Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1998-07-15
2002-05-07
Vo, Peter (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S840000
Reexamination Certificate
active
06381838
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a Ball Grid Array (BGA) package and to a method of manufacturing the same. More particularly, the present invention relates to a BGA package which prevents moisture from penetrating to the semiconductor chip of the package and which effectively vents the heat generated by the chip.
2. Description of the Related Art
Semiconductor device packages are becoming faster, smaller, and thinner to meet the pressing demands for miniature and multi-functional electronic devices. A Ball Grid Array Package (hereinafter, referred to as a “BGA package”) has been developed to meet these demands. The BGA package is a type of surface mount package which includes a printed circuit board (PCB), and solder balls (also referred to as solder bumps) instead of a lead frame for electrically connecting a semiconductor chip to the PCB. The BGA package has a high mounting density and thus employs a large number of I/O pins.
As described above, in the BGA package, a semiconductor chip is attached and electrically connected to the PCB. Circuit wiring patterns on a surface of the PCB are electrically connected to the semiconductor chip, which is mounted on the same surface of the PCB. The circuit wiring patterns are also electrically connected through signal via holes to external connections which are formed on the other surface of the PCB. Because the external connections are formed on the surface of the PCB which is opposed to that to which the semiconductor chip is mounted, the BGA package requires a mounting area that is smaller than that required by other conventional plastic packages. In conventional BGA packages, solder bumps serve as the external connections.
FIG. 1
is a cross-sectional view of a conventional BGA package and
FIG. 2
is an enlarged view of thermal emissive via holes of a PCB of the package.
With reference to these figures, the conventional BGA package
100
has a semiconductor chip
20
electrically connected through a Printed Circuit Board (PCB)
10
to solder bumps
30
. The solder bumps
30
serve as external connections for the BGA package
100
.
The PCB
10
comprises a board body
19
, and copper (Cu) pattern layers on an upper and a lower surface of the board body
19
. The Cu pattern layers electrically connect the semiconductor chip
20
to the solder bumps
30
. A plurality of signal via holes
14
penetrate the board body
19
, in order to electrically connect the Cu pattern layer on the upper surface to the Cu pattern layer on the lower surface of the board body
19
. Inner walls of the signal via holes
14
are plated with Cu.
The Cu pattern layer on the upper surface of the board body
19
comprises a chip attach area
60
and a plurality of circuit patterns
15
. The chip attach area
60
refers to a region at which the semiconductor chip
20
is mounted to the PCB. The respective circuit patterns
15
are positioned around the chip attach area
60
and respective ends of the circuit patterns
15
are referred to as “board bonding pads ”. The board bonding pads
17
are electrically connected to the semiconductor chip
20
by bonding wires
40
.
The Cu pattern layer on the lower surface of the board body
19
comprises a plurality of solder ball pads
13
, to which solder bumps
30
are attached, and circuit patterns
15
, which are electrically connected to the solder ball pads
13
. Via holes, which are formed underneath the chip attach area
60
, are referred to as “thermal emissive via holes
62
”. The thermal emissive via holes
62
vent to the outside the heat generated during the operation of the semiconductor chip
20
.
Both surfaces of the PCB
10
are coated with solder resist
16
, except at locations corresponding to the board bonding pads
17
on the upper surface and the solder ball pads
13
on the lower surface. During the coating process, the thermal emissive via holes
62
are filled with the solder resist
16
, as shown in FIG.
2
.
After that, the upper surface of the PCB
10
is encapsulated with molding compound, such as a thermosetting resin, to protect the semiconductor chip
20
and the circuit patterns
15
from external environmental stress, whereby a package body
50
is formed. After attaching solder balls to the solder ball pads
13
of the lower surface of the PCB
10
, the solder bumps
30
are formed by a reflow soldering process.
The conventional BGA package described above has the following problems. First, the PCB is extremely hygroscopic. Therefore, the BGA package absorbs moisture much more readily than do conventional packages using a metal lead frame.
Moisture is absorbed into the BGA package
100
through two routes: an absorption through the board body
19
and an absorption through the thermal emissive via holes
62
. The amount of absorption through the board body
19
depends on the physical properties of the material of the board body
19
. Therefore, problems caused by the absorption of the moisture through the board body
19
may be overcome by selecting a suitable material for the board body.
The thermal emissive via holes
62
provide the second route because the solder resist
16
with which they are filled is also extremely hygroscopic. Therefore, moisture can flow to the semiconductor chip
20
through the thermal emissive via holes
62
. Such moisture affects the reliability of the package more seriously than the moisture absorbed through the board body
19
.
The second problem is that although the thermal emissive via holes
62
are provided for transferring heat generated by the chip
20
to the outside of the package
100
, the rate of heat transfer through the thermal emissive via holes
62
is considerably low.
More specifically, as shown in
FIG. 2
, the chip attach area
60
is relatively large and comprises a Cu layer
12
. The thermal emissive via holes
62
are formed underneath this relatively large chip attach area
60
, but the solder resist
16
filling the thermal emissive holes
62
has a low coefficient of thermal conductivity. Therefore, most of the heat is drawn by the Cu layer
12
at the chip attach area
60
. The heat transfer rate is thus dictated in large part by the area of the Cu layer
12
at the chip attach area
60
.
A third problem is that because the PCB is coated with solder resist
16
, the viscosity of the solder resist
16
prevents the thermal emissive via holes
62
from being completely filled. In other words, the solder resist
16
may be left with voids
64
. The voids
64
may cause the PCB to crack during reliability tests, such as a burn-in test which is carried out under high temperature and pressure after the BGA package
100
is manufactured.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a BGA package which prevents moisture from penetrating to the chip through the thermal emissive via holes, and which possesses a comparatively high thermal emission property.
Another object of the present invention is to provide a BGA package having thermal emissive vias which are free of voids, and therefore is resistant to cracking.
The foregoing objects of the present invention are achieved by a BGA package having thermal emissive via holes filled with metal having a low melting point, excellent thermal conductivity and which is not highly hygroscopic, and by a method of manufacturing the same.
In a process of manufacturing a PCB of the BGA package of the present invention, the thermal emissive via holes are filled with metal having a low melting point while bottom ends of the thermal emissive via holes are closed by a coating of solder resist formed on the lower surface of a board body of the PCB. Metal paste, which comprises the metal having a low melting point, is forced through open top ends of the thermal emissive via holes using a screen-printing process, until the thermal emissive via holes are filled with the metal paste. A reflow soldering process causes the paste to completely fill the via holes.
Alternatively, instead of the metal past
Ahn Eun Chul
Ha Woong Ky
Lee Young Min
Samsung Electronics Co,. Ltd.
Smith Sean P.
Vo Peter
Volentine & Francos, PLLC
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