WO2021108775A1

WO2021108775A1 - Pcb fabrication with dielectric powder or suspension

Info

Publication number: WO2021108775A1
Application number: PCT/US2020/062513
Authority: WO
Inventors: James Rathburn
Original assignee: Hsio Technologies, Llc
Priority date: 2019-11-27
Filing date: 2020-11-27
Publication date: 2021-06-03

Abstract

A method of fabricating a printed circuit board (PCB) is described. The method includes providing a PCB core, the core having a first nan-conductive layer and a first plurality of conductive lines exposed at a first surface of the first non-conductive layer. A first dielectric powder is deposited on the first surface. The first dielectric powder is then heated to form a first liquified dielectric on the first surface. The first liquified dielectric is then cooled form a first hardened dielectric on the first surface of the first non-conductive layer.

Description

PCB FABRICATION WITH DIELECTRIC POWDER OR SUSPENSION RELATED APPLICATIONS [0001] This application claims the benefit of United States Provisional Application No. 62/941,310, filed on Nov.27, 2019, entitled “CUSTOMIZED DIELECTRIC POLYMER WITH PHOTO IMAGEABLE OPTION”, which is hereby incorporated herein by reference. BACKGROUND [0002] Traditional printed circuits are often constructed in what is commonly called rigid or flexible formats. The rigid versions are used in nearly every electronic system, where the printed circuit board (PCB) is a laminate of materials and circuits that when built is relatively stiff or rigid and cannot be bent significantly without damage. Flexible PCBs have become popular in many applications where the ability to bend the circuit to connect one member of a system to another has some benefit. These flexible PCBs are made in a similar fashion as rigid PCBs, where layers circuits or circuitry and dielectric are laminated. The main difference is the material set used for construction. [0003] Typical flexible PCBs start with a polymer film that is clad, laminated, or deposited with copper. A photolithography image with the desired circuitry geometry is printed onto the copper, and the copper film is etched to remove the unwanted materials. The PCB core is them built up and processed in a manner similar to that of rigid PCBs with a series of imaging, masking, drilling, via creation, plating, trimming, etc. The resulting PCB is flexible in such a way that as it is bent, the polymer film bends and supports the copper circuitry in a way that it does not crack or break. These circuits are solderable and can have devices attached to provide some desired function. They are commonly used in many electronic systems such as notebook computers, medical devices, displays, handheld devices, autos, aircraft, and many others. These flexible PCBs can be used in high frequency applications where the material set and design features can often provide better electrical performance than a comparable rigid circuit. [0004] Flexible RGBs can be connected to a larger system in a variety of ways. In most cases, a portion of the circuitry is exposed to create a connection point in terminal. Once exposed, the terminal can be connected to another circuit or component by soldering, conductive adhesive, thermosonic welding, pressure or some sort of connector. In general, the terminals are located on an end of the circuit, where edge traces are exposed or in some cases an area array of terminals are exposed. Often there is some sort of mechanical enhancement at or near the connection to prevent the joints from being disconnected during use or flexure.

BRIEF DESCRIPTION

[0005] Embodiments for a method of fabricating a printed circuit board (PCB) are described. The method includes providing a PCB core, the core having a first non-conductive layer and a first plurality of conductive lines exposed at a first surface of the first non-conductive layer. A first dielectric powder is deposited on the first surface. The first dielectric powder is then heated to form a first liquified dielectric on the first surface. The first liquified dielectric is then cooled form a first hardened dielectric on the first surface of the first non-conductive layer. [0006] Embodiments for another method of fabricating a printed circuit board (PCB) are also described. The method includes providing a PCB core, the core having a first non-conductive layer and a first plurality of conductive lines exposed at a first surface of the first non- conductive layer. A first dielectric suspension is deposited on the first, surface. The first dielectric suspension includes a first dielectric powder suspended in a solvent. The first dielectric suspension is then heated to form a first liquified dielectric on the first surface. The first liquified dielectric is then cooled form a first hardened dielectric on the first surface of the first non-conductive layer.

DRAWINGS

[0007] Understanding that the drawings depict only exemplary embodiments and are not therefore to be considered limiting in scope, the exemplary embodiments will be described with additional specificity and detail through the use of the accompanying drawings, in which: [0008] FIG. 1 is a cross sectional view of an example PCB core;

[0009] FIG. 2 is a cross-sectional view of the example PCB core of FIG. 1 having dielectric powder on a first surface thereof;

[0010] FIG. 3 is a cross-sectional view of the example PCB core of FIG. 2 wherein the dielectric powder has been thermal cycled to form a hardened dielectric layer;

[0011] FIG. 4 is a cross-sectional view of the example PCB core of FIG. 3 having a second dielectric layer formed on a second surface thereof;

[0012] FIG. 5 is a cross-sectional view of the example PCB core of FIG. 4 having one or more trenches and vias etched therein.

DETAILED DESCRIPTION

[0013] In general, flexible RGBs fill a needed function within the electronics industry. They can be considered expensive compared to some rigid PCB products. Flexible RGBs to have some limitations regarding layer count or feature registration, and they are generally used for small or elongated applications.

[0014] Rigid PGBs and package substrates experience challenges as the feature sizes and line spacing are reduces to achieve further miniaturization and increased circuit density. The use of laser ablation has become increasingly used to create the via structures for fine line or fine pitch structures. The use of lasers allows localized structure creation, where the processed circuits are plated together to create via connections from one layer to another. As density increases, the laser processed via structures can experience significant taper, carbon contamination, layer to layer shorting during the plating process due to registration issues, and high resistance interconnections that may be prone to result in reliability issues. The challenge of making fine line PCBs often relates to the difficulty in creating small or blind and buried vias. [0015] Liquid crystal polymer (LCP) has been used in applications with a method called fusion bonding where the base LCP material is laminated with sufficient heat and pressure to cause multiple layers of LCP to bond to each other to fuse to resemble a contiguous material set consisting of LCP. During lamination, the material is processed close to the melt temperature of LCP without rising into the liquidous phase where loss of definition, material movement, circuit movement and circuit embossing are more likely to occur. A limitation of this use of LCP is reached when a circuit stack beyond 4 to 5 layers is desired with fine circuit geometry contained within the stack. The sequential lamination used to build multi-layer constructions challenges the material set where previously created LCP to LCP fusion bond interfaces can weaken and embedded circuitry can be disrupted, in some cases, this many sequential laminations also results in the LCP to copper bond having inadequate reliability resulting in deiamlnation.

[0016] For high speed applications, impedance control is provided by surrounding the circuits on a PCB with a low loss dielectric to retain as much of the input signal as possible. Most dielectrics used in PCB fabrication are supplied in sheet form and the bonding layers are primarily in sheet form as well. As dielectrics get thinner to maintain impedance targets, most of those materials are of contiguous content across the sheet and the designer must live with the provided electrical and mechanical properties without much opportunity for modification. [0017] The subject matter described herein uses dielectric powder or suspension to fabricate high performance circuit structures the enable increased circuit density. The dielectric powder or suspension fabrication processes herein can be used to create dielectric layers that are thin and/or fill small spaces between conductive lines enabling use of conductive lines having a fine line and/or pitch dimension.

[0018] Figure 1 is a block diagram of an example PCB core 100 onto which a dielectric powder or suspension can be used. The PCB core 100 has a first non-conductive layer 102 and a first plurality of conductive lines 104 exposed on a first surface 106 of the first non-conductive layer 102. Solid portions of the Figures herein represent metal, such as copper. The first non- conductive layer 102 can be composed of any suitable non-conductive material including liquid crystal polymer (LCP), ajinomoto build-up film (ABF), polyimide, polyamide, Teflon, a ceramic, or a glass. The first plurality of conductive lines 104 can be composed of copper. At least a portion of the first plurality of conductive lines 104 is exposed at the first surface 106 of the first non-conductive layer. The exposed portion of the conductive lines 104 can be disposed on the first surface 104 or can be embedded flush with the first surface 104. The conductive lines 104 can also extend partially or completely through the non-conductive layer 102. A portion of the conductive lines 104 can also be exposed at a second surface 108 of the PCS core 100, wherein the second surface 108 is reverse of the first surface 106. in an example, the exposed portions of the conductive lines 104 on the first and/or second surfaces 106, 108 form a respective conductive layer of the PCB core 100.

[0019] Although only a single non-conductive layer 102 is shown with a conductive layer on each side thereof, in other examples, the PCB core 100 can have different numbers of conductive layers and non-conductive layers. For example, the PCB core can have multiple internal conductive layers with respective non-conductive layers between the conductive layers. Also, any of these PCB cores can have a conductive layer on one of the surfaces with no conductive layer on the other surface or can have a conductive layer on both the "top" and "bottom" surfaces. The PCB core 100 can also have other devices or structures thereon or in such as a packaged or bare semiconductor die or a hollow chamber.

[0020] Figure 2 is a cross-sectional view of the PCB core 100 having a dielectric powder 202 on the first surface 106. As the conductive lines and the spaces between the lines reduce, it becomes more difficult to process films of dielectric to cover and fill between the conductive lines. The subject matter described herein uses dielectric in a powder or suspension form to fill small spaces between lines and precisely control the thickness of a dielectric over a conductive line.

[0021] In Figure 2 the dielectric powder 202 is placed over and between exposed conductive lines 104 on the first surface 106. In an example, an electrostatic charge is applied to the powder to control the location of the particles. For example, an electrostatic charge can be applied to the powder 202 as it is applied to the surface 106 to control where the powder 202 accumulates on the surface 106. The electrostatic charge can push or puli the powder to move it towards or away from a desired location, in addition to or instead of applying charge while the powder is being applied, electrostatic charge can be applied after the powder 202 has accumulated on the surface 106 to hold the powder 202 in place or repel the powder from certain areas during further processing.

100221 In an example, multiple different dielectric powders 202 are applied to the same surface. Different dielectric powders can be used in order to have different electrical and/or mechanical properties in the dielectric at different locations. Thus, the resulting dielectric in the first area can have different electrical and/or mechanical properties than the resulting dielectric in the second area. In contrast to a sheet dielectric which has a uniform composition across the entire sheet, using dielectric powder 202 allows multiple different dielectric compositions to be composed on a single surface. The different compositions can be layered such that a second dielectric powder having a second composition can be placed on top of a first dielectric powder having a first composition. For example, a first dielectric powder can be placed in a first layer to fill the spaces between conductive lines in a conductive layer and optionally just above the conductive lines forming that conductive layer. A second dielectric powder can be placed in a second layer over the first dielectric layer to cover the conductive lines or to add thickness over the lines, in this way, dielectric with first electrical properties can be disposed between conductive lines in a conductive layer and dielectric with second electrical properties can be disposed overtop of those conductive lines. In another example, dielectric powder 202 can be applied at different thicknesses in different locations across the first surface 106. Thus, the resulting dielectric can be thicker or thinner in different areas as desired.

[0023] Alternatively, or in addition to, layering the different dielectric powders, the different dielectric powders can be placed in different areas on a surface. For example, a first dielectric powder 202 can be applied to a first area of the first surface 106 and a second dielectric powder 202 can be applied to a second area of the first surface 106. As should be understood, more than two different dielectric powders can be applied to a surface as well. Electrostatic charge can be used to aid in controlling the area on the surface in which a dielectric powder accumulates and/or is held.

[0024] In an alternative example, a dielectric suspension can be applied to a surface in addition to, or instead of, a dielectric powder. A dielectric suspension is a dielectric powder that is mixed with a liquid or semi-liquid solvent to create a liquid or semi-liquid suspension where the powder is suspended in the solvent. The suspension can take the form of a slurry, paste, or ink. The dielectric suspension can be applied to the first surface 106 to fill small spaces between conductive lines and allow precise control of the thickness of the dielectric. In an example, multiple different dielectric suspensions having different compositions resulting in different electrical properties can be applied to a single surface in a similar manner to that described with respect to the dielectric powders. A dielectric suspension can also be applied at different thicknesses in different locations similar to the dielectric powder 202.

[0025] The dielectric powder applied as a powder or included in a dielectric suspension can be composed of a power of at least, one of: liquid crystal polymer (LCP), ajinomoto build-up film (ABF), polyimide, polyamide, polytetrafluoroethylene (PTFE) (e.g., Teflon), a ceramic, or a glass, in an example, a dielectric powder used can be composed of a single one of the previously listed materials. In an alternative example, a dielectric powder can be composed of multiple of the previously listed materials or a combination of any one or more of the previously listed components along with one or more additional components added to achieve a desired effect. The effect achieved by the additional component can be any desired effect, such as an effect on a resulting electrical or mechanical property of the dielectric, or a processing effect, like allowing photocuring of the dielectric. Any desired electrical property can be adjusted, such as for Impedance or loss tuning of the dielectric. A wide assortment of mixtures is possible including LCP plus Teflon, LPC plus ABF, LCP plus boron nitride, LCP plus alumina, and LCP plus a bonding agent. For dielectric suspensions, any of these dielectric powders can be mixed with an appropriate solvent to form a dielectric suspension. In an example, the dielectric powder has the same composition or a different composition but the same general material as the non- conductive layer 102 (e.g., LCP) on which it is applied, which can allow good bonding between the two materials when heated, in some example, a base material (e.g., more than 50% by volume) of the dielectric powder is the same composition as the non-conductive layer 102. [0026] Figure 3 is a cross-sectional view of the PCB core 100 wherein the dielectric powder 202 or suspension has hardened into a dielectric layer 302. Once dielectric powder 202 or suspension has been applied to the first surface 106, the dielectric powder 202 can be heated and optionally pressed to melt the dielectric powder 202 forming a liquified dielectric. Advantageously, dielectric in powder form can melt at a relatively low temperature with little or no melting of existing materials of the PCB core 100. This enables the dielectric layer 302 to be formed with little or no melting of the non-conductive layer 102 in the PCB core 100. The melting temperature of the dielectric powder 202 is based on the material composition of the dielectric powder as well as the size of particles in the dielectric powder 202. Any of the available variants of LCP can be used, wherein different variants have different melt temperatures providing some latitude in lamination temperature and pressure ranges. If a dielectric suspension is placed on the first surface 106, the dielectric suspension can be heated to evaporate the solvent and form liquified dielectric.

[0027] Regardless of whether the dielectric started as a powder 202 or a suspension, the liquified dielectric can bond to surface with which it is in contact, for example, the non- conductive layer 102 and conductive lines 104. The liquified dielectric can be cooled to form a hardened dielectric layer 302 on the first surface 106. In an example, the liquified dielectric can be pressed to aid in filling the spaces on the first surface 106 and to aid in forming a planar surface, if desired, on the top of the hardened dielectric layer 302. if multiple dielectric powders 202 or suspensions are present on the first surface 106 all of the powders or suspensions can be heated and cooled simultaneously.

[0028] Advantageously, forming a dielectric layer 302 using a dielectric powder 202 or dielectric suspension enables the dielectric layer 302 to be thinner that is possible with existing dielectric pre-formed films, in an example, the gram structure of the dielectric layer 302 indicates that the dielectric layer 302 was formed from a dielectric powder 202 or suspension on the surface 106 of the PCB core 100. In such an example, the gram structure of a dielectric layer can be analyzed to determine whether the dielectric layer was formed from a dielectric powder 202 or suspension on the PCB core 100 or was formed from a pre-formed film or pre- formed bond ply material placed on the PCB core 100. In an example, a dielectric layer formed from multiple different dielectric powders or suspensions having different compositions is identifiable due because the dielectric layer is an uninterrupted structure with multiple different regions having different compositions and resulting electrical and/or mechanical properties.

[0029] The PCB core 100 with the hardened dielectric layer 302 can then be processed further as desired. For example, additional layers can be added to the PCB core 100. Figure 4 is a cross- sectional view of a second dielectric layer 402 formed on the second surface 108 of the PCB core 100. The second dielectric layer 402 can be added in the same manner as the first dielectric layer 302 by applying desired dielectric powder(s) 202 or suspension to the second surface 108 and thermal cycling. Further processing can also include adding one or more other additional layers such as metal layers, conductive lines, vias, devices (e.g., packaged or bare semiconductor dies), and/or non-conductive layers to the PCB core 100. Additional non- conductive layers can be added using the dielectric powder 202 or suspension techniques discussed herein or can be applied using existing techniques such as applying a dielectric film. [0030] Figure 5 is a cross-sectional view of the PCB core 100 having hardened dielectric layers 302 and 402 thereon with further processing. In this example, the second dielectric layer 402 had a photo-imageable material is added to the dielectric powder such that the hardened dielectric 302 is photo-imageable. This further processing, therefore, can include the dielectric layer 402 being exposed to UV light with a mask to cure certain areas. Uncured areas can be etched away to form trenches 502, vias 504, or other features as desired. Advantageously, forming features via UV light masking and etching as opposed to using lasers eliminates the need for a laser stop to be included in the PCB core 100. This also allows etching without having to apply a separate solder mask, which is a high loss material that can be difficult to register at high resolution.

[0031] Additional processing such as that described above can be repeated multiple times to add and processes additional layers forming a multi-layer stack circuit. Advantageously, using dielectric powder or suspensions any number of dielectric layers to be added to a multi-layer stack while subjecting the stack to temperatures below the melt temperature of existing non- conductive layers. This can potentially reduce damage caused for high layer stacks as compared to the repeated higher-temp thermal cycling in existing lamination processes.

Claims

CLAIMS What is claimed is:

1. A method of fabricating a printed circuit board (PCB) comprising: providing a PCB core, the core having a first non-conductive layer and a first plurality of conductive lines exposed at a first surface of the first non-conductive layer; depositing a first dielectric powder on the first surface; heating the first dielectric powder to form a first liquified dielectric; and cooling the first liquified dielectric to form a first hardened dielectric on the first surface of the first non-conductive layer.

2. The method of claim 1, wherein depositing the first dielectric powder includes depositing the first dielectric powder on a first area of the first surface; depositing a second dielectric powder on a second area of the first surface, the second dielectric powder having a different composition than the first dielectric powder, the second area being different than the first area.

3. The method of claim 1, wherein the first dielectric powder includes a powder of at least one of: liquid crystal polymer (LCP), ajinomoto build-up film (ABF), polyimide, polyamide, polytetrafluoroethylene (PTFE), a ceramic, or a glass.

4. The method of claim 3, wherein the non-conductive layer is composed of liquid crystal polymer (LCP).

5. The method of claim 3, wherein the first dielectric powder includes a photo-imageable material, exposing the first hardened dielectric to UV light with a mask to cure unmasked areas; and removing uncured portions of the first hardened dielectric.

6. The method of claim 1, wherein the first dielectric powder includes at least two different materials, a first of the two materials being a liquid crystal polymer (LCP) and a second of the two materials being at least one of: polytetrafluoroethylene (PTFE), ajinomoto build-up film(ABF), boron nitride, or a bonding agent,

7. The method of claim of 1, wherein the PCB core has a second plurality of conductive lines exposed on a second surface; depositing a third dielectric powder on the second surface; heating the third dielectric powder to form a second liquified dielectric; and cooling the second liquified dielectric to form a second hardened dielectric on the second surface.

8. The method of claim 1, comprising: applying an electrostatic charge to control a location of the powder on the first surface.

9. A method of fabricating a printed circuit board (PCB) comprising: providing a PCB core, the core having a first non-conductive layer with a first plurality of conductive lines exposed at a first surface of the first non-conductive layer; depositing a first dielectric suspension on the first surface, the first dielectric suspension including a first dielectric powder suspended in a solvent; heating the first dielectric suspension to evaporate the solvent forming a first liquified dielectric; and cooling the first liquified dielectric to form a first hardened dielectric on the first surface of the first non-conductive layer.

10. The method of claim 9, wherein depositing the first dielectric suspension includes depositing the first dielectric suspension on a first area of the first surface; depositing a second dielectric suspension on a second area of the first surface, the second dielectric suspension having a different composition than the first dielectric suspension, the second area being different than the first area.

11. The method of claim 9, wherein the first dielectric powder includes a powder of at least one of: liquid crystal polymer (LCP), ajinomoto build-up film (ABF), polyimide, polyamide, polytetrafluoroethylene (PTFE), a ceramic, or a glass.

12. The method of claim 11, wherein the non-conductive layer is composed of liquid crystal polymer (LCP).

13. The method of claim 11, wherein the first dielectric powder includes a photo-imageabie material, exposing the first hardened dielectric to UV light with a mask to cure unmasked areas; and removing uncured portions of the first hardened dielectric.

14. The method of claim 9, wherein the first dielectric powder includes at least two different materials, a first of the two materials being a liquid crystal polymer (LCP) and a second of the two materials being at least one of: polytetrafluoroethylene (PTFE), ajinomoto build-up film(ABF), boron nitride, or a bonding agent.

15. The method of claim of 9, wherein the PCB core has a second plurality of conductive lines exposed on a second surface; depositing a third dielectric powder on the second surface; heating the third dielectric powder to form a second liquified dielectric; and cooling the second liquified dielectric to form a second hardened dielectric on the second surface.