System and method for providing uniform solder jiunt height for printed circuit boards and their assemblies

There is disclosed a technique for constructing a printed circuit board assembly to provide solder joints with a uniform height. A solder mask is provided on the external surfaces of the printed circuit board to minimize the mount of conductive pad area that is exposed to solder. The solder mask includes a plurality of relatively small openings with a predetermined pattern to minimize the build up of solder, while insuring sufficient solder height to connect to the grounding component located on the chassis to insure adequate EMI protection. Preferably a polka dot pattern is used for certain conductive pads, while a single narrow strip or the solder mask opening configuration is used for rectangular pad configurations. Other configurations and patterns are also available to provide an adequate electrical connection while insuring uniform solder height.

FIELD OF THE INVENTION 
The present invention relates generally to computers and other electronic 
assemblies comprising printed circuit boards, and more particularly, to 
apparatus and methodologies for affixing printed circuit boards within a 
computer or other electronic assemblies. More particularly, the invention 
relates to the design and configuration of printed circuit boards capable 
of supporting a variety of electrical components. Still more particularly, 
the present invention relates to solder mask patterns for uniformly 
controlling the size and height of solder joints on printed circuit board 
assemblies to facilitate installation of the circuit board and to insure 
proper grounding and circuit connections on the circuit board once 
installed. 
BACKGROUND OF THE INVENTION 
Computer and other electronic assemblies typically include a plurality of 
printed circuit (PC) boards that support electronic components. Commonly, 
PC boards are secured within a steel or plastic chassis for an electronic 
assembly by means of screws extending through holes in the PC board and 
threadedly engaged to a portion of the chassis. In addition to the 
mounting screws, the assignee of the present invention also has used an 
alternative PC board mounting technique by which vertically-extending 
plastic hooks are molded into the base of the computer chassis. The hooks 
are received through slots formed in a PC board as the PC board, lying in 
a horizontal plane, is lowered onto the base of the chassis. After the PC 
board is seated on the chassis, with hooks extending through slots, the PC 
board is displaced horizontally to engage the hooks on the upper surface 
of the PC board at one end of the slots, thereby preventing vertical 
motion of the PC board. Thereafter, a small plastic catch mechanism is 
snapped into engagement with an edge of the PC board to prevent horizontal 
motion. 
Because of the sensitivity of electrical components, and the risk that 
electrical circuits may be shorted if not properly secured within the 
product, it is necessary to ground the circuit board and to provide a 
ground for the electrical components on the circuit board. In many 
instances, both of these functions are provided by the fasteners (screws 
and/or hooks) that extend through the circuit board to the chassis of the 
product. Thus, for example, in a personal computer chassis, the 
motherboard containing many of the computer's electrical components is 
fastened to the chassis by a plurality of mounting screws and/or hooks. 
Because the chassis usually is constructed of a conductive material, or 
contains a conductive layer, a path to ground is provided through the 
mounting fasteners. The grounding of the circuit board is necessary to 
provide shielding against electromagnetic interference ("EMI") from 
outside sources, which might otherwise disturb the operation of the 
components on the circuit board. 
As one skilled in the art will understand, a large variety of electrical 
components may be mounted on the circuit board, including integrated 
circuits and other discrete components. Typically, the electrical 
components are mounted on the circuit board in one of two ways. The first 
technique for mounting electrical components on a circuit board is to 
provide electrical leads on the electrical components that extend through 
the circuit board. The tips of these leads then are bent and/or soldered 
to an outer surface of the circuit board. Where space is limited, solder 
usually is preferred as a method to both secure the electrical component 
to the circuit board, and to provide a high quality electrical connection 
between the leads and the circuit paths on or in the circuit board. In 
order to perform this soldering process, usually a wave soldering machine 
is used to provide the requisite soldering of connections on one or both 
sides of the circuit board. Typically, the circuit board is placed in the 
wave soldering machine so that solder is applied to all conductive 
surfaces on the bottom of the circuit board. 
In addition to providing solder to the leads of the electrical components, 
solder also is applied to conductive pad surfaces on the solder-side of 
the circuit board during the wave soldering process. To the extent that 
conductive surfaces on the solder-side of the board are not to be 
soldered, a solder mask must be provided as the bottom layer of the 
circuit board. The solder mask resists the application of solder. Thus, in 
accordance with normal techniques, the solder-side surface of the circuit 
board is designed with conductive pads, such as copper, where solder is to 
be deposited during the wave soldering process. Some of the larger 
conductive pads that are provided on the bottom of the circuit boards are 
the grounding pads, where grounding screws and/or hooks are to be attached 
or mounted. 
Printed circuit boards typically are designed in layers, with various 
circuit paths, and in some instances, circuit elements, in each layer of 
the board. Thus, a typical circuit board has a number of different layers, 
with the upper and lower layers including circuit paths for connecting to 
components that are mounted to the top or bottom of the circuit board. In 
prior art circuit boards, typically a plurality of "plated through holes" 
are provided for receiving the grounding screws. As that term is 
conventionally used, plated through holes are holes drilled through the 
circuit board, which include a conductive material applied to the board at 
the periphery of the mounting hole so that each layer of the circuit board 
can be electrically connected to the grounding screw. In addition, a solid 
conductive (copper) pad typically is provided on external layers of the 
board adjacent the aperture for the screw. The upper pad functions to 
connect to the head of the screw, while the lower pad functions to connect 
to the body or chassis of the product in which the board is mounted. 
Solder tends to wick through the plated through holes, and to accumulate 
in large mounts on the relatively large pads. Because solder is not 
applied evenly in the wave solder process (based upon various 
considerations such as surface tension, etc.), after soldering the solder 
deposited on the conductive pads and plated through holes tends to be 
uneven, and in fact, relatively large bumps or clusters may appear at one 
portion of the pad while other portions have relatively large valleys. 
This problem is greatly exaggerated on larger pad areas, such as the 
grounding pads. 
Because of these abnormalities in solder heights at the grounding pads and 
plated through holes, the installation of the boards may prove difficult, 
if not impossible. For example, in assignee's personal computers, hooks 
are used to secure the motherboard to the chassis. The hook therefore 
functions as a clamp by sandwiching the motherboard in place. See U.S. 
application Ser. No. 08/179,806, filed Jan. 11, 1994, the teachings of 
which are incorporated by reference as if fully set forth herein. The hook 
has a certain clearance to accommodate the circuit board. If the 
conductive pads at the site of the hook are too high, the circuit board 
will not fit within the hook structure. If, conversely, the height of the 
solder is too low, the conductive pad will not make a good contact with 
the hook and/or the metal chassis of the product in which the board is 
mounted, causing potential problems with electromagnetic interference. 
Moreover, if the height of the solder varies from one pad to another, the 
board will not lie flat within the chassis, but instead will be required 
to flex and bulge to fit properly in the chassis, causing stress on the 
circuit board. To alleviate these problems it typically is necessary to 
manually touch-up the larger solder pads in order to remove excess solder, 
or to add solder to provide a uniform solder height. This manual rework of 
the PC board adds significantly to the cost of manufacturing the circuit 
boards. Consequently, it would be advantageous to develop a technique for 
uniformly depositing solder on a circuit board during a wave soldering 
process to provide a solder joint with sufficient quantities of solder to 
provide adequate EMI protection, without providing too much solder to 
create problems with installation. 
A second common technique for attaching electrical components to circuit 
boards is to surface mount the components by placing the component on 
conductive solder pads on the surface of the board. The solder then is 
heated in an appropriate chamber to reflow the solder on each pad to 
obtain a soldered connection with an associated lead or pad on the 
component. During the assembly of the circuit board, solder paste 
typically is deposited on the board by an automated (solder paste 
screening) machine, so that the solder paste is deposited in a relatively 
uniform manner to all of the conductive surfaces on the board. In addition 
to providing conductive paths for connecting to the leads of the surface 
mount components, it is common to also deposit solder paste on larger 
conductive pads where grounding is necessary, for example, where grounding 
screws and other attachments are to be located. 
In the situation where the conductive pads are relatively large, the solder 
paste deposited on these pads tends to flow unevenly over the entire pad 
boundary when the reflow process occurs, leaving minimal or no solder 
height to electrically connect to the components provided on the chassis 
for grounding. There is a need to control and maintain the flow of the 
solder during the reflow process, to achieve enough solder raised above 
the surface in a desired area to provide a high quality electrical 
connection. 
In many applications, such as computer motherboards, both types of 
components (surface mounts and through-hole) are used on the same circuit 
board. Typically, the surface mount components are soldered first on one 
side of the board, and then the board is placed in a wave solder machine 
to wave solder the other side of the board to connect the through hole 
devices to circuitry on the circuit board. In this situation, the problems 
with surface mount soldering and wave soldering are both present. 
It obviously would be advantageous to develop a method for constructing a 
circuit board which would provide a uniform solder height for the larger 
solder areas to facilitate ease of installation and to insure adequate 
grounding for EMI protection regardless of the type of soldering that is 
used or the type of circuit board that is implemented. 
SUMMARY OF THE INVENTION 
Accordingly, there is provided herein a system and process of providing 
uniform solder joint heights by limiting the amount of solder that is 
deposited, while still insuring the adequacy of the ground connection. 
Typically, large conductive pad areas are provided adjacent the apertures 
for the grounding screws and/or hooks. In the preferred embodiment, an 
upper and lower solder mask pattern is used to limit the amount of pad 
area that is exposed to the solder. 
In the preferred embodiment, a bottom solder mask layer of a particular 
configuration is used to distribute the solder in dimples on the 
conductive ground pad areas on the bottom of the PC board so that the 
amount of solder deposited during wave soldering is limited by controlling 
the amount of copper that is exposed on the surface of the circuit board. 
Similarly, on the top of the PC board, the conductive pad area adjacent the 
grounding hook aperture is covered by a top solder mask layer to limit the 
portion of the pad that is exposed. Other ground pad surfaces that tend to 
overflow during reflow of the solder paste on the surface mount side of 
the board may include patterns in the top solder mask layer with a 
particular configuration accompanied with an identical solder past 
configuration to minimize the problem with solder flowing or spreading 
over the entire area of the pad leaving no height for proper electrical 
contact during the reflow process. 
In one embodiment of the present invention, a plurality of polka dot shaped 
holes are designed in the portion of solder mask layers overlaying the 
ground pads so that relatively small dots or dimples of solder are 
deposited uniformly in the pad area on both sides of the PC board. The 
polka dots may be arranged uniformly in rows and columns, or can be 
offset. Alternatively, and especially for the pad surfaces on the surface 
mount side of the PC board adjacent the hook aperture, the solder mask 
preferably is designed with a strip opening for permitting a single, 
relatively narrow conductive strip for mating with the conductive 
grounding hook or other features provided on the chassis for grounding. 
As part of the present invention, plating is eliminated from the apertures 
for the screws and certain other components to prevent the excess 
accumulation of solder which typically results. Instead, the present 
invention uses a plurality of relatively small plated via holes. The via 
holes preferably are spaced radially around the aperture on the conductive 
external pads and include a plated through connection to connect to inner 
layers of the board. 
These and other characteristics and advantages of the present invention 
will become readily apparent to those skilled in the art upon reading the 
following detailed description and claims and by referring to the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The principles of the present invention are generally applicable to any 
type of printed circuit board, regardless of the intended use of that 
circuit board. In the preferred embodiment and in the following 
discussion, however, the focus will be on a motherboard for a personal 
computer. One skilled in the art will understand, however, that the 
present invention is not limited to use on computer-related circuit 
boards, but instead can be readily implemented on any circuit board to 
provide uniform solder joint heights. Similarly, the principles of the 
present invention are generally applicable to any type of circuit board 
that requires solder joints, including HASL (hot air solder level) boards, 
ENTEK.RTM. coated circuit boards, bare copper printed wire boards (PWB's), 
and the like. Consequently, the terms "circuit board," "printed circuit 
board" and "PC board" are used generically in this description to refer to 
the various alternative circuit board designs requiring soldered 
connections. 
Referring now to FIG. 1, the top side of a typical printed circuit board 40 
constructed generally in accordance with the preferred embodiment is 
shown. The circuit board 40 generally includes a plurality of pads 25 for 
receiving surface mounted components (not shown), and a plurality of 
plated-through holes 30 for receiving the leads of through hole 
components. Only a few such pads 25 and holes 30 have been shown for the 
sake of simplicity. In addition, a plurality of generally circular 
apertures 15 are provided in the board for receiving mounting screws, and 
a plurality of generally elongated apertures 45 are provided for receiving 
mounting hooks. As shown in FIGS. 6, 7 and 8, mounting hooks 46 and 
grounding screws 33 are used in the preferred embodiment to secure the 
printed circuit board 40 to the body or chassis 38 of the product in which 
the board is installed. Preferably, both the screws 33 and hooks 46 
include, or are constructed of, a conductive material to provide a 
conductive path to ground for the circuit board. In accordance with the 
preferred embodiment, and referring still to the top side of the board as 
shown in FIG. 1, the circuit board 40 includes a conductive pad surface 42 
adjacent the screw apertures 15 and another conductive pad surface 47 
adjacent the hook apertures 45, to provide a surface for connecting to the 
screw or hook, and to function as a ground path from the circuit board to 
the grounding screw or hook. Similarly, as shown in FIG. 1, other 
conductive ground pads 58 also may be provided as necessary to complete 
the design of the circuit board. Referring now to FIG. 2, the bottom side 
of the PC board similarly includes a conductive pad surface 44 adjacent 
the screw apertures 15 and a generally rectangular pad surface 34 adjacent 
the hook apertures 45. In the preferred embodiment, solder is deposited on 
conductive surfaces on the top side of the board by a solder paste 
screening machine and then reflowed in an oven, while solder is applied 
subsequently to the bottom side of the PC board in a wave solder machine. 
Because the present invention preferably is designed for use with grounding 
hooks 46 (see FIGS. 6 and 7), the circuit board 40 is configured with a 
specified thickness to fit within the dimensions of the grounding hook. 
Even if a hook is not used, the board must be configured with a thickness 
that is less than the length of the grounding screw, and which is 
sufficiently thin to accommodate probes during testing of the boards. In 
the preferred embodiment of the present invention, wherein the circuit 
board 40 comprises a motherboard that attaches to a computer chassis 
through grounding hooks, the circuit board must be sufficiently thin to 
fit within the hook clearance. In the preferred embodiment shown in FIG. 
5, the hook clearance is 0.080" (80 mils), so that the maximum board 
thickness x (with solder deposited on the grounding pads) must be less 
than 0.080". The bare printed wire board (PWB) 40, before solder is 
deposited, preferably is designed with a thickness y of 0.062" (62 mils), 
with a tolerance of 0.007" (7 mils). Consequently, the bare board has a 
thickness of from 0.055-0.069" (55-69 mils), prior to the soldering phase 
of assembly. 
In a worst case scenario, where the printed circuit board 40 is 0.069" (69 
mils) thick, the solder pads on both the top and bottom of the circuit 
board cannot be higher than a total of 0.011" (11 mils) in order to fit 
within the hook clearance of 0.080" (80 mils). The solder on the top of 
the board can be deposited with a fair amount of precision in the 
situation of surface mounted components, with a deposit height of 
approximately 0.007-0.008" (7-8 mils), and a reflow height (after the 
solder is melted) of approximately 0.003-0.004" (3-4 mils). Consequently, 
the solder deposited on the bottom of the circuit board preferably is less 
than approximately 0.010" (10 mils) to permit the board to fit within the 
hook clearance. 
At the same time, for proper EMI protection, and to insure a good contact 
between the grounding hook and grounding screws and the circuit board, it 
is preferred that the solder height is at least 0.005" (5 mils) on the 
bottom of the board after wave soldering. Thus, in the preferred 
embodiment, the solder joints on the bottom of the board have a uniform 
height of between 0.005-0.010" (5-10 mils) to fit within the hook 
clearance and still provide adequate EMI protection. 
The particular configuration of the pad surfaces 42, 44 (as per FIGS. 1, 2, 
3 and 4) for the mounting screw hole aperture 15 will now be discussed in 
detail in accordance with the preferred embodiment, to achieve the desired 
solder joint height. In the prior art, typically the hole aperture was 
constructed as a plated through hole, with a large circular conductive pad 
surface on both the top side of the board and the bottom side of the 
board. As noted in the Background section of this application, this prior 
art technique resulted in a large and uneven amount of solder accumulating 
on the bottom side of the board during the wave soldering process. 
As shown in FIGS. 3, 4 and 8, the screw aperture 15 in the preferred 
embodiment is not plated as was done previously in the prior art 
construction. Instead, a plurality of smaller diameter "via" holes 65 are 
provided in the preferred embodiment spaced equidistantly in a circular 
arrangement around the screw aperture 15. In the preferred embodiment, 
eight via holes 65 are provided. The via holes 65 preferably are plated to 
provide a grounding path for each of the board layers that comprise the 
circuit board 40. As shown in FIG. 3, the screw apertures 15 have a 
diameter .alpha., which in the preferred embodiment is 0.166" (166 mils). 
A circular conductive pad 21 encircles aperture 15. The via holes 65 are 
positioned radially about aperture 15, spaced apart at an angle .theta.. 
In the preferred embodiment (where eight via holes are provided), .theta. 
equals 45p. The via holes are constructed with a diameter f. According to 
the preferred embodiment of the present invention, solder paste is 
deposited uniformly over the conductive pad surface 21 by an automatic 
placement machine. 
The bottom side of the circuit board (shown in FIG. 4) preferably includes 
a solder mask layer 27 to control the portions of the circuit board that 
receive solder during the wave solder process. In accordance with the 
preferred embodiment, the solder mask layer 27 is configured with small 
diameter holes 17 that are designed to align with the via holes to permit 
solder to form in the via holes during the wave solder process and to wet 
the external pads. As solder forms in the via holes, small dimples 
preferably are provided on the solder-side of the board at the via hole 
locations. The dimples extend from the solder-side surface of the circuit 
board approximately 0.005-0.010" (5-10 mils) in accordance with the 
preferred embodiment. In the preferred embodiment, the via holes are 
designed with a diameter f of 0.035" (35 mils) and the solder mask 
includes holes 17 positioned over the via holes 65 with a diameter i of 
0.060" (60 mils). As one skilled in the art will understand, other 
dimensions can be used without departing from the principles of the 
present invention. For example, it has been found that the via holes 65 
may vary in size from 0.015-0.040" (15-40 mils), while the holes 17 in the 
bottom solder mask layer 27 may vary from 0.020-0.080" (20-80 mils). 
Referring now to FIGS. 6, 7, 10 and 11, the solder pads 34, 47 associated 
with the elongated hook apertures 45 will now be described. As best seen 
in FIG. 11, the bottom side of the circuit board 40 includes a conductive 
pad 34 adjacent the hook aperture 45 for providing an electrical 
connection to the bottom portion of the grounding hook 46 (FIGS. 6 and 7), 
or alternatively to the chassis 38. According to the preferred embodiment, 
and as shown in FIG. 11, the conductive pad 34 is configured as a 
plurality of small polka dot solder mask openings over conductive areas 
66. The polka dot conductive areas 66 preferably are obtained through the 
use of the solder mask layer 27, which forms the bottom layer of the PC 
board 40. The solder mask layer 27 includes holes or openings for exposing 
the underlying conductive surface (which preferably is copper) in the 
desired pattern. 
In the preferred embodiment, the polka dot areas are offset, but they also 
can arranged uniformly in columns and rows. The diameter of each polka dot 
area is defined as j with a pad area width of o. In the preferred 
embodiment, the diameter j is 0.060" (or 60 mils), while the pad width o 
is 0.145" (145 mils). In the preferred embodiment, the center of the polka 
dots in adjacent rows are offset a distance l, which preferably is 0.040" 
(40 mils). The center-to-center distance between adjacent dots in a row is 
shown as k, which in the preferred embodiment is 0.080" (80 mils). The 
distance between the centerlines of each row is defined by the variable n, 
which in the preferred embodiment is a distance of 0.069" (69 mils). The 
center of each dot preferably is displaced a distance m and p from the 
edge of pad 34, which in the preferred embodiment is a distance of 0.040" 
(40 mils). 
While a polka dot pattern has been described as the preferred embodiment, 
other patterns could also be used upon the condition that they provide a 
relatively uniform solder arrangement, without any bridging between 
adjacent conductive pad areas, and provided they produce a solder height 
of between 0.005-0.010" (5-10 mils). Similarly, the size and location of 
the conductive areas 66 can be modified as necessary to achieve the 
desired solder height. 
While a similar polka dot pattern can be used on the surface mount side of 
the PC board, the preferred embodiment is to use a single strip of 
conductive material as the conductive pad on the top side of the board. 
Referring now to FIG. 10, the preferred conductive pad structure 47 for 
the hook aperture 45 is a single strip of exposed conductive material 81 
which has a width that preferably is approximately one third the width of 
the available pad area. According to the preferred embodiment, this 
opening in the mask is centered in the pad area to mate with the wiping 
area of the associated chassis component. The strip preferably is obtained 
by a solder mask layer 23, which comprises the top layer of the PC board 
40. The solder mask layer 23 includes an opening that exposes the narrow 
conductive strip 81 of the conductive pad 47. In the preferred embodiment, 
solder paste is deposited on the narrow conductive strip 81 and other 
openings in the solder mask by a solder paste screening machine, and then 
the solder paste is reflowed by conventional techniques during the surface 
mount soldering process. The solder mask functions to contain the solder 
within the desired pattern to prevent overflow over the entire pad area, 
while insuring a good quality solder joint at a repeatable height. 
Referring now to FIG. 9, other conductive ground pad surfaces also may be 
required on the surface mount side of the PC board. FIG. 9 illustrates one 
possible pad configuration 58 that may be required on the surface side of 
the board, as shown in FIG. 1. This pad configuration comprises a circular 
portion 68 and a rectangular portion 72. In the preferred embodiment, the 
circular portion 68 is constructed with via holes 85 in the board, and 
with holes in the bottom solder mask layer 27 aligned with the via holes, 
substantially in accordance with the design of the screw aperture 15 as 
described in relation to FIGS. 3 and 4. In accordance with the preferred 
embodiment, the rectangular portion 72 includes top solder mask layer 23 
configured with a plurality of offset (or alternatively, uniformly 
arranged) polka dot openings to expose conductive areas 73 for soldering. 
Solder preferably is deposited with a uniform height on the conductive 
areas 73 in the rectangular portion 72 and on the grounding pad 91 
comprising the circular portion 68 of pad 58 to insure a high quality 
electrical contact raised a predetermined distance above the pad. 
Constructing the PC board with the solder mask patterns described above 
produces a high quality electrical contact with the grounding screw and/or 
hook, while insuring that the solder deposits will be relatively uniform 
in height to facilitate installation and minimize rework after the wave 
soldering is complete. As one skilled in the art will understand, the 
exposed conductive area of other pad configurations can be limited using 
the various techniques described above. 
In the foregoing specification, the invention has been described with 
reference to the presently preferred embodiment thereof. It will, however, 
be evident that various modifications and changes may be made thereto 
without departing from the broader spirit and scope of the invention as 
set forth in the appended claims. The specifications and drawings are, 
accordingly, to be regarded in an illustrative rather than a restrictive 
sense.