GB2117570A - Circuit boards - Google Patents

Abstract

Circuit assemblies are formed by selective vapour deposition of a metal on an insulating board carrying the circuit components. The metal provides a direct connection to each component thus obviating the need for subsequent soldering and may also bond each component to the board.

Description

SPECIFICATION Circuit boards This invention relates to methods of manufacturing electronic circuit assemblies and to circuit assemblies made by such methods.

At present the most commonly used method for making electrical or electronic circuitry is the manufacture of a printed circuit on a suitable insulating substrate having a pre-designed electrical conductor arrangement to which the various components and devices are electrically bonded by a soldering process.

The advantage of soldering is that all the electrical connections or bonds may be made in a single operation in a short time as compared to individually made bonds. It is common practice in this way to make over 2,000 electrical bonds within a minute or similar order of time. It will be evident that the electrical connections so made will often serve as mechanical fixtures resisting forces tending to separate the components or devices from the circuit assembly when in use.

The development of integrated circuit devices has resulted in the need to make printed circuit boards having smaller widths, closer spaced conductors. The tendency for liquid solder to freeze so as to bridge (short-circuit) the conductors increasingly restricts the use of mass soldering techniques as described above for integrated circuit device interconnection at this smaller scale.

The manufacture of complex circuitry on a very small scale, as for example Integrated Circuit devices, has for many years been performed by vacuum vapour deposition. This has been necessary because the size of the conductors is at least two orders of magnitude less than those of the smaller conventional printed circuit boards. The means for obtaining sufficiently defined conductor patterns for printed circuit boards were not sutiable to define much smaller but even more complex circuitry. The vacuum vapour processes used for making integrated circuits were therefore developed.

Normal evaporative vacuum vapour deposition is known as a line-of-sight process. This means that the atoms being deposited travel in straight lines frm the source to the substrate. Thus a surface presented to the vapour but parallel to this line will receive little or no deposit. This property is used to advantage when making integrated circuits on the flat polished surface, of a silicon wafer or chip.

However, producing an adequately conductive deposit on the edge of a connection surface having a thickness of over one thousandth of an inch, approximately 25 micrometres, or very much thicker as is often necessary for printed circuit applications, is not feasible unless the surface is presented in the beam of vapour by, for instance, tilting the substrate.

If this is done it becomes difficult to prevent deposition in areas which must not have a deposit, when defining the deposit with masks or resists as commonly used.

The further drawback to evaporative vacuum deposition is that adhesion of the deposit to many substrates, especially insulating plastics used for printed circuit boards, may be poor or even nil. Pre-deposition of a bonding material is often necessary.

The object of the invention is to minimise or to overcome these disadvantages.

According to one aspect of the invention there is provided a method of manufacturing a circuit assembly comprising a plurality of components mounted on an insulating board or substrate and interconnected via a conductor pattern, said method including selectively depositing a metal on the board via a vapour phase process to provide said conductor pattern such that the pattern forms a direct electrical connection to said components.

According to another aspect of the invention there is provided a circuit assembly, comprising an insulated substrate board in which a plurality of components are mounted, and a conductor pattern disposed on said board and providing an electrical interconnection between said components, and wherein said pattern provides a direct bond to each said component.

A variety of vacuum deposition processes, especially sputtering processes, are now available which, we have found, may be used to obtain adherent conductive metal deposits at relatively high deposition rates onto substrates which are compatible with a vacuum process, i.e. that do not outgas significantly when heated to 1 00 C or nore, sputtering processes have the advantage that alloyed metals can be deposited easily which is of use to obtain conductors of higher strength or more resistant to thermal fatigue. We have found that sputtering processes provide excellent step coverage in such applications and can thus be employed both to bond component terminations to that pattern in a single step process to form a completed circuit assembly.As no subsequent soldering is required the conductor tracks can be very closely spaced and a high component density can be achieved.

These processes employed herein for adherent vacuum deposition emit the depositing vapour in a conical beam having a significant angular spread such as a solid angle of 90 . It is therefore possible to deposit onto surface out of a line of sight and thus obtain electrical connection to the terminations of, for instance, surface mounted components preplaced on the surface of an insulating board.

It is then possible to define circuit connections using a mask or masks which limit the deposit to a desired conductor pattern and to use the masks or separate devices acting as masks to define the deposition of electrical connections to the surface mounted components and devices. It is evident that by this means a printed board assembly may be made completely ready for use in one deposition exposure only, or a few exposures if more than one mask is necessary. Further, as the material of the deposit such as copper alloys of high electrical conductivity is much more mechanically strong at service temperatures than solder there is no need to use an adhesive to secure the components and devices. If desired a deposit can be made onto a nonconducting or otherwise prepared part of the component or device to aid mechanical attachment.

Embodiments of the invention will now be described with reference to the accompanying drawings in which:~ Figure 1 is a cross-section of a portion of a circuit board provided with a mask defining a conductor or interconnect pattern; Figure 2 illustrates the technique of multiple masking; Figure 3 shows one form of component termination suitable for use with the conductor deposition techniques described herein; Figure 4 shows an alternative component termination arrangement; and Figure 5 illustrates the use of a mechanical support during the deposition process.

Referring to Fig. 1, a circuit board 11, which is made of an insulating material that does not out-gas significantly when in vacuum, is provided with a mask 12 defining a conductor interconnect pattern. The manufacture of suitable masks requires that the edges of the mask openings 13 do not come into contact with the deposition substrate. An undercut is necessary to ensure that the mask does not become attached to the substrate by the deposited metal with consequent damage to mask and substrate. As shown in Fig. 1 the mask must be undercut defining a shoulder 14 at this edge. This may conveniently be done when using a laser to cut a mask by coating the lower surface with a layer 15 of an adherent plastic or low-boiling point metal such as zinc or magnesium to the desired thickness of undercut.During laser cutting of the conductor pattern the layer material 15 close to the cut evaporates to produce the undercut of Fig. 1. Laser cutting is desirable to obtain narrow width openings in a relatively thick mask although clearly any method of manufacture having the desired result may be used.

Fig. 2 shows in diagrammatic form how a densely packed array of narrow conductors 21 may be defined by two or more masks 22, 23 each of which will have adequate mechanical strength. Whilst the masks must be applied in correct registration the overlap of conductor thickness will prevent any acceptable misregister from causing a higher resistance at the overlap.

Fig. 3 shows a convenient form of component termination for use with bondprinting as this method of assembling circuits may be called. The termination 31 which may be of any suitable conductive material and which is attached to the component 32 is biased by a compliant mask (not shown) during deposition against the substrate 33 to ensure that there is no lack of contact between the end of the termination and the substrate. This is essential to ensure a reliable contact as the thickness of the printed conductor may only need to be 5 micrometres or less and manufacturing tolerances in the termination must be allowed for.

If some terminations were in contact with the substrate before the compliant mask were applied, the pressure of the mask required to press the non-contacting termination ends downwards can cause the contacting termination ends to lift away from the substrate surface. If an adhesive 41 (Fig. 4) is used, it must flow to form a fillet at the termination edge so as to close any gap at that point (Fig.

4).

We prefer to employ plasma cleaning of the terminations and substrate prior to metal deposits. This gives adhesion, and no special finishes are required for the terminations in comparison with terminations intended for solder-bending. Plasma cleaning is also desirable at chosen intervals to remove the deposit from the mask openings which will otherwise reduce in width progressively as metal is deposited thereon.

It will be noted that because the vapour is not emitted on a line-of-sight it is possible to place substantial mechanical supports 51 on the vapour side of the mask as shown in Fig.

5. Providing that these are sufficiently far from the mask in relation to their breadth and depth no great reduction of the deposit will result as the vapour will be partially blocked only.

Claims (10)

1. A method of manufacturing a circuit assembly comprising a plurality of components mounted on an insulating board or substrate and interconnected via a conductor pattern, said method including selectively depositing a metal on the board via a vapour phase process to provide said conductor pattern such that the pattern forms a direct electrical connection to said components.

2. A method as claimed in claim 1, wherein said metal is deposited via a vacuum sputtering device.

3. A method as claimed in claim 1 or 2, wherein said metal provides a bond between each said component and the board thereby securing that component to the board or to a pre-existing conductor pattern disposed on the board.

4. A method as claimed in any one of claims 1 to 3, wherein said conductor pattern is formed by metal deposition through succes sive members of a set of masks.

5. A method as claimed in any one of claims 1 to 4, wherein said metal is copper.

6. A method of manufacturing a circuit assembly substantially as described herein with reference to the accompanying drawings.

7. A circuit assembly made by the method of any one of claims 1 to 6.

8. A circuit assembly, comprising an insulated substrate board in which a plurality of components are mounted, and a conductor pattern disposed on said board and providing an electrical interconnection between said components, and wherein said pattern provides a direct bond to each said component.

9. A circuit assembly substantially as described herein with reference to the accompanying drawings.

10. An electronic device incorporating one or more circuit assemblies as claimed in claim 7, 8 or 9.