Metal fusion bonding – Process – Preplacing solid filler
Reexamination Certificate
2000-06-15
2002-08-13
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Preplacing solid filler
C228S215000
Reexamination Certificate
active
06431432
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods for attaching solder balls to a substrate.
2. Description of the Related Art
In assembly of components including semiconductor chips, it is often necessary to attach conductors on one surface to conductors on another surface with solder balls. For example, the manufacture of a “flip chip” involves placing solder balls on the bond pads of a semiconductor chip and attaching the solder balls to corresponding conductive traces on a substrate, thereby electrically connecting the chip to the substrate.
A conventional technique for creating a flip chip is illustrated in
FIGS. 1
a
-
1
b.
FIG. 1
a
is an overhead view of an unbonded flip chip with a die
100
having an active circuit surface
102
on which are arranged a plurality of solder balls
104
, which typically are an alloy comprising lead and tin.
FIG. 1
b
is an overhead view of a substrate
108
to which the die
100
will be attached. The substrate comprises a plurality of bond pads
106
, often comprising copper with a thin top layer of nickel/gold, each of a plurality of traces
118
, typically copper, connected to a corresponding one of the plurality of bond pads
106
. Frequently, substrates are produced by vendors that ship the substrates to whomever performs final assembly of the package. To protect the traces
118
from oxidizing and from mechanical damage during the shipment process, substrate vendors often apply an organic solderability preserve (“OSP”) to the traces
118
.
Prior to bonding the die
100
to the substrate
108
, solder flux is applied to the plurality of solder balls
104
or the plurality of bond pads
106
on the substrate
108
. Typical flux materials include low-solids, no-clean fluxes such as TAC-10 (produced by Indium Corp.) and Kester 9601 (produced by Kester Corporation). The flux serves primarily to aid the flow of the solder, such that the plurality of solder balls
104
make good contact with the plurality of bond pads
106
. The flux may be applied in a variety of ways, including brushing or spraying, or dipping the die
100
into a thin film, thereby coating the plurality of solder balls
104
with flux.
As shown in
FIG. 1
c,
the substrate
108
is overlaid with a solder mask
110
. The solder mask
110
is typically 35 um thick and often comprises a dielectric material. As shown, the solder mask
110
has a plurality of holes
112
therein, each of the plurality of holes
112
corresponding to one of the plurality of solder balls
104
. Each of the plurality of holes
112
exposes a corresponding one of the plurality of bond pads
106
; the exposed bond pads contact the plurality of solder balls
104
when the die
100
and substrate
108
are moved toward each other. The solder mask
110
keeps the solder in the area of each of the bond pads
106
and thus prevents the solder from flowing onto the plurality of traces
118
. Also, in the area outside the die placement area, the solder mask
110
provides mechanical protection and surface insulation resistance to the plurality of traces
118
.
FIG. 1
d
is a cross sectional side view of an assembly
114
comprising the substrate
108
and the die
100
after the two have been brought together. The assembly
114
is heated, causing the plurality of solder balls
104
to reflow and thus to mechanically and electrically couple a pad on the die
100
to a corresponding one of the plurality of bond pads
106
. During the reflow process, the solder mask
110
prevents the solder from flowing onto the plurality of traces
118
. The assembly
114
is heated within a temperature range for a certain time to activate the flux (“soak time”). The temperature is then increased to cause the solder to reflow. After cooling, underfill is then dispensed into a gap
116
(shown in
FIG. 1
d
) between the die
100
and the substrate
108
. Since the solder mask
110
is typically 35 um thick, it reduces the gap
116
for underfill by about 15 um to 20 um.
This reduction in the gap between a chip and a substrate increases the difficulty of dispensing the underfill; specifically, a smaller gap impedes the flow of underfill between the chip and the substrate and thereby reduces the adhesion of the underfill. Reduced adhesion, in turn, results in decreased reliability of the attachment of die to substrate under conditions of stress, such as temperature cycling and moisture preconditioning. Solder masks have other drawbacks. For example, dimensional tolerances of the solder mask openings for the pads on the substrate can limit the density (pitch) between solder balls due to substrate manufacturing defects and defects involved in the assembly of the flip chip. These defects lower manufacturing yields and increase the cost of the substrate and the cost of flip chip assembly.
Therefore, it would be desirable to control solder spread by means other than a solder mask.
SUMMARY OF THE INVENTION
The present invention satisfies the above need to control solder spread on a substrate with bond pads by means other than a solder mask. According to one aspect of the present invention, a solder mask is placed on a substrate but this solder mask will not be used to control solder spread but merely helps to protect traces that are distant from the bond pads. The solder mask has an opening that is preferably greater than the area of a die to be attached; this opening exposes both the bond pads and at least portions of traces proximate to the bond pads. Since the traces proximate to the bond pads are not protected by a solder mask, the present invention employs at least one of two methods to control the flow of solder.
According to one of the methods, an appropriate combination of a flux and an OSP is selected along with particular process parameters (e.g. the shape of the solder reflow profile) to control solder spread. According to the other of the methods, the portions of the traces that are proximate to the bond pads are oxidized, thereby preventing solder from flowing onto these portions of the traces during the solder reflow process.
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patent: 5156997 (1992-10-01), Kumar et al.
patent: 5345056 (1994-09-01), Frei et al.
patent: 5467253 (1995-11-01), Heckman et al.
patent: 5523920 (1996-06-01), Machuga et al.
patent: 5898992 (1999-05-01), Annable
patent: 5920123 (1999-07-01), Moden
patent: 5953814 (1999-09-01), Sozansky et al.
patent: 5954751 (1999-09-01), Chen et al.
patent: 5956605 (1999-09-01), Akram et al.
patent: 5964395 (1999-10-01), Glovatsky et al.
patent: 5973406 (1999-10-01), Harada et al.
patent: 6118180 (2000-09-01), Loo et al.
Desai Kishor V.
McCormick John Pierre
Dunn Tom
Johnson Jonathan
LSI Logic Corporation
Westman Champlin & Kelly
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