Semiconductor device manufacturing: process – Packaging or treatment of packaged semiconductor – Assembly of plural semiconductive substrates each possessing...
Reexamination Certificate
2001-01-13
2002-09-17
Talbott, David L. (Department: 2827)
Semiconductor device manufacturing: process
Packaging or treatment of packaged semiconductor
Assembly of plural semiconductive substrates each possessing...
C438S121000, C438S124000, C438S125000, C438S126000, C438S127000
Reexamination Certificate
active
06451625
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to integrated circuit packaging technology, and more particularly, to a method of fabricating a FCBGA (Flip-Chip Ball-Grid-Array) package with molded underfill.
2. Description of Related Art
BGA (Ball Grid Array) is an advanced type of integrated circuit packaging technology which is characterized by the use of a substrate whose front side is mounted with a semiconductor chip and whose back side is mounted with a grid array of solder balls. During SMT (Surface Mount Technology) process, the BGA package can be mechanically bonded and electrically coupled to a printed circuit board (PCB) by means of these solder balls.
FCBGA (Flip-Chip Ball-Grid-Array) is a more advanced type of BGA technology which is characterized by that the semiconductor chip is mounted in an upside-down (i.e., clip chip) manner over the substrate and bonded to the same by means of a plurality of solder bumps attached to the I/O pads thereon.
As the flip chip is readily bonded in position over the substrate, however, an undergap would be undesirably left between the flip chip and the substrate. If this flip-chip undergap is not underfilled, it would easily cause the flip chip to suffer from fatigue cracking and electrical failure due to thermal stress when the entire package structure is being subjected to high-temperature conditions. As a solution to this problem, it is an essential step in flip-chip package fabrication to fill up the flip-chip undergap with an electrically-insulative material into the flip-chip undergap. The underfilled material, when hardened, can serve as a mechanical reinforcement for the flip chip to cope against thermal stress.
Conventionally, there are many methods that can be used to perform the above-mentioned flip-chip underfill process. One method is the molded-underfill process, which can fill the flip-chip undergap incidentally through the molding process to fabricate the required encapsulation body (or called molded compound). One example of the molded-underfill process is depicted in the following with reference to
FIGS. 1A-1D
(note that
FIGS. 1A-1D
are simplified schematic diagrams showing only the parts related to the invention; the actual layout on the FCBGA package may be much more complex).
FIG. 1A
is a schematic sectional diagram showing a semi-finished FCBGA package assembly before molding; and
FIG. 1B
shows a bottom view of the semi-finished FCBGA package assembly of FIG.
1
A. As shown, the semi-finished FCBGA package assembly includes: (a) a substrate
110
having a front surface
110
a
and a back surface
110
b;
(b) a semiconductor chip
120
mounted in an upside-down (i.e., flip chip) manner by means of a plurality of solder bumps
121
over the front surface
110
a
of the substrate
110
; (c) an array of solder-ball pads
130
formed over the back surface
110
b
of the substrate
110
, which are used for subsequent attachment of an array of solder balls (not shown) thereon; and (d) a solder mask
140
which is predefined with a plurality of openings
141
to expose the solder-ball pads
130
.
As the semiconductor chip
120
is readily mounted in position over the substrate
110
, however, a flip-chip undergap
120
a
would be undesirably left between the semiconductor chip
120
and the substrate
110
. If this flip-chip undergap
120
a
is riot underfilled, it would easily cause the semiconductor chip
120
to suffer from fatigue cracking and electrical failure due to thermal stress when the entire package structure is being subjected to high-temperature conditions. One solution to this problem is to perform a molded-underfill process.
To facilitate the molded-underfill process, it is required to drill a vent hole
111
through the substrate
110
and the solder mask
140
at the central point of the area where the semiconductor chip
120
is mounted. Conventionally, the vent hole
111
is formed by drilling through the vent hole
111
and the solder mask
140
, so that the part of the vent hole
111
in the solder mask
140
is equal in inside diameter as the part in the substrate
110
.
Referring further to
FIG. 1C
together with
FIG. 1D
, during the molding process, the semi-finished FCBGA package assembly is disposed in a molding tool (not shown), and an encapsulation material, such as epoxy resin, is injected into the molding tool (not shown) to form an encapsulation body
150
to encapsulate the semiconductor chip
120
.
During the forgoing molding process, the encapsulation material will also infiltrate into the flip-chip undergap
120
a.
Owing to the provision of the vent hole
111
, the air in the flip-chip undergap
120
a
can escape to the outside atmosphere, thus allowing the encapsulation material to infiltrate unresistingly into the entire flip-chip undergap
120
a
and thereby form a molded underfill layer
122
between the semiconductor chip
120
and the substrate
110
.
One problem to the foregoing molded-underfill process, however, is that the encapsulation material would further infiltrate all the way through the vent hole
111
to the bottom side of the solder mask
140
(the marching path is indicated by the arrows in FIG.
1
C), and thus flash over the exposed surface of the solder mask
140
and possibly over the nearby solder-ball pads
130
. Since the encapsulation material is electrically-insulative, the mold flash over the solder-ball pads
130
would degrade the bonding between the solder-ball pads
130
and the subsequently attached solder balls (not shown) thereon.
Related patents, include, for example, the U.S. Pat. No. 6,038,136 entitled “CHIP PACKAGE WITH MOLDED UNDERFILL”. This patent discloses a FCBGA package that is underfilled through molded-underfill process. The utilization of this patent, however, still has the above-mentioned problem of mold flash.
SUMMARY OF THE INVENTION
It is therefore an objective of this invention to provide a method for fabricating a FCBGA package with molded underfill, which can help to prevent mold flash over exposed surface of the resulted package through the vent hole, so as to assure the quality of the outer appearance of the resulted package.
It is another objective of this invention to provide a method for fabricating a FCBGA package with molded underfill, which can help to prevent mold flash over exposed surface of the resulted package, so as to assure the quality of the bonding between the solder-ball pads and the solder balls attached thereon.
In accordance with the foregoing and other objectives, the invention proposes an improved method for fabricating a FCBGA package with molded underfill.
Broadly recited, the method of the invention comprises the following steps: (1) preparing a substrate having a front surface and a back surface; the substrate being formed with a vent hole at a predefined location; the vent hole having an exit in the back surface of the substrate, (2) forming a plurality of solder-ball pads over the back surface of the substrate; (3) forming a solder mask over the back surface of the substrate; wherein the solder mask is predefined with an array of pad openings to expose the solder-ball pads and a mold-buffering opening aligned to the vent hole; and wherein the mold-buffering opening is dimensioned to be greater in width than the inside diameter of the vent hole; (4) mounting a flip chip over the front surface of the substrate; wherein a flip-chip undergap is left between the flip chip and the substrate; and (5) performing a molding process to form an encapsulation body through the injection of a encapsulation material to encapsulate the flip chip; wherein the vent hole allows the air in the flip-chip undergap to escape to the outside atmosphere, thereby allowing the encapsulation material to infiltrate unresistingly into the flip-chip undergap and form a molded-underfill layer; and wherein as the encapsulation material flows to the exit of the vent hole, the encapsulation material is contained with the solder mask's mold-buffering opening.
By the invention, there would s
Ho Tzong-Da
Pu Han-Ping
Corless Peter F.
Edwards & Angell LLP
Jensen Steven M.
Siliconware Precision Industries Co. Ltd.
Talbott David L.
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