GB2384119A

GB2384119A - A printed circuit board

Info

Publication number: GB2384119A
Application number: GB0200509A
Authority: GB
Inventors: Anthony Banks
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 2002-01-10
Filing date: 2002-01-10
Publication date: 2003-07-16
Anticipated expiration: 2022-01-10
Also published as: GB2384119B; GB0200509D0

Abstract

A printed circuit board comprises a first layer of electrically conducting tracks 12' and electrically conducting contact pads 14'' and a solder resist layer having a plurality of apertures 20 forming conduits to the contact pads 14'' of the first layer to allow solder to pass through the solder resist layer and bond to the contact pads 14'' of the first layer. At least one side of at least one aperture 20 of the solder resist layer is substantially linear with respect to an adjacent track 12' of the first layer, thus reducing the risk of solder forming an unwanted electrical connection between a contact pad 14 and a track 12' in the event that the apertures 20 of the solder resist layer are mis-aligned with the contact pads 14'' of the first layer.

Description

A PRINTED CIRCUIT BOARD This invention relates to a printed circuit board, a mask for curing the solder resist layer of a printed circuit board and a method of manufacturing a printed circuit board.

Modern electronic equipment, for example mobile phones, make use of electronic devices with advanced packaging technologies in order to achieve size and weight reduction. One such technology is known as a micro Ball Grid Array (BGA) packaging. A pBGA package is connected to a printed circuit board (PCB) by an array of solder balls which project below the surface of the package in an X/Y grid. During assembly of the BGA package onto the PCB, the solder balls are melted to make an electrical and mechanical connection to the respective contact pads on the PCB.

We have appreciated that a problem with the known manufacturing techniques for PCBs is that manufacturing yield is dependent on precise alignment of the various layers of the printed circuit board to ensure that the process of attaching the BGA package to the PCB by melting the BGA solder balls does not inadvertently result in a short circuit between a contact pad and a track on the PCB.

This is particularly problematic where there is misalignment of the solder resist layer so that the apertures in the solder resist layer for connecting the solder balls to the contact pads are offset from contact pads. In this situation, the solder from the balls of the BGA tends to spread out to cover the area of PCB which is bare of solder resist and the solder may spread over and become electrically connected to an adjacent track causing a short circuit between the package output, contact pad and track of the PCB. We have appreciated that the yield problem is exacerbated by the known design of the solder resist layer of PCBs.

The present invention in its various aspects aims to ameliorate this problem by providing an improved design of solder resist layer. The present invention in its various aspects is defined in the independent claims. Advantageous features are set forth in the dependent claims.

More particularly, in accordance with the invention in a first aspect, there is provided a printed circuit board comprising a first layer having electrically conducting tracks and electrically conducting contact pads for connecting the leads of an electrical component to the appropriate track or tracks, and a solder resist layer at least partially covering the first layer, the solder resist layer having a plurality of apertures forming conduits to the contact pads of the first layer to allow solder to pass through the solder resist layer and bond to the contact pads, wherein at least one side of at least one aperture is substantially linear with respect to an adjacent track to reduce the risk of solder forming an unwanted electrical connection between a contact pad and a track in the event that the apertures of the solder resist layer are misaligned with the contact pads of the first layer. As the tracks of a PCB are generally set out in a substantially linear fashion, apertures with substantially linear sides may be provided to maximise the distance between the edge of the aperture and an adjacent track to which the contact pad of the aperture is not intended to be electrically coupled.

Preferably the apertures in the solder resist layer are substantially rectangular or substantially square. Since the side length of a square aperture is smaller than the diameter of a circular aperture of the same area as the square aperture, the distance between a contact pad and an adjacent, but electrically unconnected, track is increased. Improved manufacturing yield may be obtained by virtue of this increased distance.

Preferably although substantially square apertures are provided in the solder resist layer, the electrical contact pads of the PCB are circular. Preferably the area of the substantially square apertures in the solder resist layer are approximately equal to the area of the contact pads of the PCB regardless of the shape of the contact pads. The use of substantially square apertures in the solder resist layer is particularly preferable for attaching uBGA packages where the pitch between adjacent solder balls is O. 5mm or less.

In accordance with the invention in a second aspect, there is provided a mask for curing a solder resist layer of a printed circuit board by exposure to a curing medium, the mask comprising at least one region permeable to a curing medium and a plurality of regions impermeable to the curing medium, wherein at least one impermeable region comprises an area having at least one substantially linear side.

Preferably the impermeable regions in the mask are substantially rectangular or substantially square.

In accordance with the invention in a third aspect, there is provided a method of manufacturing a printed circuit board, the method comprising the steps of building a first layer comprising electrically conducting tracks and electrically conducting contact pads for connecting the leads of an electrical component to the appropriate track or tracks, covering the first layer with a layer of uncured solder resist material, creating a mask for exposing the solder resist material to a curing medium, the mask comprising at least one region permeable to a curing medium and a plurality of regions impermeable to the curing medium, wherein each impermeable region is suitable for forming apertures in a solder resist layer, aligning the mask with respect to the printed circuit board by aligning the impermeable regions of the mask with the contact pads of the first layer of the printed circuit board, curing regions of

the solder resist material to form a solder resist layer by exposing the unmasked solder resist material to a curing medium, and removing the uncured areas of the solder resist material to leave a plurality of apertures in the solder resist layer, each aperture forming a conduit through the solder resist layer to a contact pad of the first layer, at least one of the apertures having at least one substantially linear side. Preferably at least one impermeable region in the mask is substantially rectangular or substantially square and any corresponding apertures formed in the solder resist layer are substantially rectangular or substantially square.

In accordance with the invention in a fourth aspect, there is provided a method of operating a laser for curing a solder resist layer of a printed circuit board, the method including the steps ofdefining a direct image pattern for controlling illumination'of solder resist material on a printed circuit board, the direct image pattern having at least one first region and a plurality of second regions, the first regions corresponding to areas where the solder resist material is to be cured by the laser and the second regions corresponding to areas where the solder resist material is to remain uncured, for generation of apertures in the cured solder resist layer; at least one second region having at least one substantially linear side, rasterising the direct image pattern to determine co-ordinate data describing when the light from the laser is to be directed onto solder resist material of a printed circuit board and when light from the laser is to be prevented from reaching the solder resist material, and operating the laser according to the co-ordinate data. Preferably at least one aperture is substantially rectangular or substantially square. Preferably the PCB is mounted in known relation to the laser beam and the laser beam is directable and/or the PCB mounting is moved to allow the laser beam to be directed onto different areas of the PCB.

Preferred embodiments of the invention will now be described in more detail with reference to the accompanying drawings in which: Figure 1 is a schematic of a printed circuit board having a typical routing pattern for a micro Ball Grid Array (uBGA) integrated circuit; Figure 2 is a schematic of the printed circuit board of figure 1 where there is a misalignment in the horizontal direction of the solder resist apertures for the electrical contact pads; Figure 3 is a diagram comparing the radius of a circle of unit area with the distance from the centre of a square of the same area to its edge; and Figure 4 is a schematic of a printed circuit board having the same routing pattern as that of figure 1 but having square apertures in the solder resist layer to act as conduits through the solder resist layer to the electrical contact pads.

Figure 1 shows schematically a printed circuit board 10 with a routing pattern of electrically conducting tracks or tracks 12 and contact pads 14 connected to the appropriate track. The routing pattern of tracks 12 is typical for a PCB 10 to which a uBGA package is to be attached. The tracks 12 and contact pads 14 are laid down on the PCB 10 in a first layer and the first layer is covered by a layer of curable solder resist material. The solder resist material remains soft and may be washed away, or otherwise removed, until it is cured by exposure to a curing medium. Typically the solder resist material is photo-sensitive and the solder resist material is cured by exposing areas of the solder resist material to a light source and masking other areas of

the solder resist material to prevent the light from curing the material in these areas. A mask for curing solder resist material may, therefore, comprise regions permeable to a curing medium and regions impermeable to the curing medium.

Once the solder resist material has been exposed as required to the curing medium, the uncured material is removed, for example by simply washing away, leaving a solder resist layer at least partially covering the first layer.

The solder resist layer 16 is formed with apertures aligned with the contact pads 14 of the first layer. The apertures form conduits through the solder resist layer 16 and allow solder to pass through the solder resist layer and bond with the contact pads 14 of the PCB 10. In this way, the leads or solder balls of an electrical component may be soldered to the contact pads 14 of the PCB 10 to connect the component electrically to the tracks 12 of the PCB 10.

When the solder resist layer 16 is aligned perfectly with the first layer such that the apertures in the solder resist layer 16 correspond exactly to the contact pads 14 of the PCB 10 the risk of the soldering operation of connecting an electrical component to the PCB 10 causing an unwanted electrical connection between a first track 12'and an adjacent contact pad 14'to which it is not intended to be connected is minimised.

From Figure 1 it is, however, clear that for a uBGA package, the gap between a contact pad, for example contact pad (BGA #1) 14', and the adjacent track (trace #4) 12'is small as indicated by arrow 17. For a pBGA package with a pitch of 0. 5mm between the balls, the gap is less than O. lmm. This gives rise to the possibility that alignment error within a typical alignment tolerance could leave the adjacent track 14'exposed as a copper surface. During the soldering

operation, this could lead to a short circuit occurring between the collapsing solder ball and the adjacent track.

This is shown schematically in Figure 2 in which the alignment of the apertures 18 of the solder resist layer 16 is offset from the contact pads 14 of the first layer increasing to an unacceptable level the risk that an unwanted connection between a first track 12'and adjacent, supposedly electrically unconnected, contact pad 14'will be caused when the electrical component is mounted on the PCB 10. In the example of Figure 2, the offset of the solder resist layer 16 relative to the first layer is such that each contact pad is likely to become connected to an independent track when the electrical component is soldered onto the PCB 10. From Figure 2 it is clear that the operation of soldering a BGA onto the PCB 10 is likely to cause an unwanted electrical connection between, for example, contact pad 14' (BGA pad #1) and track 12' (trace #4) etc.. This is indicated schematically on figure 2 by a *.

Generally speaking for a PCB having tracks which are substantially linear, the distance between an aperture of the solder resist layer and an adjacent track is maximised when the side of the aperture is linear with respect to the adjacent track and in particular when the side of the aperture is parallel to the adjacent track. The shape of the top and bottom of the aperture are not critical in this respect. See for example figure 4 where provision of linear side 24 in the aperture 20 maximises the distance between the edge of the aperture 14"and track 12' (trace #4) whereas the shape of side 22, top 26 and bottom 28 is less important. In practice this can be achieved by forming substantially rectangular or square apertures in the solder resist layer. This is particularly important for apertures corresponding to contact pads which are positioned at a small distance from an adjacent track such as the aperture

corresponding to contact pad 14'on figure 1. It is less important for the row of apertures corresponding to the lower row of contact pads on figure 1 where the distance between the contact pads and the adjacent tracks is greater.

Thus, rectangular or square apertures in the solder resist layer may be used as conduits to some or all of the contact pads of a printed circuit board.

Figure 3 shows a circle of unit radius and a square, centred on the centre of the circle, of the same area. From Figure 3, and basic geometry, it is clear that the distance from the centre of the square to the centre of an edge is 0.886 (i. e. 0.5 x n '). Thus using a substantially square aperture in the solder resist layer as opposed to a circular aperture may result in an increase of over 11% on the possible solder resist short circuit route between a contact pad and an adjacent unconnected track when there is no offset of the apertures of the solder resist layer to the contact pads of the PCB. Taking into account this increased distance, the probability of a short circuit occurring from a solder ball of an electrical component to an adjacent track of the PCB to which it is not intended to be electrically connected during the soldering operation is much reduced even where there is some inaccuracy in the alignment of the apertures in the solder resist layer with the underlying contact pads, resulting in an increase in manufacturing yield. This is particularly the case for uBGA packages where the pitch between the solder balls is approximately 0. 5mm or less.

Although the distance from the corner of the square to its centre is greater than the radius of the circle of the same area, we have appreciated that this does not adversely affect the manufacturing yield when a typical rectilinear routing pattern is used. Similarly, use of a substantially rectangular aperture orientated to maximise the distance between the edge of the aperture and the adjacent track may also reduce the risk of short circuits when the apertures of

the solder resist layer are offset from the contact pads of the PCB 10.

Figure 4 shows such a typical routing pattern for a PCB with the round contact pads of the PCB being overlaid by square apertures 20 in the solder resist layer 16. The minimum distance of solder resist between a contact pad and the adjacent track, e. g. contact pad 14" (BGA pad #1) and track 12' (trace #4), is greater than the minimum distance of solder resist between a corresponding pad and track when the solder resist apertures are circular. Although the square apertures shown in Figure 4 are of smaller area than the area of the circular pads, square apertures of equal area to the circular contact pads would still provide a larger spacing between the edge of the aperture and the adjacent track than would circular apertures. In this respect, it is important to note that typical routing patterns for PCBs follow rectilinear layouts with the tracks being formed of interconnecting substantially straight sections.

It is important to note that the advantage of the present invention is achieved by the provision through the solder resist layer of apertures having substantially linear sides and that the shape of the contact pads of the first layer of the PCB may remain circular. There is sufficient overlap between the substantially straight-sided apertures of the solder resist layer and the circular contact pads of the first layer of the PCB to ensure adequate bonding of the electrical component to the contact pads whilst reducing the risk of unwanted short circuits between a contact pad and an independent track should the apertures of the solder resist layer be mis-aligned with, and therefore offset from, the contact pads. Use of a substantially square aperture provides the additional advantage of maximising the overlap between the aperture and the contact pad whilst providing the optimum gap between the edge of the aperture and the adjacent track.

A possible advance in the curing of solder resist material is the use of line direct imaging (sometimes referred to as direct line imaging). Line Direct Imaging (LDI) uses a laser to cure solder resist material into the desired configuration of solder resist layer, the beam of the laser being controlled by a microprocessor to illuminate directly those areas which are to be cured and to prevent illumination of areas where the solder resist material is not to be cured. A negative mask is not required in the LDI process. Rather than produce a negative image mask, line direct imaging (LDI) of a solder resist layer would use rasterised data describing the desired solder resist pattern to control a laser directly to cure the solder resist material in the appropriate regions. Generally, the laser is not switched on and off but rather an acousto-optic modulator (AOM) is interposed between the laser beam and the PCB to turn the beam on or off and therefore control whether or not the beam hits the PCB. The AOM is typically controlled by a high frequency sound wave which either makes the AOM transparent to the laser beam to allow the beam to pass through and hit the PCB or makes the AOM opaque to the laser beam to prevent illumination of the particular region of the PCB by the laser.

Data describing the desired solder resist pattern would be stored in a computer used to control the AOM and hence to control the illumination of the solder resist material on the PCB by the laser. The PCB is generally mounted on a vacuum platen and co-ordinates of a given origin determined. By controlling the movement of the platen, and hence the PCB, relative to any movement of the laser beam, the solder resist material is illuminated as required to cure first regions and to leave uncured second regions corresponding to the shape of apertures desired in the solder resist layer.

The uncured areas of solder resist material may then be removed in any appropriate manner to leave a PCB having a solder resist layer partially covering the first layer.

Alternative methods of manufacturing PCBs and in particular of laying down solder resist layers may be, or become, known. Any such methods which are used to provide substantially straight sided, substantially rectangular, and in particular substantially square, apertures in a solder resist layer corresponding to the locations of the contact pads of the PCB where the apertures act as solder conduits through the solder resist layer to the contact pads are considered to fall within the scope of the present invention. Whilst the invention has been described with particular reference to PCBs using uBGA packages it will be readily understood by the skilled man that it is equally applicable to any type of PCB where the distance between the electrical contacts of an electrical component to be soldered to the PCB is small, and is of particular use where the distance or distances are approximately 0. 5mm or less.

With respect to the above description, it is to be realised that the equivalent apparatus and methods are deemed readily apparent to one skilled in the art, and all equivalent apparatus and methods to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention. Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes have already occurred with those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents may be resulted to, falling within the scope of the invention. It should also be noted that the features described by reference to particular figures and at different points of the description may be used in

combinations other than those particularly described or shown. All such modifications are encompassed within the scope of the invention as set forth in the following claims.

Claims

CLAIMS 1. A printed circuit board comprising a first layer having electrically conducting tracks and electrically conducting contact pads for connecting the leads of an electrical component to the appropriate track or tracks, and a solder resist layer at least partially covering the first layer, the solder resist layer having a plurality of apertures forming conduits to the contact pads of the first layer to allow solder to pass through the solder resist layer and bond to the contact pads, wherein at least one side of at least one aperture is substantially linear with respect to an adjacent track to reduce the risk of solder forming an unwanted electrical connection between a contact pad and a track in the event that the apertures of the solder resist layer are misaligned with the contact pads of the first layer.
2. A printed circuit board according to claim 1, wherein at least one side of each aperture is substantially linear with respect to an adjacent track.
3. A printed circuit board according to either claim 1 or claim 2, wherein the at least one side of the at least one aperture is substantially parallel to the adjacent track.
4. A printed circuit board according to any of the preceding claims wherein at least one aperture is substantially rectangular and is orientated to reduce the risk of solder forming an unwanted electrical connection between a contact pad and an adjacent track in the event that the apertures of the solder resist layer are misaligned with the contact pads of the first layer.

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5. A printed circuit board according to claim 4, wherein the at least one aperture is substantially square.
6. A printed circuit board according to any of the preceding claims, wherein the area of each aperture in the solder resist layer is substantially the same as the area of the corresponding electrical contact pad of the first layer.
7. A printed circuit board according to any of the preceding claims, wherein the electrical contact pads of the first layer are substantially circular.
8. A printed circuit board according to any of claims 1 to 7 wherein the electrical contact pads of the first layer are substantially square.
9. A printed circuit board according to any of the preceding claims, wherein an electrical component is soldered to the printed circuit board, the leads of the electrical component being aligned with the apertures of the solder resist layer and soldered to the electrical contact pads of the first layer through the apertures in the solder resist layer.
10. A printed circuit board according to claim 9, wherein the electrical component is housed in a ball grid array package.
11. A printed circuit board according to claim 10, wherein the distance between adjacent leads on the ball grid array integrated circuit is approximately 0. 5mm or less.
12. A mask for curing a solder resist layer of a printed circuit board by exposure to a curing medium, the mask comprising at least one region permeable to a curing

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medium and a plurality of regions impermeable to the curing medium, wherein at least one impermeable region comprises an area having at least one substantially linear side.
13. A mask according to claim 12, wherein the at least one impermeable region is of substantially rectangular cross-section suitable for forming at least one substantially rectangular cross-section aperture in a solder resist layer to act as a conduit to an electrical contact pad on a printed circuit board.
14. A mask according to claim 13, wherein the at least one impermeable region is of substantially square cross- section.
15. A method of manufacturing a printed circuit board, the method comprising the steps of: a) building a first layer comprising electrically conducting tracks and electrically conducting contact pads for connecting the leads of an electrical component to the appropriate track or tracks; b) covering the first layer with a layer of uncured solder resist material; c) creating a mask for exposing the solder resist material to a curing medium, the mask comprising at least one region permeable to a curing medium and a plurality of regions impermeable to the curing medium, wherein each impermeable region is suitable for forming apertures in a solder resist layer; d) aligning the mask with respect to the printed circuit board by aligning the impermeable regions of the mask with the contact pads of the first layer of the printed circuit board; e) curing regions of the solder resist material to form a solder resist layer by exposing the unmasked solder resist material to a curing medium; and

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f) removing the uncured areas of the solder resist material to leave a plurality of apertures in the solder resist layer, each aperture forming a conduit through the solder resist layer to a contact pad of the first layer, at least one of the apertures having at least one substantially linear side.
16. A method according to claim 15, wherein the at least one impermeable region is substantially rectangular for forming at least one substantially rectangular aperture in the solder resist layer.
17. A method according to claim 16, wherein the at least one impermeable region of the mask is substantially square for forming at least one substantially square aperture in the solder resist layer
18. A method according to any of claims 15 to 17, further comprising the steps of : a) locating an electrical component on the printed circuit by aligning the leads of the electrical component with the apertures in the solder resist layer; and b) soldering the leads of the electrical component to respective contact pads of the first layer through the apertures in the solder resist layer, whereby in the event of misalignment of the apertures of the solder resist layer with the contact pads of the first layer the risk of solder forming an unwanted electrical connection between a lead of the electrical component and a track of the first layer is reduced.
19. A method of operating a laser for curing a solder resist layer of a printed circuit board, the method including the steps of: a) defining a direct image pattern for controlling illumination of solder resist material on a printed circuit board, the direct image pattern having at least

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one first region and a plurality of second regions, the first regions corresponding to areas where the solder resist material is to be cured by the laser and the second regions corresponding to areas where the solder resist material is to remain uncured for generation of apertures in the cured solder resist layer; at least one second region having at least one substantially linear side, b) rasterising the direct image pattern to determine co- ordinate data describing when the light from the laser is to be directed onto solder resist material of a printed circuit board and when light from the laser is to be prevented from reaching the solder resist material; and c) operating the laser according to the co-ordinate data.
20. A method according to claim 19, wherein the at least one second region of the direct image pattern has a substantially rectangular cross-section for generating a substantially rectangular aperture in the cured solder resist layer.
21. A method according to claim 20, wherein the at least one second region of the direct image pattern is substantially square for generating at least one substantially square aperture in the cured solder resist layer.
22. A method of curing a coating of solder resist material on a printed circuit board using a laser operated in accordance with any of claims 19 to 21, the method comprising the steps of: a) mounting the printed circuit board in known relation to the laser; b) selecting a direct image pattern which corresponds to the desired solder resist layer of the mounted printed circuit board;

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c) operating the laser to direct light onto first areas of the solder resist material curing the solder resist material in the first regions and leaving uncured the solder resist material in the second regions; d) removing the uncured areas of the solder resist material to leave apertures in the cured solder resist layer to act as conduits through the solder resist layer to the electrical contacts of the printed circuit board, at least one of the apertures having at least one substantially linear side.
23. A direct image pattern for curing solder resist material on a printed circuit board by laser direct imaging, the direct image pattern comprising at least one first region corresponding to an area of a printed circuit board where the solder resist material is to be cured by a laser, and a plurality of second regions corresponding to areas of a printed circuit board where electrical contact pads for an electrical component are located, the second regions forming apertures in a solder resist layer of a printed circuit board to act as conduits through the solder resist layer to the electrical contact pads, at least one second region having at least one substantially linear side.
24. A direct image pattern according to claim 23, wherein the at least one second region has a substantially rectangular cross-section for forming a substantially rectangular aperture in a solder resist layer of a printed circuit board.
25. A direct image pattern according to claim 24, wherein the at least one second region is substantially square for forming a substantially square aperture in the solder resist layer.

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26. A printed circuit board substantially as hereinbefore described with reference to the accompanying drawings.
27. A method of manufacturing a printed circuit board substantially as hereinbefore described with reference to the accompanying drawings.
28. A mask for a solder resist layer of a printed circuit board substantially as hereinbefore described with reference to the accompanying drawings.
29. A direct image pattern for a solder resist layer of a printed circuit board substantially as hereinbefore described with reference to the accompanying drawings.