Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2002-06-17
2004-08-31
Chambliss, Alonzo (Department: 2827)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S613000, C228S180220
Reexamination Certificate
active
06784087
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a cylindrical bonding structure and its method of manufacture. More particularly, the present invention relates to a cylindrical bonding structure for a flip chip package and a method of fabricating the cylindrical bonding structure.
2. Description of Related Art
In this information-saturated society, working with electronic products has become an integral part of our daily life. Currently, integrated circuit products are used for doing business, educating our children or providing us with games for recreation. As a result of rapid progress in electronic technologies, devices having powerful functions and personalized designs have been developed. Moreover, most electronic products have light and compact design. Nowadays, high-density integrated circuits are frequently housed within compact semiconductor packages such as a flip-chip package and a ball grid array (BGA) package.
In the flip-chip technique, bumps are formed on the bonding pads of a chip so that the bumps may be attached to corresponding contact points on a substrate after flip over. Compared with conventional wire bonding and tape automatic bonding (TAB) packaging techniques, a flip-chip package has the shortest signal transmission path between the chip and the substrate and hence has superior electrical properties. In addition, a flip-chip package may be designed to have its back exposed so as to increase heat dissipation rate. Due to the above reasons, flip-chip packaging techniques are widely adopted in the semiconductor fabrication industry.
FIG. 1A
is a partially magnified view showing a connection configuration between a bump on a chip and a contact point on a substrate in a conventional flip-chip package. A chip
110
normally has a plurality of bonding pads
112
(only one is shown in FIG.
1
A). Each bonding pad
112
has a bump
114
. In general, the bump
114
is a solder bump so that the flip-over chip
110
may directly connect with one of the bonding pads
122
(only one is shown in
FIG. 1A
) on the substrate
120
. Since the chip
110
and the substrate
120
each has a different coefficient of thermal expansion (CTE), a standoff distance must be provided between the chip
110
and the substrate
120
so that differential thermal expansion will not accumulate too much shear stress to break the bumps
114
prematurely.
Thus, to prevent shear stress from damaging the bumps
114
, bumps
114
having a great height are often attached to the bonding pads
112
of the chip
110
so as to increase the distance of separation between the chip
110
and the substrate
120
as much as possible. However, increasing the overall height of the bumps
114
must be accompanied by a corresponding increase in outer diameter and volume of the bumps. Moreover, to prevent short-circuiting, pitch between neighboring bumps
114
must be increased. Ultimaately, distance between neighboring bonding pads
112
on the chip
110
is hard to reduce.
In addition, pre-solder material is often applied on the junction pads
122
of the substrate
120
before the lower end of the bumps
114
are put against the pads
122
. In a reflow operation, the low melting point pre-solder melts and joins the bumps
114
and the junction pads
122
together. Because an additional step of applying low melting point solder over the junction pads
122
of the substrate
120
has to be conducted, cost of fabricating the substrate
120
is increased Furthermore, to increase the distance of separation between the chip
110
and the substrate
120
, high lead solder is a principle ingredient of the bumps
114
. Since a high temperature treatment of the bump material to form a spherical shape bump often produces oxide material near the surface, the bumps
114
and the junction pads
122
often have poor adhesion after the solder reflow process. Poor adhesion often leads to bad electrical connections between the chip and the substrate and a low overall yield of the flip chip package.
FIG. 1B
is a partially magnified view showing an alternative connective configuration between a bump on a chip and a contact point on a substrate in a conventional flip-chip package. A solder mask
124
is formed over the substrate
120
to pattern out contact area around the junction pads
122
. In fact, there are two major patterning techniques that employ the solder mask
124
. The first one is called a ‘solder mask define’ (SMD) and the other one is called a ‘no solder mask define’ (NSMD). In
FIG. 1A
, a ‘solder mask define’ (SMD) technique is used. An opening
126
in the solder mask
124
exposes a portion of the junction pad
122
so that a bump on the chip
110
is in a corresponding position over the junction pad
122
on the substrate
120
. In
FIG. 1B
, a ‘no solder mask define’ (NSMD) technique is used. An opening
126
in the solder mask
124
completely exposes a junction pad
122
so that a bump is completely connected to the junction pad
122
. The most commonly used material for forming the solder mask
124
is, for example, green lacquer.
To shorten pitch between neighboring junction pads
122
, SMD technique such as the one shown in
FIG. 1A
is often employed. Only a portion of the junction pad
122
is exposed through the solder mask
124
for contact with the lower edge of a bump
114
(shown in profile by dash lines
114
a
). However, because actual dimension of a bump
114
may vary from the standard dimension by ±10%, variation in positional accuracy between the bump
114
and the junction pad
122
of up to 10 &mgr;m is possible. Furthermore, the opening
126
in the solder mask layer
124
may have an intrinsic diametrical variation of about 15 &mgr;m. Hence, when the bump
114
and the junction pad
122
are laid on top of each other, the lower edge of the bump
114
may not come into direct contact with the surface of the junction pad
122
. In extreme cases, part of the outer edge of the bump
114
may lean upon the upper corner of the opening
126
of the solder mask layer
124
shown by the dash line
114
b
in FIG.
1
A. Hence, after a solder reflow operation, the bump
114
may not be properly bonded with the junction pad
122
to form a good electrical connection. To ensure proper bonding between the lower edge of the bump
114
with the junction pad
122
, diameter of the opening
126
of a conventional solder mask
124
is generally larger than the external diameter of the bump
114
. Since distance between neighboring junction pads
122
must be increased to accommodate the extension, ultimate level of integration is greatly reduced.
SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a cylindrical bonding structure and its method of manufacture capable of reducing the separation between neighboring bonding pads on a chip while increasing distance of separation between the chip and a substrate. Ultimately, reliability of the junctions connecting the chip and the substrate is improved and post-packaging life of the chip is extended.
A second object of this invention is to provide a cylindrical bonding structure and its method of manufacture capable of reducing the diameter of openings on a solder mask for exposing a junction pad so that distance of separation between neighboring junction pads on the substrate is reduced. Consequently, the distance of separation between neighboring bonding pads (bumps) on the chip is also reduced.
A third object of this invention is to provide a cylindrical bonding structure and its method of manufacture that requires no application of low melting point solder material on the junction pads of a substrate or the surface of bumps before conducting a reflow process. Thus, production cost of a flip-chip package is reduced.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a cylindrical bonding structure and its method of manufacture. A ball contact metallic layer is formed ove
Chou Chien-Kang
Kuo Hsi-Shan
Lee Jin-Yuan
Lin Shih-Hsiung
Chambliss Alonzo
J.C. Patents
Megic Corporation
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