Metal fusion bonding – Process – Preplacing solid filler
Reexamination Certificate
2000-07-12
2002-06-18
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Preplacing solid filler
C228S180210, C228S254000
Reexamination Certificate
active
06405920
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to printed circuit boards (PCBs), and more particularly to printed circuit boards having enhanced mounting pads thereon.
2. Disclosure Information
Surface-mount electronic components
30
are typically connected to printed circuit boards as illustrated in FIG.
1
. First, a substrate
10
is provided which has a plurality of circuit traces
12
and mounting pads
20
thereon. The mounting pads
20
are arranged on the substrate
10
in matched relation with the terminations
32
of the electronic components
30
that are to be attached to the substrate. Next, depositions of solder paste
40
are placed atop each mounting pad
20
, such as by screenprinting or dispensing, followed by placement of the components
30
atop the substrate such that each component termination
32
rests on the solder deposition
40
atop a respective mounting pad
20
. Finally, the assembly is subjected to reflow soldering, laser soldering, or other processing so as to melt the solder depositions
40
and then allow them to cool to form solid solder joints
50
which mechanically and electrically connect each component termination
32
with its respective mounting pad
20
.
The typical approach in depositing solder paste atop the mounting pads
20
is to deposit the paste in a 1:1 or less ratio with respect to each pad. For example, a rectangular 80×120-mil mounting pad may have a solder deposition thereatop that is a 50×90-mil rectangle generally centered atop the pad. However, the inventors of the present invention have discovered that it is advantageous in some applications to intentionally overprint the solder paste; that is, to deposit the solder in such a way that the deposition
40
extends outside the outer edges
22
of the mounting pad, as illustrated in FIG.
2
. This is done, for example, to promote the formation of certain solder joint shapes during reflow.
Although overprinting provides certain advantages, it presents certain problems as well. One problem is that when using conventional, industry-standard design rules for circuit board construction, the overprinted solder deposition
40
extends partly atop the solder mask
14
which typically surrounds each mounting pad
20
. (The solder mask
14
is a non-solder-wettable layer used on most PCBs for covering essentially all of the substrate surface, except for apertures
16
that are formed therein located about each mounting pad
20
, as illustrated in
FIGS. 1-2
. The edges
18
of each aperture
16
are generally spaced outward from the corresponding mounting pad edges
22
by a certain distance P, called the “solder mask pullback”; for screenprinted solder mask, the pullback P is typically 10-20 mils, whereas for liquid photoimageable solder mask (LPISM) the pullback is typically 0-5 mils.) When the solder paste deposition
40
extends beyond the mounting pad edges
22
and onto the surrounding solder mask
14
as illustrated in
FIGS. 3-4
(or, when no solder mask is used, onto the top surface of the surrounding bare substrate
10
), there is a tendency to form solder balls
60
that are separate from the solder joint
50
. The formation of solder balls
60
is undesirable for at least two reasons: (1) solder balls
60
take solder away from the solder joint
50
, which often affects the joint
50
detrimentally, and (2) the solder balls
60
may roll across the substrate/solder mask and lodge between adjacent pads/terminations/joints, thereby causing unwanted solder bridges and short-circuits. These problems are presented regardless of whether the substrate
10
has a solder mask layer
14
thereon.
It would be desirable, therefore, to provide a way of allowing overprinting without the aforementioned drawbacks.
SUMMARY OF THE INVENTION
The present invention overcomes the disadvantages of the prior art approaches by providing a printed circuit board having enhanced mounting pads useful for solder paste overprinting. The PCB comprises: a dielectric substrate having at least one mounting pad thereon, wherein each mounting pad is arranged in matched relation with a respective termination of an electronic component. Each mounting pad includes a main body portion and one or more fingerlike extensions extending outward from the main body portion and away from a projected footprint of the electronic component. A method for overprinting using this construction is also disclosed, as is a method for repairing a PCB having this construction.
It is an object and advantage that the present invention helps to minimize solder ball formation when overprinting solder paste.
Another advantage is that the present invention may be readily and inexpensively used with conventional circuit board designs.
These and other advantages, features and objects of the invention will become apparent from the drawings, detailed description and claims which follow.
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Brunner Bjoern Erik
Jairazbhoy Vivek Amir
McMillan Richard Keith
Dunn Tom
Johnson Jonathan
Kajander John
Visteon Global Technologies Inc.
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