Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1996-12-19
2001-03-13
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S842000, C174S250000, C174S259000, C174S260000, C174S262000
Reexamination Certificate
active
06199273
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a connector structure for mounting an electronic package such as a semiconductor or IC package on a printed-circuit board and a method and process for forming such a connector structure. More particularly, it pertains to a connector structure for an electronic package of the ball-grid array (BGA) type to be mounted on a printed-circuit board and a method and process for forming the connector structure.
Electronic packages which are primarily used at present include quad flat packages and pin-grid array packages.
As shown in
FIG. 10
, a quad flat package (QFP) has a package body
31
containing an electronic device such as an IC chip
32
and a lead frame having leads
33
which project away from all four sides of the package. The package is mounted on a printed-circuit board
34
, and the electrical connection between the IC chip
32
and the board
34
(package-to-board interconnection) is attained through the leads
33
which each have one end electrically connected to the chip
32
and another end electrically connected to a conductor pattern formed on the board
34
.
In a pin-grid array (PGA) package, as shown in
FIG. 11
a package body
31
containing an IC chip
32
has pins
35
which are electrically connected to the IC chip
32
and protrude at right angle from the lower surface of the package body
31
. IC chip
32
is electrically connected to a printed-circuit board
34
by inserting the pins
35
into through-holes formed in the board
34
followed by soldering.
However, the designs of these packages are not advantageous for coping with an increase in the number of I/O terminals resulting from an increase in the level of circuit integration of LSIs in recent years, as described below.
In a QFP, since the leads
33
serving as connecting terminals extend away from all four sides of the package body
31
, the dimension available for terminals is limited to the total length of the four sides, i.e., 4L (L=length of each side of the package). Therefore, the number of I/O terminals which the package can have is limited.
Compared to a QFP, a PGA package is better able to cope with an increase in the number of I/O terminals, since the pins
35
serving as connecting terminals extend away from the flat lower surface of the package body
31
rather than from the four sides thereof. However, the diameter of the pins
35
cannot be reduced too much since sufficient mechanical strength is required for these pins to be inserted into a printed-circuit board
34
. Furthermore, a certain area around each pin
35
should be kept in order to support the pin at the base of the package body. Therefore, there is a limitation on reduction of the pin pitch (center-to-center distance between adjacent pins), and the packaging density on a printed-circuit board attainable by a PGA package is still insufficient to cope with recent increases in the number of I/O terminals.
In view of these situations, BGA packages as shown in
FIGS. 12
a
and
12
b
have been attracting attention in recent years. In the BGA package shown in these figures, a package body
31
has electrode pads
36
A which are electrically connected to an IC chip
32
. The package is positioned on a printed-circuit board
34
having electrode pads
36
B such that the electrode pads of the package are in registration with those of the board, and each pair of electrode pads
36
A and
36
B facing each other is connected through a metallic ball
37
, in place of a pin
35
used in the PGA package shown in FIG.
11
. The metallic balls
37
are usually solder balls so as to make it possible to attain package-to-board interconnection by melting (reflowing) at a relatively low temperature.
Due to the use of metallic balls
37
, a BGA package can have a pad pitch (center-to-center distance between adjacent electrode pads) smaller than the pin pitch of a PGA package, which requires that a relatively large area be kept around each pin for support. Therefore, for a BGA package, it is possible to increase the number of connecting terminals over that attainable by a PGA package, leading to an increase in packaging density on a printed-circuit board. At present, the smallest I/O pitch (pin pitch) for a PGA packages is 1.27 mm, while it is possible for a BGA package to reduce the I/O pitch (pad pitch) to 0.6 mm.
In a BGA package, a metallic ball is usually pre-set on each electrode pad of an electronic package with the aid of an adhesion force given by a flux, which is typically a mixture of a solvent, rosin, and an activator. The metallic ball is then heated for reflowing (remelting) to join it to the electrode pad, thereby making it possible to create an electrical connection between the electrode pads of the package and the printed-circuit board through the ball, as described in European Patent No. 236,221. U.S. Pat. No. 5,060,844 discloses a connector structure using a solder ball having a high melting point via a solder having a low melting point.
A plastic substrate for an electronic package normally has hollow through-holes which penetrate the substrate from one surface to the other surface. Since it is difficult to form a connecting terminal directly atop such a hollow through-hole, many attempts have been made to form a through-hole connection using a metallic ball.
FIG. 13
a
shows a convention typical connector structure in cross section using metallic balls on a plastic substrate having hollow through-holes. A plastic substrate
41
has a through-hole
42
and an electrode
43
formed so as to cover the wall of the hollow through-hole
42
in order to electrically connect the two surfaces of the substrate
41
. The electrode
43
slightly extends onto the two surfaces of the substrate so as to form an electrode pad which surrounds the open end of the through-hole on each surface of the substrate. The electrode
43
is electrically connected to an IC chip
32
mounted on the upper surface of the substrate
41
. As can be seen from the plan (bottom) view of
FIG. 13
b,
the electrode
43
has an extension
43
A on the lower surface of the substrate
41
facing away the IC chip
32
. A metallic ball in the form of a solder ball
44
is joined to a second electrode pad
45
formed in contact with the extension
43
A of the electrode
43
on the lower surface of the substrate. This connector structure is called a dog bone because of its shape in the plan view shown in
FIG. 13
b.
However, the dog bone-type connector structure has the following problems.
(1) In order to prevent the solder ball
44
from flowing toward the substrate around the electrode
43
including its extension
43
A when melting, it is necessary to form a solder resist layer which repels molten solder over the electrode
43
. The formation of the solder resist layer requires a complicated process comprising the steps of applying a solder resist solution and forming holes in the resulting solder resist layer by use of photoetching technology, and it is time-consuming.
(2) Since extra space is required to locate the second electrode pad
45
apart from the through-hole
42
, such a BGA package provides only a limited increase in packaging density and limited size reduction.
A process has been proposed to form another structure for through-hole connection by filling a through-hole with a resin which may be electrically conductive, curing the filled resin by heating, plating the surface of the cured resin, and joining a metallic ball to the plated coating. This process is also complicated and not suitable for practical application.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a connector structure for a BGA package which has a high bonding (shear) strength and a good ability to endure thermal cycling, which can increase the packaging density, and which can be formed by a relatively simple process.
Another object of the present invention is to provide a method and process for forming such a connector structure.
In general, the present invention provides a connector s
Hidaka Akihiro
Ito Takuji
Kubo Toshihiko
Takamichi Hiroshi
Arbes Carl J.
Burns Doane , Swecker, Mathis LLP
Sumitomo Metal Industries Ltd.
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