Metal fusion bonding – Process of disassembling bonded surfaces – per se
Reexamination Certificate
2000-11-15
2002-07-09
Elve, M. Alexandra (Department: 1725)
Metal fusion bonding
Process of disassembling bonded surfaces, per se
C228S008000, C228S013000, C324S765010, C356S072000, C216S002000
Reexamination Certificate
active
06415975
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to IC (integrated circuit) packages, and more particularly, to a method and system for determining the quality of bonding between a conductive ball and a conductive pad of an IC (integrated circuit) die within an IC package by etching the conductive ball from the conductive pad and analyzing a magnified image of the bottom of the conductive ball.
BACKGROUND OF THE INVENTION
An IC (integrated circuit) die typically is housed within an IC (integrated circuit) package having leads that are coupled to conductive pads on the IC die for providing connection to nodes of the integrated circuit. Referring to
FIG. 1
, a cross sectional view of a lead frame strip
100
of an IC package includes an IC die
102
mounted on a die frame dap
104
. The die frame dap
104
is coupled to the lead frame strip
100
via tie bars
108
. The structures of the lead frame strip
100
are part of an IC package and are known to one of ordinary skill in the art of IC package manufacture.
Further referring to
FIG. 1
, a lead interconnect
112
of the lead frame strip
100
is coupled to a conductive pad on the IC die
102
for providing connection to a node of the integrated circuit of the IC die
102
. A conductive ball
114
is bonded to a conductive pad
116
on the IC die
102
. The conductive ball
114
is coupled to the lead interconnect
112
via a wire
118
.
FIG. 1
is a cross sectional view along a line A—A of a top view of the lead frame strip
100
of FIG.
2
.
The performance of the integrated circuit of the IC die
102
is determined by the quality of bonding between the conductive ball
114
and the conductive pad
116
. Referring to
FIG. 3A
, the conductive ball
114
is bonded to the conductive pad
116
using an ultrasonic and heating process to form an intermediary material
120
that bonds the conductive ball
114
to the conductive pad
116
, as known to one of ordinary skill in the art of IC package manufacture. In the ultrasonic and heating process, the intermediary material
120
is formed from a first conductive material of the conductive ball
114
and a second conductive material of the conductive pad
116
. For example, if the conductive ball
114
is comprised of gold and the conductive pad
116
is comprised of aluminum, then the intermediary material
120
is comprised of an intermetallic alloy (Au
x
Al
y
) formed from the gold of the gold ball
114
and the aluminum of the aluminum pad
116
.
FIG. 3A
illustrates an example of poor bonding between the conductive ball
114
and the conductive pad
116
. In
FIG. 3A
, a relatively low amount of material of the conductive ball
114
and the conductive pad
116
have been used to form a low amount of the intermediary material
120
. With such low amount of intermediary material
120
bonding the conductive ball
114
to the conductive pad
116
, the conductive ball
114
may have high resistance poor contact with the conductive pad
116
. Such high resistance degrades the speed performance of the integrated circuit within the IC die
102
.
FIG. 3B
illustrates an example of good bonding between the conductive ball
114
and the conductive pad
116
. In
FIG. 3B
, a relatively high amount of material of the conductive ball
114
and the conductive pad
116
have been used to form a high amount of the intermediary material
120
. With such high amount of intermediary material
120
bonding the conductive ball
114
to the conductive pad
116
, the conductive ball
114
may have low resistance good contact with the conductive pad
116
. Such low resistance enhances the speed performance of the integrated circuit within the IC die
102
.
Because the performance of the integrated circuit within the IC die
102
depends on the quality of bonding between the conductive ball
114
with the conductive pad
116
, the quality of bonding is monitored during manufacture of IC packages. Referring to
FIG. 4A
, in the prior art, a cross section along line B—B is made to result in the cross sectional view of
FIG. 4B
of the intermediary material
120
between the conductive ball
114
and the conductive pad
116
. Such a cross sectional view is analyzed to determine the quality of bonding between the conductive ball
114
and the conductive pad
116
. For example, a resin material is formed around the conductive ball
114
and the conductive pad
116
, and the conductive ball
114
and the conductive pad
116
are polished from the side inward to the cross section B—B to result in the cross sectional view of FIG.
4
B.
For consistent analysis, the cross section B—B should consistently be through the center of the conductive ball
114
as multiple IC packages are examined. However, polishing down to the exact center of the conductive ball
114
in the prior art is difficult to control. Referring to
FIG. 5A
, if the cross section B—B is not at the center of the conductive ball
114
, then the cross sectional view of
FIG. 5B
results with an inaccurate representation of the amount of intermediary material
120
between the conductive ball
114
and the conductive pad
116
. Thus, the quality of the bonding between the conductive ball
114
and the conductive pad
116
cannot be consistently determined with the polishing method of the prior art.
In addition, a cross section of the intermediary material
120
may not be an accurate representation of the quality of bonding between the conductive ball
114
and the conductive pad
116
. For example, if a void or a contaminant is present within the intermediary material
120
, and if the cross section is not through such a void or contaminant, then the cross section alone may not represent the poor contact between the contact ball
114
and the contact pad
116
from such a void or contaminant.
Nevertheless, because bonding between the conductive ball
114
and the conductive pad
116
in an IC package determines the performance of the integrated circuit, accurate and consistent determination of the quality of bonding is desired.
SUMMARY OF THE INVENTION
Accordingly, in a general aspect of the present invention, the quality of bonding between a conductive ball and a conductive pad of a lead frame strip of an IC package is determined by etching the conductive ball from the conductive pad and analyzing the bottom of the conductive ball.
The conductive ball is comprised of a first conductive material and a conductive pad is comprised of a second conductive material. The conductive ball is bonded to the conductive pad by formation of an intermediary material formed from the first conductive material of the conductive ball and the second conductive material of the conductive pad.
In one embodiment of the present invention, the lead frame strip is immersed within an etching solution such that the intermediary material is etched between the conductive ball and the conductive pad until the conductive ball may be decoupled from the conductive pad. The conductive ball has a first color in areas where the intermediary material was not abutting the conductive ball and has a second color in areas where the intermediary material was abutting the conductive ball when the conductive ball was bonded to the conductive pad. The intermediary material was abutting an intermediary material area of the conductive ball when the conductive ball was bonded to the conductive pad. A magnified image of the intermediary material area on the conductive ball is captured by a microscope with the intermediary material area being substantially centered within the magnified image when the conductive ball is decoupled from the conductive pad.
The intermediary material area appears as the second color within the magnified image, and an area of the conductive ball wherein the intermediary material was not abutting the conductive ball appears as the first color within the magnified image. The magnified image of the intermediary material area on the conductive ball is analyzed by a data processor to determine quality of bonding between the conductive ball and the conductive pad
Sabyeying Watana
Vijchulata Prakorn
Advanced Micro Devices , Inc.
Choi Monica H.
Cooke Colleen P.
LandOfFree
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