Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2002-04-11
2003-04-08
Abrams, Neil (Department: 2839)
Metal working
Method of mechanical manufacture
Electrical device making
Reexamination Certificate
active
06543128
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a ball grid array (BGA) package and its fabricating process, and more particularly to a substrate for a ball grid array package and the fabricating process of the substrate.
2. Description of Related Art
The IC (integrated circuit) packaging is the last step of the fabrication of the IC products. The purpose of the IC packaging is to provide the chip a medium for electrical connection to the PCB (Printed Circuit Board) or other appropriate devices, and additionally, to protect the chip from being damaged or short-circuit.
The IC circuit is generally encapsulated in a package and is then bonded to the PCB or other substrates, and the BGA package is one of the package structure being used very often. As the degree of integration of the IC is getting higher and higher, a lot of wires are led out, and hundreds of connections are required to constitute an integrated circuit. Therefore, it becomes necessary for the improvement on the design and fabrication process etc. of the chip carrier used for carrying the chip in order to perform packaging, and of the substrates of PCB and circuit carrier used for the connection and assembly of electronic components.
FIG. 1
is a top view of a substrate of a ball grid array package according to the prior art presented in 1994 by Intel Company with U.S. Pat. No. 5,519,580. A surface of a substrate
100
comprises a ball pad
102
, a via
114
, and a solder mask
110
. The ball pad
102
further comprises a center area
104
, and tabs
106
which is symmetrically outward extended in radial direction. The peripheral of the via
114
is provided with a via land
116
which is electrically connected to the center area
104
by a conductive connecting bar
108
. The solder mask is used for covering the circuit to prevent unnecessary electrical connection. There is a ball pad opening
112
concentrically disposed with the ball pad
102
with a diameter of the ball pad opening
112
greater than that of the center area
104
.
FIG. 1B
is a cross-sectional view of a portion of the structure of the BGA package shown in FIG.
1
A. As shown in
FIG. 1B
, the center area
104
on the surface
101
of the substrate
100
is positioned at the center of the ball pad opening
112
, and is not covered by the solder mask
110
. The solder ball
118
is disposed on the center area
104
and is connected to the surface
101
and the side wall of the ball pad
102
. Thereby, the contact area between the solder ball
118
and the ball pad
108
is relatively large, consequently, a relatively more robust solder joint can be obtained.
FIG. 1C
shows that when the solder joint is not robust enough to sustain the external forces, the center area
104
of the ball pad
102
would crack to become cracked center area
104
′ and
104
″. Consequently, the solder ball
118
and the center area
104
′ would separate from the surface
101
of the substrate
100
which would result in poor reliability on the bonding between the solder ball
118
and the substrate
100
.
FIG. 2A
is a top view of a substrate of a ball grid array package according to the prior art presented in 1996 by Intel Company with U.S. Pat. No. 5,706,178. As shown in the
FIG. 2A
, elliptically shaped ball pads
202
are disposed on the substrate
200
and are parallel to the long axis. Vias
204
and
204
′ are also set up in the area of the ball pad
202
. The solder mask
206
and the ball pad
202
are both on the same side surface of the substrate
200
. And the solder mask
206
has a set-up of a ball pad opening
208
with a diameter which is equal to the short axis
212
of the elliptically shaped ball pads
202
. Therefore, the ball pad opening
208
can expose a portion of the surface of the elliptically shaped ball pads
202
while the remaining portion will be covered by the solder mask
206
.
FIG.
2
B and
FIG. 2C
are the cross-sectional views of
FIG. 2A
along the cross-section
2
B—
2
B and the cross-section
2
C—
2
C respectively. Since
FIG. 2B
is also a cross-sectional view along the short axis
212
of the ball pad
202
, the ball pad
202
is not covered by the solder mask
206
, i.e. the diameter of the ball pad opening
208
(in
FIG. 2A
) is equal to the length of the short axis
212
of the elliptical ball pad
202
. Since
FIG. 2C
is also a cross-sectional view along the long axis
214
of the elliptical ball pad
202
, a portion of the ball pad
202
is covered by the solder mask
206
, i.e. the diameter of the ball pad opening
208
(in
FIG. 2A
) is smaller than the length of the long axis
214
of the elliptical ball pad
202
. The solder ball
216
is disposed in the ball pad opening
208
, and is attached to the ball pad
202
.
This type of package employs elliptically shaped ball pad
202
in order to increase the routing space available in between the ball pads, and further to raise the routing density of the conductive trace
210
in between the ball pads. A portion of the surface of the elliptical ball pad
202
is covered by the solder mask
206
which can reinforce the solder joint between the ball pad
202
and the substrate
200
, and increase the peel strength of the ball pad
202
. But limiting by the ball pad opening
208
, there is only a surface contact without the side wall connection between the solder ball
216
and ball pad
202
, therefore, the solder joint between them is not quite strong.
FIG. 3A
is the top view of a type of ball pad and conductive trace arrangement on the substrate according to the prior art. There are ball pads
302
, conductive traces
304
, and a solder mask
306
attaching to the a substrate
300
made of Bismaleimide-Triazine (BT) resin. The spacing between the solder balls
312
is 1,270 &mgr;m (micron), and the ball pad diameter
310
is 800 &mgr;m, hence the distance between the edges of the ball pads is 470 &mgr;m. As the width
314
of the conductive trace
304
is 100 &mgr;m, at least 50 &mgr;m is needed for the spacing between each of the conductive trace
304
and each ball pad
302
and for the spacing between the adjacent conductive traces
304
. Thereby, only two conductive traces
304
can be disposed between two adjacent traces
304
. Consequently, the routing capability of the substrate
300
will be affected such that its routing density is unable to increase.
Additionally,
FIG. 3B
is a cross-sectional view of
FIG. 3A
along section
3
B—
3
B. As shown in FIG.
3
A and
FIG. 3B
, the solder mask
306
covers all over the top of substrate
300
including the conductive traces
304
. The solder mask
306
also covers up to the peripheral portion of the ball pads
302
, leaving only a center portion of the ball pads
302
that is exposed by the ball pad openings
308
. The ball pad openings
308
having 600 &mgr;m in diameter are concentrically disposed with the ball pad
302
. In this way, the anchor force of the solder mask
306
covering the peripheral portion of the solder pads
302
can strengthen the solder joint between the solder pads
302
and the substrate
300
. The solder balls
320
are attached to the ball pad
302
through the ball pad openings
308
. Similar to the situation in
FIG. 2B
, limiting by the dimension of the ball pad opening
308
, there is only a surface contact without the side wail connection between the solder ball
320
and ball pad
302
, therefore, the solder joint between them is not quite strong.
What is more, shown in
FIG. 3C
is the top view of another type of ball pad and conductive trace arrangement on the substrate according to the prior art. Similar to those in
FIG. 3A
, same component numbers are used in FIG.
3
C. As shown in
FIG. 3C
, three conductive traces
304
′ can be disposed in between the adjacent ball pads
302
if the width
314
′ of the conductive traces
304
′ is reduced down to 90 &mgr;m rather than 100 &mgr;m as shown in FIG.
3
A. In this way, the routing density can be increased only that reducing the width of the conductive trace
Huang Chien-Ping
Yang Grace
Abrams Neil
Dinh Phuong
J.C. Patents
Siliconware Precision Industries Co. Ltd.
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