GB2258183A

GB2258183A - Solder mask defined printed circuit board

Info

Publication number: GB2258183A
Application number: GB9216263A
Authority: GB
Inventors: Carl N Garcia
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1991-08-01
Filing date: 1992-07-30
Publication date: 1993-02-03
Also published as: GB9216263D0

Abstract

The printed circuit board comprises a dielectric material (403) having a plurality of solder pads (401). A solder mask (405) is photographically defined on the dielectric material (403) and the plurality of solder pads (401) such that the solder mask (405) overlaps the edges of the solder pads (401) and fiducials. The solder mask (405) thus defines the solder pads (401), providing a more uniform size and shape for the pads (401). <IMAGE>

Description

SOLDER MASK DEFINED PRINTED CIRCUIT BOARD Field of the Invention The present invention relates generally to the field of printed circuit board manufacturing.

Background of the Invention A solder mask on a printed circuit board prevents solder from connecting traces to pads that should not be connected.

The mask can be applied by screening the material on the board, leaving solder pads free from the mask for a later soldering operation.

In applying the mask to the board, a border is left around the solder pads where there is neither solder mask nor solder pad. This accommodates any inaccuracies in applying the solder mask by allowing for a shifting of the solder mask.

Without the border, if the mask was shifted in one direction too far, the mask would cover too much of the solder pad. The border allows the mask to be shifted and still enable all of the solder pad to be exposed. This concept is illustrated in FIGs.

1A and B. FIG. 1A shows the ideal situation of a solder mask (a) applied without error, leaving a border (c) between the solder pad (b) and mask (a). FIG. 1B shows the typical situation where the mask has shifted, exposing portions of other solderable surfaces attached to the solder pads. An electronic part (d), shown in dotted lines, may now shift due to the irregular shaped pad. This can cause two parts to short together or a part may lift up on one end during the soldering process due to non-uniform surface tension.

FIG. 2 illustrates another prior art example of a solder mask applied on a printed circuit board. This board has an optical fiducial that is known in the art to be used as a reference point in the placement of the solder pads during the circuit board fabrication.

Another problem of typical solder mask application is illustrated in FIG. 3; the application of solder paste is more difficult. FIG. 3 shows a cross section of a printed circuit board where there is a gap between the solder pad (e) and the solder mask (f). When the solder paste stencil (g) is placed over the printed circuit board, it will be higher on the side resting with the mask over the trace (i) and there will be an air gap (h) on the sides with no trace. This allows the solder paste to flow through this air gap, creating non-uniform solder paste application and unwanted paste residue on the bottom surface of the stencil. This is caused by the non-planar surface surrounding the pad, preventing a good gasket seal between the bottom of the stencil and the board.Solder paste can also flow onto the trace (i) attached to the pad, where there should have been solder mask material and onto the exposed dielectric (j) on three sides of the pad. There is a resulting need for more uniform solder mask application.

Summary of the Invention This invention addresses at least some of the deficiencies which prevail in the prior art described above. In accordance with the present invention there is provided a circuit board which comprises a dielectric material with a plurality of solderable areas thereon. A solder mask material is disposed on the dielectric material and the plurality of solderable areas such that substantially no dielectric material is exposed between the solderable areas and the solder mask material.

An exemplary embodiment of the present invention will now be described with reference to the accompanying drawings.

Brief Description of the Drawings FIGs. 1A and B show a top view of the prior art application of a solder mask.

FIG. 2 shows a top view of the prior art application of a solder mask on a printed circuit board having a fiducial.

FIG. 3 shows a cross section of the prior art application of a solder mask.

FIG. 4 shows a cross section view of a preferred embodiment of a solder mask used to define solder pads in accordance with the present invention.

FIG. 5 shows a top view of solder mask of Fig. 4 used to define solder pads and fiducials in accordance with the present invention.

Detailed Description of the Preferred Embodiment FIG. 5 illustrates a printed circuit board arrangement comprising a printed circuit board and a preferred embodiment of a solder mask used to define solder pads and fiducials thereon. The preferred embodiment of the solder mask overlaps the areas of the printed circuit board designated for the solder pads and fiducials. Moreover, the use of the preferred embodiment of the solder mask results in the formation of more uniformly sized solder pads.

FIG. 5 shows typical solder pads 301-303 for a gull wing lead shape integrated circuit package. One of the solder pads 302 is connected to a trace 304 while another is connected to a plated through hole 305. These items, as well as the outline shown around the pads 306-308 and fiducial 320, is shown in dotted lines as they are the part of the copper covered by the solder mask 310.

The solder pads 301-303 and fiducial 312 fabricated using the preferred embodiment of the solder mask are slightly larger than typical, prior art solder pads. The increase in size is substantially equal to the border of the prior art. This enables the solder mask 310 to overlap 311 the solder pad 303 and fiducial 312 while maintaining the same surface area of the solder pad and fiducial as in the prior art. The increase in the size of the solder pad and fiducial does not reduce the useable real estate on the board since the size increase is equal to the unusable border between the pad or fiducial and the solder mask of the prior art.

Plated through holes can actually be placed closer to the pads using the solder mask of the present invention since there is no danger of the mask not covering the hole. This is also true of traces placed between the solder pads. Since there is no longer a border between the solder mask and the solder pad, the traces can be placed closer to the pad without danger of the mask missing the trace, thereby allowing solder to mistakenly connect the pad to the trace.

Another benefit of the present invention is illustrated in FIG. 4. This drawing shows a cross section of a solder pad 401 connected to a trace 402 on the printed circuit board dielectric 403. It can be seen that the solder mask 404 overlaps 405 the pad on all sides. The mask 404 also completely covers the trace 402 connected to the pad 401. The mask 404 now forms a coplanar, level surface for the solder paste stencil 410 to rest on, forming a gasket between the board and the stencil 410. This prevents the solder paste from flowing to unwanted parts of the board.

It can be seen from both FIGs. 4 and 5 that the solder mask now completely encapsulates the solder pad. This provides the benefit of holding the pad down while an electronic component soldered to the pad is removed or a solder joint is repaired.

The solder mask defined pads and fiducials also provide a very precise geometry. Since the solder mask application is one of the last processes in board fabricating, it can be used to cover up dimensional changes in the board from all prior processes. By defining the pads and fiducials by the solder mask, all prior board fabrication process dimensional changes are eliminated. The soldermask defined pads and fiducials thus provide a precisely defined board for subsequent board assembly processes (solder paste print, part placement, etc.).

While the solder mask of the present invention can be applied using a screening method, the preferred embodiment uses a photo-defining method. This results in cleaner lines between the mask and the pad.

Claims

1. A printed circuit board having a plurality of solder mask-defined solderable areas,the board comprising: a dielectric material with the plurality of solderable areas coupled thereon; and solder mask material disposed on the dielectric material and the plurality of solderable areas such that substantially no dielectric material is exposed between the solderable areas, and the solder mask material.

2. The printed circuit board of claim 1 wherein the plurality of solderable areas include solder pads, plated through holes, fiducials and traces.

3. A printed circuit board having at least one soldermask-defined solder pad, the board comprising: a dielectric material with at least one of solder pad coupled thereon; and solder mask material disposed on the dielectric material and at least one solder pad such that the solder mask material substantially covers the edges of the solder pad.

4. A printed circuit board in accordance with claim 1, or 3, wherein the solder-mask material is deposited on the dielectric material by a photo-defining method.

5. A printed circuit board substantially as described herein with reference to Figs. 4 and 5 of the accompanying drawings.