Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2001-09-25
2003-02-25
Nguyen, Ha T. (Department: 2812)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S612000, C438S615000, C438S618000
Reexamination Certificate
active
06524943
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a bump forming method. More particularly, the present invention relates to a method of forming metal bumps on electrodes provided on a printed circuit board, a wafer or a ceramic board by utilizing a resin film as a mask.
2. Description of the Related Art
Recently, there is an increasing demand for mounting electronic components on a substrate (e.g. a printed circuit board) at high densities. For meeting such a demand, much attention is focused on bear chip mounting. The bear chip mounting includes the face-up bonding which utilizes wire-bonding for providing electrical connection between a chip and a wiring pattern on a circuit board, and the face-down bonding which utilizes metal bumps for providing electrical connection. Recently, the face-down bonding increasingly replaces the face-up bonding.
The face-down bonding which utilizes metal bumps is capable of connecting electronic components at a low resistance. In forming metal bumps, the following requirements should be satisfied.
When the electrodes of an electronic component are arranged at a high density, metal bumps need be correspondingly arranged at a small pitch to be precisely positioned on the electrodes. This is particularly true in forming metal bumps on the electrodes of a semiconductor device. Further, all metal bumps need to have an equal height for ensuring reliable connection between electronic components. In addition, there is also an inherent demand for a decrease in the manufacturing cost.
Conventionally, metal bumps for face-down bonding have been formed by plating or vapor deposition. However, the formation of metal bumps by such methods requires much cost for the equipment while providing difficulties in controlling the height and composition of the bumps. Recently, therefore, the conventional methods are increasingly replaced with a metal mask printing method and a resin mask loading method for realizing a cost reduction while providing a higher freedom in controlling the composition of metal bumps.
FIGS. 2
a
through
2
e
illustrate a prior art metal mask printing process for forming metal bumps. According to the metal mask method, as shown in
FIG. 2
a,
use is made of a board
20
provided with electrodes
21
, and a metal mask
22
formed with openings
22
a
corresponding to the electrodes
21
. As shown in
FIG. 2
b,
the metal mask
22
is placed on the board
20
for bringing the openings
22
a
in conformity with the electrodes
21
. Then, as shown in
FIG. 2
c,
a solder paste
23
25
containing solder powder is loaded in each of the openings
22
by printing. Then, as shown in
FIG. 2
d,
the metal mask
22
is removed from the board
20
. Subsequently, as shown in
FIG. 2
e,
the solder powder contained in the solder paste
23
is melted by heating, thereby providing generally spherical metal bumps
24
on the electrodes
21
of the board
20
. The metal bump formation by such a metal mask printing method is disclosed in JP-A-7-302972 for example.
FIGS. 3
a
through
3
e
illustrate a prior art resin mask loading process for forming metal bumps. First, as shown in
FIG. 3
a,
a resin film
32
is formed on a board
30
provided with electrodes
31
. Then, as shown in
FIG. 3
b,
the resin film
32
is partially etched away for forming openings
32
a
for exposing the electrodes
31
of the board
30
. Then, as shown in
FIG. 3
c,
a solder paste
33
containing solder powder is loaded in each of the openings
32
a.
Subsequently, as shown in
FIG. 3
d,
solder powder contained in the solder paste
23
is melted by heating, thereby providing generally spherical metal bumps
34
on the electrodes
31
of the board
30
. Finally, as shown in
FIG. 3
e,
the resin mask
32
is removed from the board
30
.
The metal mask printing method described above has a drawback that it has difficulties in controlling the height of the metal bumps formed at a relatively small pitch. Specifically, when the openings
22
a
of the metal mask
22
are arranged at a small pitch, removal of the metal mask
22
may cause part of the solder paste
23
filled in the openings
22
a
to be removed together. As a result, the metal bumps
24
may vary significantly in height, which may hinder reliable mounting of electronic components.
In the resin mask loading method, on the other hand, the resin film
32
as a printing mask is removed after the solder paste
33
are melted by heating. Therefore, it is possible to reliably form each of the metal bumps
35
with a predetermined amount of solder paste even when electrodes are arranged at a small pitch. Thus, in comparison with the metal mask printing method, the resin mask loading method is preferable for forming metal bumps at a small pitch which is necessary for realizing high density mounting of electronic components.
However, the prior art resin mask loading method has the following problems. In melting the solder powder contained in the solder paste
33
in the step shown in
FIG. 3
d,
the solder powder is generally heated at a temperature which is 30~50° C. higher than the melting point of the solder metal. In this heating step, however, the resin film (typically made of a thermosetting resin) hardens to some extent under heating. Therefore, in removing the resin film in the subsequent step shown in
FIG. 3
e,
part of the resin film thus hardened may remain on the surface of the board, which hinders reliable mounting of electronic components.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method of forming metal bumps using a resin film, which is capable of easily removing the resin film for forming good bumps which allow reliable mounting of electronic components.
According to a first aspect of the present invention, a method of forming metal bumps is provided which comprises the steps of: forming a resin film on a surface of a substrate provided with electrodes; forming openings in the resin film for exposing the electrodes; loading a bump material into the openings, the bump material containing a metal material which melts only partially at a first temperature, the metal material melting entirely at a second temperature higher than the first temperature; heating the bump material to the first temperature for melting only part of the metal material; cooling the bump material below the first temperature; removing the resin film; and heating the bump material to the second temperature for entirely melting the metal material.
With the above-described method, it is possible to remove the resin film from the surface of the substrate more reliably than in the prior art method. Specifically, when the bump material loaded in the openings is heated to the first temperature (hereinafter referred to as “primary heating”), only part of the metal material changes from solid phase to liquid phase. At this time, due to surface tension, the liquid phase part tends to cohere for integration with the remaining solid phase part. When the bump material is thereafter cooled below the first temperature, the liquid phase part returns to solid phase for fixing to the electrodes (hereinafter referred to as “provisionally fixing”). The cooling herein includes natural cooling wherein no positive cooling using a coolant is performed. According to the present invention, the resin film formed on the surface of the board as a mask is removed after the provisional fixing.
In the prior art method, since the bump material is heated immediately to such a high temperature as to completely melt the metal material contained therein, the resin film hardens undesirably due to such heating, consequently leading to difficulty in removing the resin film. According to the present invention, by contrast, the bump material is provisionally fixed to the electrodes by the primary heating followed by subsequent cooling, and the resin film is removed before the solder material is heated at the second temperature (hereinafter referred as “secondary heating”) which is higher than the
Armstrong Westerman & Hattori, LLP
Fujitsu Limited
Nguyen Ha T.
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