Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Utility Patent
1998-06-08
2001-01-02
Gaffin, Jeffrey (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S256000, C174S260000, C361S748000, C361S760000, C361S767000, C361S768000, C361S771000, C029S830000, C029S840000, C228S180210, C228S180220
Utility Patent
active
06169253
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to solderable electronic circuit assemblies, and more particularly to electronic circuit assemblies having solder resist windows which facilitate solder paste overprinting.
2. Disclosure Information
Traditional electronic circuit assemblies (such as printed circuit boards) generally includes a substrate
20
with conductive circuit traces
22
and mounting pads
24
thereon, as shown in FIG.
1
. Once the traces and pads have been placed on the substrate, a thin, solder-resistant layer of material known as “solder resist”
28
is typically applied over the surface of the substrate.
The solder resist layer
28
typically covers substantially all of the circuit traces, but not the mounting pads; this is accomplished by forming windows
30
in the solder resist such that each window
30
generally conforms in shape with and is situated concentrically about a respective mounting pad
24
. The edges
35
of each window
30
generally conform with and run generally parallel to adjacent, corresponding edges
25
of the mounting pad
24
, as shown in FIG.
1
. Typically, all the window edges
35
are spaced apart from their respective pad edges
25
by the same distance P; this distance P is commonly referred to as the “pullback”. For screenprinted solder resist, the pullback P is typically about 10-20 mils, whereas for liquid photoimageable solder resist (LPISM) the pullback P is typically 0-5 mils.
While it is preferred to keep the solder resist window at least as large as the respective pads at a minimum, it is common industry practice—and, in fact, a tenet of industry design standards (e.g., ANSI/IPC-D-275)—to keep the pullback P as small as possible. One reason for this is to conserve real estate on the substrate; making the solder resist openings/pullback larger rather than smaller takes up valuable board space. It is also common practice and teaching to use the same phototool to produce the masks/stencils needed for laying down the mounting pads, the solder resist layer/windows, and the solder paste depositions. For example, a given phototool may be (1) non-magnified to produce the mounting pad mask, (2) magnified to produce the solder resist layer/window mask, and (3) de-magnified to produce the solder paste stencil. This avoids having to independently design three separate masks/stencils, and making them from the same phototool provides better registration, and better avoids tool-to-tool discrepancies, than if three separate phototools were used.
Once the solder resist is applied to the substrate surface, the mounting pads and any other circuit structures exposed through the windows may be tinned, followed by solder paste being applied thereto, typically by screen-printing or deposition. When solder paste is applied to mounting pads, the common practice is to print or deposit paste-to-pad in a 1:1 or less ratio, as illustrated by the smaller-than-mounting pad deposition
40
illustrated in FIG.
1
. For example, for a typical 90×140-mil rectangular mounting pad, the solder deposited atop such a pad will usually be a similarly shaped deposition measuring 90×140 mils or less (e.g., 60×100 mils) that is centered with respect to the pad. After the paste has been printed/deposited, and the substrate populated with electronic components, the populated assembly is then subjected to reflow processing.
Applicants have discovered that it may be advantageous in some applications to go against standard industry practice and to intentionally overprint the mounting pads. For example, for a 90×140-mil pad, a 120×180-mil solder paste deposition may be desired. Such overprinting may be useful, for instance, in the creation of specially shaped solder joints during reflow. Applicants believe it is preferable to position the overprinted deposition such that substantially all of the deposition lies on the pad
24
and on one or more outboard edges
26
of the pad. (As used herein, the “outboard” edges
26
/
36
of the pad
24
and window
30
are those which are located generally outside the component footprint
50
, while the “inboard” edges
27
/
37
are those which are generally within the footprint
50
. For example, in
FIG. 1
, each rectangular mounting pad
24
and window
30
has one inboard edge
27
/
37
and three outboard edges
26
/
36
.)
However, if one were to follow standard industry practice in providing a 0- to 20-mil pullback between the pad and window edges, there would not be enough room between the pad edges
26
and the window edges
36
in which to overprint the paste without printing onto the adjacent outboard solder resist layer
28
, as illustrated by reference numeral
42
in FIG.
1
. Overprinting onto the solder resist layer
28
is undesirable because there would then be an increased risk of forming migratory solder balls and/or solder bridges between adjacent pads during reflow.
Therefore, it would be advantageous to provide a way of accommodating the desire to overprint which overcomes the aforementioned drawback.
SUMMARY OF THE INVENTION
The present invention overcomes the disadvantages of prior art approaches by providing an electronic circuit assembly offering improved overprintability. One configuration of the assembly comprises: (a) a dielectric substrate having a plurality of circuit traces and at least one mounting pad disposed thereon, wherein each mounting pad is arranged in matched relation with a respective termination of a surface-mount electronic component; and (b) a solder resist layer generally covering the substrate and having at least one window therethrough, wherein each window has at least one inboard window edge generally within a projected footprint of the electronic component and at least one outboard window edge generally outside the footprint, wherein each window generally conforms in shape with and is arranged about a respective one of the mounting pads. Each inboard window edge is spaced a first predetermined pullback distance P
1
away from a respective adjacent inboard mounting pad edge, and each outboard window edge is spaced a second predetermined pullback distance P
2
away from a respective adjacent outboard mounting pad edge, such that P
2
>P
1
for at least one of the at least one outboard window edge.
It is an object and advantage that the solder resist windows of the present invention provide one or more enlarged pullback zones outboard from the component footprint into which solder paste may be deposited, thereby facilitating solder paste overprinting.
These and other advantages, features and objects of the invention will become apparent from the drawings, detailed description and claims which follow.
REFERENCES:
patent: 5132864 (1992-07-01), Takemura et al.
patent: 5357060 (1994-10-01), Yamashita
patent: 5386087 (1995-01-01), Lee et al.
patent: 5425647 (1995-06-01), Mencik et al.
patent: 5453581 (1995-09-01), Liebman et al.
patent: 5486657 (1996-01-01), Bell et al.
patent: 5644475 (1997-07-01), Woychik et al.
patent: 5707714 (1998-01-01), Furutatsu et al.
Jairazbhoy Vivek Amir
McMillan Richard Keith
Gaffin Jeffrey
Hodges Leslie C.
Vigushin John B.
Visteon Global Technologies Inc.
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