U.S. patent application number 09/822551 was filed with the patent office on 2001-08-09 for soldered integrated circuit connections.
Invention is credited to Blue, Carey, Butler, Scott, Regner, Richard.
Application Number | 20010011676 09/822551 |
Document ID | / |
Family ID | 25532533 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010011676 |
Kind Code |
A1 |
Regner, Richard ; et
al. |
August 9, 2001 |
Soldered integrated circuit connections
Abstract
Solder paste is deposited using a solder stencil with an
aperture having a concave edge in order to reduce the formation of
solder balls. The solder deposit may be formed on a pad such that
the paste has a concave edge. Solder paste deposits may be made on
adjacent pads with the concave surfaces facing one another.
Inventors: |
Regner, Richard; (Meridian,
ID) ; Butler, Scott; (Greenleaf, ID) ; Blue,
Carey; (Kuna, ID) |
Correspondence
Address: |
TROP PRUNER & HU, PC
8554 KATY FREEWAY
SUITE 100
HOUSTON
TX
77024
US
|
Family ID: |
25532533 |
Appl. No.: |
09/822551 |
Filed: |
March 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09822551 |
Mar 30, 2001 |
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09550784 |
Apr 17, 2000 |
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6248452 |
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09550784 |
Apr 17, 2000 |
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09270695 |
Mar 17, 1999 |
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6094832 |
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09270695 |
Mar 17, 1999 |
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08986541 |
Dec 8, 1997 |
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6012231 |
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Current U.S.
Class: |
228/248.1 ;
228/33; 428/596; 428/599 |
Current CPC
Class: |
H05K 3/3442 20130101;
Y10T 428/12361 20150115; H05K 2201/10636 20130101; Y10T 428/12063
20150115; H05K 2203/0545 20130101; Y10T 428/12382 20150115; H05K
3/3485 20200801; Y02P 70/50 20151101; H01L 2224/11472 20130101;
H01L 2924/00014 20130101; H01L 2924/00014 20130101; H01L 2224/0401
20130101 |
Class at
Publication: |
228/248.1 ;
428/596; 428/599; 228/33 |
International
Class: |
B23K 020/08; B32B
003/10; B21C 027/00; B21D 028/00; B21F 027/00 |
Claims
What is claimed is:
1. A solder stencil comprising a body having at least one solder
aperture, said aperture having a concave edge profile.
2. The stencil of claim 1 wherein said concave edge is curved.
3. A solder stencil for depositing solder paste on a pad comprising
a plate having at least one solder aperture, said aperture having
an area less than the area of said pad.
4. The stencil of claim 3 wherein said aperture has a concave edge
profile.
5. The stencil of claim 3 wherein said stencil includes a pair of
apertures having concave edges, said concave edges being
opposed.
6. A method of depositing solder paste comprising the step of
forming a deposit of solder paste on a pad, said deposit having an
area less than the area of said pad, and reflowing said solder
paste.
7. The method of claim 6, including the step of forming a solder
stencil with an aperture having a concave edge.
8. The method of claim 6, including the step of forming said
deposit with an area approximately 70 to 80% of the area of said
pad.
9. The method of claim 6, including the step of forming a pair of
adjacent deposits of solder paste, each of said deposits having a
concave edge, the concave edges of said adjacent paste deposits
being opposed.
10. A pad for forming a soldered contact comprising: a pad; and a
layer of solder paste on said pad, said layer having a concave
edge.
11. The pad of claim 10, wherein said pad is incompletely covered
by said layer of solder paste.
12. The pad of claim 10, wherein said concave edge is curved.
13. A pad for forming a soldered contact comprising: a pad; and a
layer of solder paste on said pad, said layer leaving a concave
portion of said pad uncovered by solder paste.
14. The pad of claim 13, wherein said layer of solder paste has a
curved concave edge.
15. The pad of claim 13, wherein said pad is generally rectangular,
said pad having a width and a length, the width of said pad being
greater than the width of the solder paste.
16. The pad of claim 15 wherein said pad is formed of a size
selected from the group consisting of 0603, 0805 and 1206
configurations.
17. The pad of claim 16 wherein said layer of paste has a radius of
curvature of from about 0.0590 inches to about 0.092 inches.
18. The pad of claim 17 wherein the pads are rectangular and said
layer is formed having a width and length, the length of said layer
being from about 0.0125 to about 0.029 inches.
19. A method of depositing solder paste comprising the steps of:
forming a deposit of solder paste with a concave edge; and
depositing said paste on the pad.
20. The method of claim 19, including the step of forming said edge
in a circular arc.
21. The method of claim 19, including the step of reflowing said
paste and soldering a surface mount component to said pad.
22. A method of depositing solder paste comprising the steps of
forming a solder paste deposit and depositing said solder paste on
a pad such that the area of solder paste is less than the area of
said pad.
23. The method of claim 22, including the step of forming a curved
concave edge on said solder paste.
24. The method of claim 22, including the step of forming a pair of
opposed solder paste deposits on a pair of adjacent pads, said
solder paste deposits having concave edges, said concave edges of
said adjacent solder paste deposits being opposed.
25. The method of claim 22 wherein said pad is formed of a size
selected from the group consisting of 0603, 0805 and 1206
configurations.
26. The method of claim 25 including forming said deposit with a
radius of curvature of from about 0.0590 to about 0.092 inches.
27. The method of claim 26 wherein the pads are rectangular and
said deposit is formed having a width and length, the length of
said deposit being from about 0.0125 to about 0.029 inches.
Description
[0001] This invention relates generally to a solder stencil for
applying a solder paste to a pad in the manufacture of integrated
circuits. More particularly, the present invention is directed to
reducing the creation of solder balls.
BACKGROUND OF THE INVENTION
[0002] Solder stencils are generally utilized in the manufacture of
contacts on printed circuit boards to define the shape of a solder
paste on the surface to be soldered. Solder paste is a suspension
of flux and small spheres of solder particles. When heated, the
paste forms a solder which secures two metal pieces together.
Solder paste is screen printed onto a circuit board using a
stencil. The stencil is a thin sheet of metal that has a desired
hole pattern formed in the stencil.
[0003] In a reflowing process, the solder melts, forming a
mechanical and electrical connection between printed circuit board
land pads and an electrical component. Flux encourages the
formation of this connection. Flux may be removed through a
cleaning process or left on the board with the so-called no clean
process.
[0004] One problem which may occur with solder paste is the
formation of solder balls. Solder balls are small spherical solder
particles which are formed away from the main solder pool during
reflow. If these balls become loose at any time after the product
is manufactured, they can roll to a position where they can short
an electrical component. Solder balling may occur around components
and over the board. Large solder balls may be associated with small
and low clearance passive components such as 0805 and 1206 resistor
and capacitor connection sites. The large solder balls may not be
removed in the cleaning process and will persist with the no-clean
process.
[0005] In addition, excessive solder balling may prevent the solder
from making a good solder joint fillet. Thus, the formation of
solder balls may create reliability issues. There has been a
continuous effort to minimize the occurrence of solder balling. See
C. A. Harper, Electronic Packaging & Interconnection Handbook,
2d Edition, McGraw Hill (1997), at p. 5.54.
[0006] It would be highly desirable to have a technique for forming
soldered connections using solder paste which reduces the
occurrence of solder balling.
SUMMARY OF THE INVENTION
[0007] In accordance with one aspect of the present invention, a
solder stencil includes a body having at least one solder aperture.
The aperture has a concave edge profile.
[0008] In accordance with another aspect of the present invention,
a solder stencil for depositing solder paste on a pad includes a
plate having at least one solder aperture. The aperture has an area
less than the area of the pad.
[0009] In accordance with still another aspect of the present
invention, a method of depositing solder paste includes the step of
forming a deposit of solder paste on the pad. The deposit has an
area of less than the area of the pad.
[0010] In accordance with another aspect of the present invention,
a pad for forming a soldered contact includes a pad and a layer of
solder paste on the pad. The solder paste has a concave edge
profile.
[0011] In accordance with yet another aspect of the present
invention, a pad for forming a soldered contact includes a pad and
a layer of solder paste on the pad. The layer leaves a concave
portion of the pad uncovered by solder paste.
[0012] In accordance with but another aspect of the present
invention, a method of depositing solder paste includes the step of
forming a deposit of solder paste with a concave edge. The solder
paste is then deposited on the pad.
[0013] In accordance with another aspect of the present invention,
a method of depositing solder paste includes the step of forming a
solder paste deposit. The solder paste is deposited on the pad such
that the area of the solder paste is less than the area of the
pad.
[0014] These aspects of the present invention may be advantageous
for a number of reasons including reducing the occurrence of solder
balls.
DESCRIPTION OF THE DRAWING
[0015] FIG. 1 is a top plan view of a solder stencil having solder
apertures;
[0016] FIG. 2 is a top plan view of a pair of pads on a printed
circuit board;
[0017] FIG. 3 is a ton plan view of solder paste on top of the
solder pad shown in FIG. 2; and
[0018] FIG. 4 is a cross-sectional view taken generally along the
line 4-4 in FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] Referring to the drawing wherein like reference characters
are used for like parts throughout the several views, a solder
stencil 10 shown in FIG. 1 may be used to apply solder paste to
pads on printed circuit boards. The solder stencil 10 is
conventionally made of a thin sheet of metal such as brass,
stainless steel, molybdenum, alloy-42, or nickel.
[0020] The stencil 10 includes a hole pattern represented by the
two holes 12 and 14 in FIG. 1. Each of the holes 12 and 14 is of a
generally rectangular shape with facing edges 16 having a concave
configuration. The holes 12, 14 may be formed by conventional
techniques including chemical etching, laser cutting,
electropolishing, electroplating and electroforming.
[0021] As used herein, "concave" refers to an inwardly directed
configuration that would include curved configurations, V-shaped
configurations and other like configurations.
[0022] A typical pair of bonding pads 18 of a type used for small
passive components such as a capacitor or resistor may be
positioned on a printed circuit board 19, as shown in FIG. 2. Each
pad 18 has a width "X" and a length "Y". The pads 18 may, for
example, be land pads on a conventional printed circuit board. The
land pads are generally coated with a deposited layer of solder
paste. The solder paste is reflowed to electrically connect a
component to the pad 18. The soldering process creates both an
electrical and a mechanical connection.
[0023] In accordance with one aspect of the present invention, the
length Y of the bond pad 18 corresponds generally to the length Y
of the solder stencil apertures 12, 14. However, the width X of the
bond pad 18 is generally greater than the width of the stencil
apertures 12, 14 as indicated in FIG. 2. Generally the width
dimensions X of the solder apertures 12, 14 are about 20-30% less
than the width X of the corresponding bond pad 18.
[0024] The radius of curvature "R" of the concave edge 16 extends
from about 0.05 inches to about 0.1 inches and is roughly about
twice the length Y of the aperture 12, 14. The distance between the
outside edges of the land pads is indicated as "Z". Exemplary
approximate dimensions in inches for a 0.006 inch stencil thickness
are as follows:
1 X Y Z R 0603 Pad 0.0175 0.035 0.075 Stencil 0.0140 0.035 0.0591
0805 Pad 0.035 0.055 0.1 Stencil 0.0290 0.055 0.0781 1206 Pad 0.035
0.065 0.15 Stencil 0.0290 0.060 0.092
[0025] For a 0.008 inch stencil thickness the following are
exemplary approximate dimensions for conventional 0603, 0805, 1206
components:
2 X Y Z R 0603 Pad 0.0175 0.035 0.075 Stencil 0.0125 0.035 0.0590
0805 Pad 0.035 0.055 0.1 Stencil 0.025 0.055 0.092 1206 Pad 0.035
0.065 0.15 Stencil 0.025 0.065 0.092
[0026] As shown in FIGS. 3 and 4, the deposited paste 20 does not
completely cover the bond pad 18 but, instead, a concave-shaped
exposed area 22 remains. As a result of the concave configuration
of the deposited paste 20 and the apertures 12 in the stencil 10,
the occurrence of solder balls decreases. This is particularly
important in situations where no clean fluxes are utilized.
[0027] The prevention of solder balling is particularly applicable
to forming solder connections with surface mount packaging. The
surface mount components are placed onto a printed circuit board
and into the deposited paste and then the paste is reflowed. As a
result the surface mount components are secured to the pads.
[0028] While this invention has been described with respect to a
limited number of preferred embodiments, those skilled in the art
will appreciate numerous modifications and variations. It is
intended that the appended claims cover all such modifications and
variations as fall within the true spirit and scope of the present
invention.
* * * * *