Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1998-07-24
2001-08-14
Gaffin, Jeffrey (Department: 2841)
Metal working
Method of mechanical manufacture
Electrical device making
C174S260000, C174S267000, C029S843000
Reexamination Certificate
active
06272741
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to interconnect devices used in electronic assemblies, and more particularly, to pin grid arrays used to connect a large number of contacts from one circuit to another circuit.
Microelectronic circuits such as microprocessors are frequently packaged so that their large number of electrical contacts can be accessed via a pin grid array (PGA). The pin grid array comprises rows and columns of tiny conductive pins that extend generally perpendicular from the planar underside of a thin rectangular housing. It is not uncommon to have well over two hundred pins in a pin grid array. The housing of the microprocessor is made of ceramic or plastic and encases an integrated circuit chip. Microscopic wire leads electrically connect the upper ends of the pins to individual contacts on the chip. The pins may be plugged into corresponding receptacles in a socket connector mounted on a circuit board. The pin grid array has the advantage of providing a reliable mechanical and electrical interconnection between the microprocessor and the circuit board, while at the same time allowing the microprocessor to be removed for repair or replacement. For example, a mother circuit board for a personal computer may have a microprocessor with a pin grid array that can be unplugged so that an enhanced microprocessor, e.g. one having a math coprocessor, can be installed in its place.
An alternative to the pin grid array is the ball grid array. It comprises rows and columns of tiny solder balls attached to the underside of a microprocessor. These solder balls register with corresponding conductive pads or traces on the circuit board. Mechanical and electrical connection is achieved by subjecting the board and microprocessor to infrared or convective heating to achieve solder reflow. While this is a reliable method for surface mounting microelectronic components on a circuit board, the components cannot be easily removed for repair or replacement. However, it is a less expensive approach than the pin grid array since the cost of pins and socket connectors is eliminated.
Pin grid arrays have also been used on separate carrier boards to interconnect a large number of electrical contacts on a first circuit board to a second circuit board.
FIG. 1
is an exploded diagrammatic view illustrating a prior art technique widely used in the personal computer industry to connect a first plurality of electrical contacts on the underside of a multi-chip module (MCM) board
10
to a second plurality of corresponding electrical contacts on computer mother board
12
. A carrier board
14
is provided that has a plurality of pins
16
that extend through the carrier board. The pins
16
are arranged in a grid array and their upper ends are soldered to corresponding pads or traces on the underside of the MCM board
10
. The lower ends of the pins
16
can then be plugged into a socket connector
17
attached to the upper side of the mother board
12
. The socket connector
17
provides a means that can be mounted on, or directly to, the mother board
12
for individually receiving and providing electrical connection with the pins
16
. The prior art pin grid array interconnect arrangement of
FIG. 1
allows computer manufacturers to make a common mother board that can be utilized to make a variety of different computer configurations. It also allows computer users to easily upgrade their systems, e.g. to run more efficiently and at higher speeds, by simply plugging a suitable MCM
10
into the mother board
12
.
The prior art circuit board interconnection technique illustrated in
FIG. 1
, while serviceable, has reliability problems.
FIG. 2
illustrates a greatly enlarged vertical cross-sectional view of a so-called “butt joint” solder connection
18
between the upper end of one of the pins
16
and a conductive pad
20
on the underside of the MCM board
10
. A pattern of solder is typically screened onto the conductive pads
20
on the MCM board.
When this solder undergoes reflow, a fillet
22
of solder is formed around the upper end of the pin
16
. However, the fillet
22
is only slightly larger than the radial width of the pin
16
. In addition, the solder joint
24
between the upper end of the pin
16
and the conductive pad
20
is relatively narrow in vertical height. The result is a delicate mechanical interconnection between the pin
16
and the conductive pad
20
. It is often necessary to straighten one or more pins
16
in the grid array on the carrier board
12
. This can lead to fractures through the corresponding fillet
22
and joint
24
, resulting in intermittent electrical contacts or open circuits that impair proper operation of the associated circuits. The MCM board
10
and mother board
12
are typically made of FR-4 material, a laminate of fiberglass, epoxy and etched copper circuit traces. The carrier board
14
is typically made of a high temperature thermoplastic. The differences in the coefficients of thermal expansion of the different materials can stress the solder connections between the upper ends of the pins
16
and the conductive pads
20
on the underside of the MCM board
10
, leading to fractures through the fillet
22
and joint
24
, resulting in intermittent electrical contacts or open circuits. Lateral loads on the MCM board
10
, such as pushing by the user, can also break one or more of the solder connections between the pins
16
and the MCM board
10
.
Reliability problems with the prior art circuit board interconnection technique illustrated in
FIG. 1
are compounded if there is any substantial departure from true coplanarity between the MCM board
10
and the grid array of pins
16
.
FIG. 3
illustrates a greatly enlarged vertical cross-sectional view of an alternate defective solder connection
26
between the upper end of one of the pins
16
′ and the conductive pad
20
′ on the underside of the MCM board
10
. The solder connection
26
occurs when lack of true co-planarity between the MCM board
10
and the carrier board
12
places the upper end of the pin
16
′ too far away from its corresponding conductive pad
20
′. An excessively elongated fillet
22
′ of solder is formed around the upper end of the pin
16
′.
In addition, the solder joint
24
′ between the upper end of the pin
16
′ and the conductive pad
20
has been excessively elongated. The result is an even more delicate mechanical interconnection between the pin
16
′ and the conductive pad
20
.
It would therefore be desirable to provide an improved pin grid array interconnect system and method that would overcome the above-noted deficiencies of the prior art technique illustrated in
FIGS. 1-3
.
SUMMARY OF THE INVENTION
It is therefore the primary object of the present invention to provide an improved inexpensive electronic interconnect system that can reliably connect a plurality of electrically and/or thermally conductive metal contacts such as pins inserted in a carrier substrate to a corresponding plurality of electrically and/or thermally conductive metal contacts such as conductive pads or traces formed on a circuit substrate.
It is a further object of the present invention to provide an improved system for connecting a plurality of pins inserted in a carrier board to form a first pattern to corresponding conductive pads or traces formed on a circuit board and arranged in a second complementary pattern.
It is another object of the present invention to provide an improved system for interconnecting a multi-chip-module board to a computer mother board via a carrier board.
It is another object of the present invention to provide a method for more reliably connecting an array of pins on a carrier board to corresponding array of conductive pads or traces on a circuit board utilizing solder balls, thereby avoiding weak solder fillet connections that often fracture due to lack of co-planarity of the boards and/or differential thermal expansion there between.
In accordance with one embodiment of
Hart Julian Curtis
Kennedy Craig M.
Ramirez Fernando J.
Autosplice, Inc.
Gaffin Jeffrey
Jester Michael H.
Norris Jeremy
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