA, lignin, and at least one additive.

In addition to additives, the purity of the feedstock also affects whether the substrate material comprises lignin and PLA or the substrate material can be considered to consist of lignin and PLA. The substrate material may comprise lignin, PLA, additives, and impurities.

The substrate material of an embodiment comprises PLA and lignin that consists of Kraft lignin extracted from wood. The substrate material of an embodiment consists of PLA and Kraft lignin extracted from wood. The trade name for the used PLA is NatureWorks Ingeo™ Biopolymer 3251 D. This biopolymer comprises PLLA and PDLA; however, the PDLA content is very low (about 1 .5 %), whereby the melting point of this biopolymer is relatively low. The Kraft lignin is extracted from wood in the Kraft pulping process. The used PLA has a high melt flow rate. The melt flow rate a measure of the ease of flow of the melt of a thermoplastic polymer. As the PLA has a high melt flow rate, other materials, such as lignin, are easily mixed with the used PLA.

When mixing lignin with a PLA polymer it was noticed that the mixing is sensitive to the alkalinity (i.e. pH-value) of the lignin. The pH-value is defined as the pH-value of a water-lignin solution with 50 mass-% dry matter content. It was noticed that alkali lignin could not be used, since the high pH-value of the lignin affected the PLA polymer in such a way that a smooth mixture could not be obtained. In contrast, Kraft lignin, having an almost neutral pH- value, could be used in the PLA-lignin composition for a circuit board. In addition, a lignin mixture comprising at least one of Kraft lignin, sulphite lignin, and soda lignin can be used as the lignin for the PLA-lignin circuit board. The alkalinity (pH-value) of the lignin mixture may depend on the composition of the lignin mixture. For example, a mixture comprising Kraft lignin and soda lignin can be used, since Kraft lignin may be acid or neutral, while soda lignin is alkaline. It was noticed that the alkalinity (pH-value) of the lignin (or lignin mixture) used in the PLA-lignin composition for a circuit board should be less than 8.5, preferably less than 8. In addition, it was noticed that the acidity (pH-value) of the lignin (or lignin mixture) used in the PLA-lignin composition for a circuit board should be more than 5, and preferably more than 6. An almost neutral lignin (having the pH-value between 6 and 8, e.g. around 7) is preferred, since an alkaline or an acid lignin requires durability of the extruder (or mixer) used to produce the substrate material. In addition, an alkaline or an acid lignin requires durability of the equipment used to process the PLA-lignin composite. Durability can be achieved with more durable materials, which commonly imply more expensive equipment. The pH-value of Kraft lignin is commonly within the specified range. Kraft lignin may be acid or neutral. In addition, the availability of Kraft lignin is good. Respectively, it may be preferably use Kraft lignin, not only because its neutrality, but also because its price is low. In addition, it was noticed, that the lignin, which is mixed with PLA, should be substantially dry. Substantially dry lignin is mixed with PLA more easily than wet lignin. Moreover, if water vaporizes is the mixing process, steam has to be led out from the process. In an embodiment, the dry matter content of the lignin is for example at least 90 %, preferably at least 95 %, and more preferably at least 97 %.

However, some humidity was also noticed to soften the lignin. Such soft lignin mixes well with PLA. Furthermore such soft lignin may have more appropriate mechanical properties for a circuit board than a hard, totally dry lignin. In an embodiment, the dry matter content of the lignin is at most 99 %. In an embodiment, the dry matter content of the lignin is at most 98 %. In these or other embodiments, the dry matter content of the lignin may be in the range from 90 % to 99 %, or in the range from 95 % to 99 %, or in the range from 97 % to 99 %.

The mixing of lignin to PLA may be performed in an elevated temperature, wherein the PLA is in liquid (viscous) form. Therefore, the materials should be mixed in a temperature above the melting temperature of PLA. However, when the temperature exceeds 200 °C, the lignin stats to degrade. Therefore, the temperature should be below 200 °C. The crystalline melt temperature for the PLA used was in the range from 155 °C to 170 °C. Therefore, the substrate material could be made by mixing lignin with PLA in a temperature from 170 °C to 200 °C. The substrate material used in the embodiment of Figs. 1 a-1 d was formed by mixing lignin with PLA in a temperature of 185 °C. This particular temperature was found to give very good viscosity for the PLA-lignin mixture for lignin contents from 0 to 40 vol-%. A temperature range from 180 °C to 190 °C may also give adequate viscous properties. Mixing was done in an extruder. The mixing and/or melting of lignin with the PLA can be affected by lowering the glass transition temperature (Tg) of lignin. The glass transition temperature can depend on the type of lignin, and can be affected by using softeners, and by modifying lignin. As for softeners for lignin, at least one of polyethylene glycol (PEG), polyvinyl alcohol (PVA), urea, and sorbitol can be used. Lignin may be modified using at least one of esterification, acetylation, maleation, and resination. Different lignin contents were tested for the substrate material for a circuit board 100. The lignin content was measured as a volumetric percentage (vol-%). The substrate material consists of ρ^χ 100% vol-% lignin. In an embodiment, rest of the substrate material, i.e. (1 -ρ)^χ 1 00% vol-% (since the polymers do not mix in such a way that the total volume decreases, was poly-L-lactide. As discussed above, lignin and/or poly-L-lactide may further comprise impurities. More specifically, when the substrate material is manufactured by mixing a volume V_PLA of poly-L-lactide and a volume V_L of lignin, the lignin content, p, equals to V_L/(V_L+VPLA)- Therefore, the material may be produced by mixing (1 -p) parts in volume of polylactide (PLA) with p parts in volume of lignin at the elevated temperature.

Here p is the content of lignin (as measured in vol-%); it is a number greater than zero (greater, since the substrate comprises lignin). Thus defined, the content in volume, p, is related to the content is mass through the densities of the materials. The volumes may be written in terms of mass and density as V_L=m_L/pL and

Here m stands for mass and p for density, respectively. The subscripts L and PLA stand for lignin and polylactide, respectively. The densities of the materials are PPLA=1 240 kg/m³ and pL=1 349.5 kg/m³. The given density for polylactide applies only to the specific polylactide, NatureWorks Ingeo™ Biopolymer 3251 D. The given density for lignin applies for the used Kraft lignin. Using the equation, the mass percentage can be calculated as p_m=pL/(pL+pPLA(1 -p)/p), wherein p_m is the ratio of the mass of lignin in the PLA-lignin composite to the total mass of the composite. As the density of lignin is greater than the density of PLA, the mass content of lignin, p_m, is slightly more than the volume content, p of lignin.

At the 1 0 vol-% lignin content, the decrease in material costs due to the addition of lignin exceed the increased manufacturing costs due to mixing different materials. Therefore, preferably the lignin content is at least 0.10 (i.e. 1 0 vol-%). The surface roughness of the material was seen to increase when the lignin content exceeded 40 vol-%. Therefore, preferably the lignin content is at most 0.40 (i.e. 40 vol-%). However, it was also observed, that when the lignin content exceeded 20 vol-%, the substrate material became brittle. Therefore, to enhance the fracture resistance of the substrate material, the lignin content should be at most 0.20 (i.e. 20 vol-%). The mechanical properties of the lignin-PLA composite may depend on the type of the lignin used, and whether the lignin has been modified or not. The (volume) contents 0 vol-%, 1 0 vol-%, 20 vol-%, and 40 vol-% of lignin correspond to the mass contents 0 m-%, 1 1 m-%, 21 m-%, and 42 m-% of lignin, respectively.

The mechanical and electrical properties of the substrate 1 1 0 depends on the lignin content p, as will be discussed later. Moreover, the mechanical and electrical properties of the substrate 1 1 0 may depend on the type of PLA used in the circuit boards and on the type of lignin used in the circuit board.

The substrate can be moulded, injection moulded, or extruded, from the substrate material.

In injection moulding, the substrate material may be injected to mould. The temperature of the material may be in the range from 1 70 °C to 200 °C, preferably from 1 75 °C to 1 85 °C in order to have a reasonably viscosity for the substrate material.

A manufacturing process is shown in Fig. 4. The process starts by obtaining a volume V_L of lignin and a volume of V_PLA of polylactide. The polylactide is melted, and the lignin is mixed with the polylactide. In this way, substrate material is produced. The volume of substrate material, V, is the sums of the volumes of the materials, V=V_L+V_PLA- The lignin content p of the substrate material as measured in vol-% is p=V_L/V. A substrate 1 1 0 is injection moulded or extruded (or formed by other means) from the substrate material. Injection moulding can be used to form non-planar, i.e. three-dimensional, objects, such as circuit boards with arbitrary shape. The non-planar object may be e.g. a cover of an electronic device. In addition, a non-planar circuit board bay be adapted to fit into an arbitrarily formed device. The substrate may be formed by injection moulding substrate material. Extrusion, on the other hand, is a relatively cheap way to manufacture planar objects. Extrusion does not require a mould, and thus extrusion is cheaper than moulding or injection moulding. Extrusion can be used to form a film-like substrate, wherein the substrate is relatively thin (as discussed above). The substrate may be formed by extruding substrate material.

The substrate 1 10 may have a planar shape. A first surface of the substrate is metalized to from the first at least partly electrically conducting layer 120. A second surface of the substrate is metalized to from the second at least partly electrically conducting layer 130. In Fig. 4, the layers 120 and 130 are fully electrically conductive. However, as discussed with relation to Fig. 2b, also partly electrically conductive layers could be metalized. In addition, one partly electrically conductive layer and one fully electrically conductive layer could be made. Subsequent process steps depend on the type of circuit board to be manufactured.

1 ) The circuit board 100 can be marketed as such; or

2) At least one partially conducting layer can be patterned before marketing or before attaching a further substrate. Patterning refers to the process of selectively etching away part of the metalized layer, as discussed above; or

3) A further substrate can be attached onto the first or the second layer 120 or 130 (cf. Fig. 3c). The further substrate may comprise an at least partly electrically conducting layer, or such an at least partly electrically conducting layer can be metalized later. After attaching the further substrate, the circuit board may be

a. marketed (e.g. in case the further substrate comprises a fully electrically conductive layer or a partly electrically conductive layer; it is also possible that a surface of a circuit board comprising at least two substrates is not even partly metalized), b. metalized (in case the further substrate does not comprise a conductive layer)

c. patterned (in case the further substrate comprises a fully electrically conductive layer), or

d. returned for the attachment of a subsequent further substrate. The process is schematically shown in Fig. 4. The processing of vias 1 1 2 is not shown in the figure. Vias may be processed to the substrate or substrates 1 1 0. Finally, at least on surface of the circuit board may be partly covered with a solder-resist mask.

Another manufacturing process is shown in Fig. 5. The process starts by obtaining a volume V_L of lignin and a volume of V_PLA of polylactide. The polylactide is melted, and the lignin is mixed with the polylactide. In this way, as in Fig. 4, substrate material is produced. However, the substrate material is granulated to substrate material granules 51 0. As discussed above, when mixing lignin and PLA, the substrate material is in melted form. When granulating the melted composite material, droplets from the melted composite material may formed and cooled, to solidify the droplets to the substrate material granules 51 0. When granulating the melted composite material, a cylindrical bar of melted composite material may formed and cooled, and the bar may be cut to the substrate material granules 51 0.

The substrate material granules 510 can be marketed as such. Therefore, a PLA-lignin circuit board can be made from the substrate material granules. In addition, PLA-lignin substrate material granules can be made according to the process, as described above for the formation of substrate material. In particular, the preferable type of PLA, the preferable type of lignin, the applicable pH-value, lignin contents, and mixing temperatures apply equally well to the substrate material granules 51 0 as to the substrate material, since the substrate material granules consist of the substrate material. The substrate material granules 51 0 are suitable for substrate material for a biodegradable circuit board.

The size of a granule 51 0 may be, for example, in the range from 1 mm to 6 mm. The granule is typically cylindrical having a diameter and a height. At least one of these measures is in the specified range. Preferably both these measures are in the specified range. Preferably, the diameter is in the specified range. More preferably, the diameter is from 3 mm to 5 mm. The granules are not necessarily cylindrical. Granules of arbitrary shape can be characterized in terms of mass or volume. In an embodiment, the granules are cylindrical having a height of 2 mm and a diameter of 5 mm. The mass of hundred granules 510 may be, for example, in the range from 1 g to 10 g. Preferably the mass of hundred granules 510 is from 3 g to 6 g. Thus the mass of a granule is from 10 mg to 100 mg, preferably from 30 mg to 60 mg. In the above example, the mass of hundred granules was about 5 g, whereby the mass of a granule was about 50 mg.

In principle, the granules may be smaller, whereby such granules may be referred to as microgranules. Manufacturing microgranules may be more expensive than manufacturing granules.

The PLA-lignin composite granules 510 can be re-melted to form melted substrate material. The melted substrate material can be processed, such as extruded, to form the substrate 1 10. Thereafter the substrate can be metalized, patterned, attached to another substrate and/or marketed, as discussed in more detail above, with reference to Fig. 4.

The tensile yield stress and the tensile elongation (at yield) were measured for a PLA-lignin substrate material. The results are collected in table 1 . In table 1 , an elastic modulus is also calculated from these values, assuming that the material is linear up to the yield point, whereby the elastic modulus equals the tensile yield stress divided by tensile elongation.

Lignin Tensile Tensile yield Elastic

content, p elongation stress (MPa) modulus

(%) (MPa)

0 % 2.6 60 2300

10 % 1.8 48 2700

20 % 1 32 3200

Table 1 : Mechanical properties of PLA-lignin substrate as function of lignin content.

As can be seen from the Table, increasing lignin content decreases the strength of the substrate. Moreover, increasing lignin content increases the elastic modulus. Therefore, is a non-rigid or flexible circuit board is required, decreasing the lignin content improves the flexibility in two ways: through the modulus and through the elongation. Conversely, for rigid circuit boards a high lignin content may be used. The thicknesses for flexible, non-rigid and rigid substrates were discussed above. The bending stiffness of a circuit boards (i.e. a plate or a beam) generally depends e.g. on the elastic modulus, E, and the thickness, T_s. As is well known from theories of plates and beams, the stiffness (i.e. flexural rigidity) is proportional to the elastic modulus (E), and to the third power of the thickness (T_s ³). (Note: the flexural rigidity is defined as Ex l, wherein I is the moment of inertia, depending e.g. on the thickness as discussed above). The relative permittivity and the loss factor were measured using a low frequency (1 MHz) and a high frequency (1 GHz) signal. Generally, for alternating signals having the angular frequency co, a material has a complex relative permittivity of the form ε_τ (ω) = ε_τ ^τ (ω) + ίε_ι.' (ώ) where ε_Γ(ω) is the complex relative permittivity, ε (ω) is the real part of the complex relative permittivity, εΙ(ω) is the imaginary part of the complex relative permittivity, and / is the imaginary unit. The loss factor is commonly denoted by tan δ, and is defined by tan δ =—

As is clear from the above discussion, the loss factor also depends on the angular frequency ω of the measuring signal.

The dielectric properties of PLA-lignin substrates are given in Table 2.

Lignin

content, p ε_Γ at tan δ at tan δ at

1MHz 1MHz ε_Γ at 1GHz 1GHz

0 % 4.6 0.008 3.1 0.003

10 % 4.6 0.009 3.3 0.005

20 % 4.9 0.011 3.4 0.008

30 % 5.1 0.013 3.6 0.011

40 % 5.3 0.015 3.7 0.013 Table 2. Dielectric properties of PLA-lignin substrates

for different lignin contents p and for different frequencies. From the table it can be seen that increasing the lignin content increases the relative permittivity. At the low frequency, the relative permittivity seems to become approximately 1 .15 fold, when increasing the lignin content from zero to 40 vol-%. At the high frequency, the relative permittivity seems to become approximately 1 .19 fold, when increasing the lignin content from zero to 40 vol-%. The permittivity is important particularly in RF applications. The wavelength of an EM signal depends on the relative permittivity. The wavelength of an EM signal in a material is λ=λ₀Λ/ε,., wherein λ is the wavelength of an EM signal in a material having the relative permittivity e_r, and λ₀ is the wavelength of the EM signal in air. Therefore, for some applications a high lignin content is beneficial.

Moreover, it can be seen that the loss factor increases with increasing lignin content. At the low frequency, the loss factor seems to become approximately two fold, when increasing the lignin content from zero to 40 vol-%. At the high frequency, the loss factor seems to become approximately four fold, when increasing the lignin content from zero to 40 vol-%.

Both the permittivity and the loss factor seem not to be heavily dependent on the lignin content. Therefore it seems, that the dielectric properties of the PLA are not significantly deteriorated by the addition of lignin. Thus it seems that a lignin-PLA composite can be used as a substrate material in circuit boards as well as pure PLA. However, the cost of lignin is significantly lower than PLA. Both lignin and PLA and biodegradable. In addition, the composition of PLA and lignin is biodegradable. Therefore, the lignin-PLA composite may serve as a cheaper alternative for biodegradable circuit boards as a pure PLA.

As discussed above, the lignin-PLA composite circuit board can be preferably manufactured through a multiple of selections relating to the process parameters including:

- type, alkalinity, and/or dry matter content of lignin, - type of PLA,

- type of functional additives for PLA and/or lignin,

- mixing temperature,

- lignin content,

- substrate thickness, and

- material and means for metallization.

The invention is not restricted to the enclosed embodiments, but can be applied within the scope of the claims.

Claims

Claims:

1 . A circuit board comprising

- a substrate,

- a first at least partly electrically conductive layer, and

- a second at least partly electrically conductive layer, wherein

- the second at least partly electrically conductive layer is arranged a distance apart from the first at least partly electrically conductive layer, and

- the substrate comprises polylactide and lignin.

2. The circuit board of claim 1 , wherein

- the substrate is a planar object having a first surface and a second surface,

- the first at least partly electrically conductive layer is arranged on the first surface of the substrate, and

- the second at least partly electrically conductive layer is arranged on the second surface of the substrate.

3. The circuit board of claim 1 or 2, wherein the polylactide of the substrate comprises one of

- a polylactide consisting essentially of poly-L-lactide (PLLA),

- block polymers of poly-D-lactide (PDLA) and poly-L-lactide (PLLA), and

- stereocomplexes of poly-D-lactide (PDLA) and poly-L-lactide (PLLA).

4. The circuit board (100) of any of the claims 1 to 3, wherein the circuit board comprises

- at least one via, conductor or a wire arranged to conduct electrical signal and/or energy from the first at least partly electrically conductive layer to the second at least partly electrically conductive layer.

5. The circuit board of any of the claims 1 to 4, wherein

- the lignin content, p, of the circuit board is greater than 0 vol-% and at most 40 vol-%.

6. The circuit board of claim 5, wherein

- the lignin content, p, of the circuit board is greater than 0 vol-% and at most 20 vol-%.

7. The circuit board of claim 5, wherein

- the lignin content, p, of the circuit board is from 10 vol-% to 40 vol-%.

8. The circuit board of claim 5, wherein

- the lignin content, p, of the circuit board is from 10 vol-% to 20 vol-%.

9. The circuit board of any of the claims 1 to 8, wherein

- the thickness of the substrate, T_s, is less than 1 .5 mm.

10. The circuit board of any of the claims 1 to 9, wherein

- the thickness of the substrate, T_s, is at least 10 μιτι.

1 1 . The circuit board of any of the claims 1 to 10, wherein

- the thickness of the substrate , T_s, is from 50 μιτι to 500 μιτι.

12. The circuit board of any of the claims 1 to 1 1 , wherein

- at least one of the first and the second least partly electrically conductive layers comprises at least one of silver and copper.

13. The circuit board of any of the claims 1 to 12, wherein the circuit board comprises

- another substrate and

- a third at least partly electrically conductive layer, wherein

- the third at least partly electrically conductive layer is arranged a first distance apart from the first at least partly electrically conductive layer and a second distance apart from the second at least partly electrically conductive layer.

14. An electronic device comprising

- at least one electronic component and

- a circuit board of any of the claims 1 to 13.

15. An electronic device of claim 14, wherein the electronic component is selected from the group comprising resistors, thermistors, capacitors, inductors, switches, relays, displays, conductors, sensors, transducers, actuators, and semiconductors such as diodes, varactors, light emitting diodes, transistors, lasers, and integrated circuits.

16. An electronic device of claim 14 or 15, wherein the electronic device is selected from the group of sensor devices and portable electronic devices such as mobile phones, MP3 players, laptop computers, electronic keys, and remote controllers.

17. A substrate material granule for manufacturing a substrate for a circuit board of any of the claims 1 to 16, comprising

- substrate material for a biodegradable circuit board, wherein

- the substrate material comprises polylactide and lignin.

18. The substrate material granule of claim 17, wherein the polylactide of the substrate material granule comprises one of

- a polylactide consisting essentially of poly-L-lactide (PLLA),

- block polymers of poly-D-lactide (PDLA) and poly-L-lactide (PLLA), and

- stereocomplexes of poly-D-lactide (PDLA) and poly-L-lactide (PLLA).

19. The substrate material granule of claim 17 or 18, wherein

- the substrate material granule comprises Kraft lignin.

20. The substrate material granule of any of the claims 17 to 19, wherein

- the lignin content, p, of the substrate material granule is greater than 0 vol-% and at most 40 vol-%.

21 . The substrate material granule of claim 20, wherein

- the lignin content, p, of the substrate material granule is greater than 0 vol-% and at most 20 vol-%.

22. The substrate material granule of claim 20, wherein

- the lignin content, p, of the substrate material granule is from 10 vol-% to 40 vol-%.

23. The substrate material granule of claim 20, wherein - the lignin content, p, of the substrate material granule is from 10 vol-% to 20 vol-%.

24. The substrate material granule of any of the claims 17 to 23, wherein - the diameter of the substrate material granule is from 3 mm to 5 mm.

25. The substrate material granule of any of the claims 17 to 24, wherein

- the mass of the substrate material granule is froml O mg to 100 mg.

26. A method for manufacturing substrate material granules of any of the claims 17 to 25, comprising

- mixing polylactide (PLA) with lignin in an elevated temperature to form substrate material, and

- forming substrate material granules from the substrate material.

27. The method of claim 26, wherein the polylactide of the substrate material comprises one of

- a polylactide consisting essentially of poly-L-lactide (PLLA),

- block polymers of poly-D-lactide (PDLA) and poly-L-lactide (PLLA), and - stereocomplexes of poly-D-lactide (PDLA) and poly-L-lactide (PLLA).

28. The method of claim 26 or 27, wherein

- the lignin, when solved with water such that the dry matter content of the solution is 50 %, has an alkalinity (pH-value) less than 8.5.

29. The method of claim 28, wherein

- the lignin, when solved with water such that the dry matter content of the solution is 50 %, has an alkalinity (pH-value) less than 8.

30. The method of any of the claims 26 to 29, wherein

- the lignin, when solved with water such that the dry matter content of the solution is 50 %, has an acidity (pH-value) more than 5.

31 . The method of claim 30, wherein

- the lignin, when solved with water such that the dry matter content of the solution is 50 %, has an acidity (pH-value) more than 6.

32. The method of any of the claims 26 to 31 , wherein

- the dry matter content of the lignin is at least 90 %.

33. The method of any of the claims 26 to 32, wherein

- the dry matter content of the lignin is at most 99 %.

34. The method of any of the claims 26 - 33, wherein

- the lignin comprises Kraft lignin.

35. The method of any of the claims 26 - 34, wherein

- the elevated temperature is from 170 °C to 200 °C.

36. The method of claim 35, wherein

- the elevated temperature is from 180 °C to 190 °C.

37. The method of any of the claims 26 - 36, comprising

- mixing polylactide (PLA) with lignin in the elevated temperature to form the substrate material such that the lignin content of the substrate material is at most 40 vol-%.

38. The method of claim 37, comprising

- mixing polylactide (PLA) with lignin in the elevated temperature to form the substrate material such that the lignin content of the substrate material is at most 20 vol-%.

39. The method of claim 37, comprising

- mixing polylactide (PLA) with lignin in the elevated temperature to form the substrate material such that the lignin content of the substrate material is at least 10 vol-% and at most 40 vol-%.

40. The method of claim 37, comprising

- mixing polylactide (PLA) with lignin in the elevated temperature to form the substrate material such that the lignin content of the substrate material is at least 10 vol-% and at most 20 vol-%.

41 . A method for manufacturing the circuit board of claim 1 , the method comprising

- forming a substrate from substrate material,

- at least partly metalizing a first surface and a second surface of the substrate to form a first at least partly electrically conductive layer and a second at least partly electrically conductive layer, wherein

- the substrate material comprises polylactide (PLA) and lignin.

42. The method of claim 41 , comprising

- mixing polylactide (PLA) with lignin in an elevated temperature to produce the substrate material.

43. The method of claim 42, wherein the lignin comprises one of

- a polylactide consisting essentially of poly-L-lactide (PLLA),

- block polymers of poly-D-lactide (PDLA) and poly-L-lactide (PLLA), and

- stereocomplexes of poly-D-lactide (PDLA) and poly-L-lactide (PLLA).

44. The method of claims 42 or 43, wherein

45. The method of claim 44, wherein

46. The method of any of the claims 42 to 45, wherein

- the lignin, when solved with water such that the dry matter content of the solution is 50 %, has an acidity (pH-value) more than 5.5.

47. The method of claim 46, wherein

48. The method of any of the claims 42 to 47, wherein

- the lignin comprises Kraft lignin.

49. The method of any of the claims 42 to 48, wherein

- the dry matter content of the lignin is at least 90 %.

50. The method of any of the claims 42 to 49, wherein

- the dry matter content of the lignin is at most 99 %.

51 . The method of any of the claims 42 to 50, wherein

- the elevated temperature is from 170 °C to 200 °C.

52. The method of claim 51 , wherein

- the elevated temperature is from 180 °C to 190 °C.

53. The method of any of the claims 42 to 52, comprising

54. The method of claim 53, comprising

55. The method of claim 53, comprising

56. The method of claim 53, comprising

57. The method of claim 41 , comprising

- producing the substrate material using substrate material granules of any of the claims 17 to 25.

58. The method of any of the claims 41 to 57, comprising

- forming a planar substrate from the substrate material.

59. The method of any of the claims 41 to 58, comprising

- forming the substrate by extruding or injection moulding the substrate material.

60. The method of any of the claims 41 to 59, comprising

- using a metallization means for at least partly metalizing a first surface and a second surface of the substrate to form a first at least partly electrically conductive layer and a second at least partly electrically conductive layer and

- selecting the metallization means from the group comprising printing, sputtering, evaporation, and spraying.

61 . The method of any of the claims 41 to 60, comprising

- using a metal, an ink or an adhesive for at least partly metalizing a first surface and a second surface of the substrate to form a first at least partly electrically conductive layer and a second at least partly electrically conductive layer, wherein

- the metal, the ink or the adhesive comprises at least one of copper and silver.

62. Use of the substrate material granule of any of the claims 17 to 25 for manufacturing a circuit board of any of the claims 1 to 13.

63. Use of the substrate material granule of any of the claims 17 to 25 for manufacturing an electronic device of any of the claims 14 to 16.