Metal fusion bonding – Process – With pretreating other than heating or cooling of work part...
Reexamination Certificate
2001-05-17
2004-04-20
Edmondson, Lynne (Department: 1725)
Metal fusion bonding
Process
With pretreating other than heating or cooling of work part...
C228S019000, C228S206000, C228S208000, C228S180220, C134S002000, C134S003000, C148S022000, C148S023000
Reexamination Certificate
active
06722557
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a flux cleaning process suitable for processing semiconductor devices, and to a method of manufacturing the semiconductor devices.
2. Related Art
Along with a recent development in a high density packaging technology for packaging electronics components and parts, a further compactness in a semiconductor package is demanded. Now, as such an example of semiconductor packaging for providing a compacter design of electronic devices, a flip-chip connection process is drawing attention and becoming to be an indispensable process therefor, whereby a solder bump which is formed on an electrode pad on a semiconductor chip is electrically and physically connected with an electrode terminal provided on a packaging printed circuit board.
This method of forming the solder bump can be divided roughly into two approaches: one to form solder bumps in batch on a wafer; the other to form solder bumps respectively on an individual semiconductor chip after it was diced from the wafer. However, it is preferable to form solder bumps in batch on the wafer from the viewpoint of economy of a production cost.
As an example of forming solder bumps in batch in a stage of the wafer for reducing the production cost significantly, there is such a process of forming solder bumps whereby, by using a high precision printing method using a solder paste and a screen mask, solder is transferred onto a semiconductor chip electrode to form a solder bump.
Now, with reference to
FIGS. 8A through 10B
, a method of forming solder bumps by printing will be described by way of example of a solder paste
32
printed on a semiconductor wafer
1
.
FIGS. 8A
to
10
B show cross-sections of a screen mask
41
and a wafer
44
in each step of the solder printing method.
With reference to
FIG. 8A
, at first, a bottom surface
42
of the screen mask
41
having perforation
43
formed in a predetermined pattern is placed on a wafer
44
in face-to-face contact. Then, a solder paste
32
is supplied to an upper surface
37
of the screen mask
41
. In the next step of
FIG. 8B
, a squeegee
30
with its tip end contacting the upper surface
37
of the screen mask is moved in a direction of an arrow under application of a downward pressure, so as to roll and fill the solder paste
32
into the perforation
43
of the screen mask
41
.
Then, as illustrated in
FIG. 9A
, the screen mask
41
is lifted off from the wafer
44
. As a result, as shown in
FIG. 9B
, the solder paste
32
filled in the perforation holes
43
is transferred onto the wafer
44
, thereby depositing the solder paste
32
on an electrode pad
36
formed on the wafer
44
in a predetermined pattern.
Subsequently, as illustrated in
FIG. 10A
, the wafer
44
is wholly heated with a heating system such as a heater
45
or the like. A rosin group flux contained in the solder paste
32
, when heated by the heater
45
, is activated to effectuate removal of an oxide on the electrode pad
36
by reduction reaction so as to provide a clean metal surface. Then, a powder solder contained in the solder paste
32
is melted to form a round swell by a surface tension, which then wets and spreads only on the surface of the electrode pad
36
which is cleaned by the flux, then to form a bump electrode
35
. This heating treatment is generally referred to as a “wet back”.
As illustrated in
FIG. 10B
, compositions of the flux contained in the solder paste
32
which spill out while heating adhere, as a flux residue
34
, to a surface of the solder bump electrode
35
and a peripheral area in the vicinity of a bordering region between the solder bump electrode
35
and the wafer
44
. Although the flux residue
34
adhered to the surface of the solder bump electrode
35
can be removed by a normal flux cleaning process using, for example, a hydrocarbonic group cleaning agent, the other flux residue
35
adhered to the peripheral area in the vicinity of the bordering region between the solder bump electrode
35
and the wafer
44
cannot be removed.
Then, as illustrated in
FIG. 11
, a semiconductor chip
38
diced from the wafer
44
into an individual semiconductor chip is soldered onto an electrode portion
40
of a printed circuit board
39
via a bump electrode
35
. At the time of soldering, because the flux residue
34
remains on the surface of the solder bump electrode
35
, unless cleaning thereof is done sufficiently, the flux residue
34
is included between the surface of the printed circuit board electrode
40
and the surface of the bump electrode
35
, and is left as inclusion in a portion of the solder bump connection, thereby deteriorating an adhesion strength and reliability, and further causing an insufficient wettability portion
46
.
Generally, in order to increase adhesion strength in the solder bump connection portion, a resin
31
is filled in a gap between the semiconductor chip
38
and the printed circuit board
39
to seal therebetween. However, if a sticky flux residue
34
is left in the vicinity of the surface of the semiconductor chip
38
, a filling performance of the sealing resin
31
is affected to deteriorate the adhesion strength between the semiconductor chip
38
and the sealing resin
31
, thus degrading the reliability of the device. For this reason, it is especially important and indispensable completely to remove the flux residue
34
from the periphery of the solder bump electrode
35
prior to mounting the semiconductor chip on the printed circuit board
39
.
As a rosin group flux cleaning agent, solvents such as Freon (chlorofluorocarbon) or trichlene (trichloroethylene) cleaning agents have been used heretofore, however, as they may involve environmental problems such as destruction of the ozone layer, global pollution or the like, hydrocarbon group and terpene group cleaning agents are now being used instead of the Freon or trichlene cleaning agents. By the way, when using these cleaning agents they are heated. Although a cleaning performance improves at higher temperatures, in consideration of various factors such as an increasing amount of evaporation at higher temperatures (in terms of economy), flammability, safety and the like, they cannot be used at temperatures higher than 80° C. or so.
If these flux residues are tried to be removed by cleaning using these cleaning agents, however, these flux residues cannot be removed completely permitting some of the flux residues to remain adhering to the bordering and adjacent area between the solder bump electrode and the semiconductor chip. Normally, a thixotropic substance is contained in the solder paste for the purpose of securing its printed pattern to be retained, wherein an amido-fatty acid therein which is widely used because of its excellent performance cannot be removed easily with the normal cleaning agents for use in the normal flux cleaning. Although the flux residue may be removed by mechanically rubbing the wafer or by applying a ultrasonic cavitation (vibration) thereto, it is not permitted to give such a mechanical shock or damage to the semiconductor chip having bumps attached thereto.
SUMMARY OF THE INVENTION
The present invention has been contemplated to solve the above-mentioned problems associated with the related art, and to provide a novel method capable of efficiently cleaning flux residue which is produced in process of fabricating semiconductor devices, and a method of manufacturing semiconductor devices including this novel cleaning method.
Namely, the present invention is directed to a method of flux cleaning for cleaning a solder electrode portion which is formed using the flux, comprising: a step of coating a flux on the solder electrode portion, a step of a heat treatment thereof, and a step of the cleaning. Further, the present invention is directed to a method of manufacturing a semiconductor apparatus, comprising: a step of mounting a semiconductor chip on a printed circuit board using the solder electrode portion after the flux cleaning.
According to the flu
Depke Robert J.
Edmondson Lynne
Holland & Knight LLC
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