Solder column tip compliancy modification for use in a BGA socket connector

A socket connector system for forming a separable electrical contact between a first circuit substrate and a second circuit substrate. A dendrite interposer is disposed between the first circuit substrate and the second circuit substrate. A solder body is disposed between the first circuit substrate and the dendrite interposer. The solder body may include one of several types of solder columns or a solder ball. The solder body has a contact end which engages the dendrite interposer. The contact end has a void. An area of the contact end engages the dendrite interposer when compressive forces are exerted on the first circuit substrate and the second circuit substrate. This provides for all areas of a plurality of contact ends to engage and form reliable electrical contacts with the contact pads.

FIELD OF THE INVENTION 
The present invention relates generally to socketing single chip or multi 
chip modules by solder columns or balls and, more particularly, to a 
system for forming a separable and reliable electrical connection between 
circuit substrates. 
BACKGROUND 
In some electrical circuit packaging designs, it is desirable to provide a 
separable electrical connection between a single or multi-chip module 
substrate and the circuit board on which it is mounted. Such an electrical 
connection is generally considered "separable" if the chip module 
substrate can be easily unplugged from the circuit board and subsequently 
attached to another circuit board. The need for such separable connections 
is due to economies, in that it is cost-effective to use the same chip 
module substrates on a variety of circuit boards. 
A conventional socket connector system is illustrated in FIG. 1. A first 
circuit substrate 10 and a second circuit substrate 12 are provided. A 
dendrite interposer 14 is disposed between first circuit substrate 10 and 
second circuit substrate 12. Dendrite interposer 14 is typically on the 
order of several millimeters thick. The Flexiposer.TM. interposer by 
International Business Machines Corporation is one such type of dendrite 
interposer 14. Dendrite interposer 14 includes: (a) a carrier 15 made of 
polyamide film or other similar type of circuit board material, and (b) 
pairs of electrical contact pads 16a and 16b attached to carrier 15. For a 
given pair, contact pads 16a and 16b are attached to carrier 15 on 
opposing sides and electrically coupled to one another through vias 18 
formed in carrier 15. On a given contact pad 16a or 16b, a plurality of 
ridges 26 extend out from contact pads 16a and 16b toward one of the 
circuit substrates 10 or 12. Ridges 26 are typically needle-like 
structures of palladium which are grown off circuit contact pad 16a or 
16b. Ridges 26 generally function to penetrate dust and debris in forming 
an electrical contact between mating surfaces of circuit substrates 10 and 
12. Ridges 26 are typically on the order of 0.025 to 0.051 mm (1 to 2 
mils) in height. 
A solder body 20 is disposed between first circuit substrate 10 and 
electrical contact pad 16a attached to dendrite interposer 14. Mounting 
end 30 of solder body 20 is mounted to first circuit substrate 10. Contact 
end 32 of solder body 20 faces contact pad 16a. Compressive forces are 
exerted, in the direction of arrows A of FIG. 1, on first circuit 
substrate 10 and second circuit substrate 12 to force all areas of contact 
ends 32 to engage and form reliable electrical contacts with all of the 
contact pads 16a (see area 22 of FIG. 1). 
According to the conventional socket connector system illustrated in FIG. 
1, however, rarely do all of the solder bodies 20 form reliable electrical 
contacts with the contact pads 16a. Often, solder body 20 makes only a 
poor or non-existent electrical contact with contact pad 16a (see area 24 
of FIG. 1). This results from at least two inherent tolerance variations 
in the conventional socket connector system of FIG. 1. First, there are 
often variations in height between the two circuit substrates 10 and 12 to 
be mated due to inconsistencies in the flatness of one or both of the 
circuit substrates 10 or 12. These variations may also arise due to 
inconsistencies in the thickness of any hardware located between the 
circuit substrates 10 and 12. On a typical system, where two circuit 
substrates 10 and 12 are to be mated, a variation in height between 
approximately 0.05 to 250 mm (2 to 10,000 mils) may exist between 
electrical contact pads 16a and solder bodies 20. Second, the rigid nature 
of the solder material in solder body 20 renders it unlikely that all of 
the solder bodies 20 throughout the system will deform to the extent 
necessary under typical compressive forces to provide uniformly the 
desired reliable electrical connections. 
Accordingly, a need remains for an improved socket connector system which 
forms a separable and reliable electrical connection between circuit 
substrates. 
SUMMARY OF THE INVENTION 
To meet this and other needs, and in view of its purposes, the present 
invention provides a socket connector system forming a separable 
electrical contact between a first circuit substrate and a second circuit 
substrate. A dendrite interposer is disposed between the first circuit 
substrate and the second circuit substrate. A solder body is disposed 
between the first circuit substrate and the dendrite interposer. The 
solder body has a contact end which engages the dendrite interposer. The 
contact end has a void. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary, but are not restrictive, of 
the invention.

DETAILED DESCRIPTION 
Referring now to the drawing, wherein like reference numerals refer to like 
elements throughout, FIG. 2 illustrates a socket connector system 100 
according to the present invention. The system 100 forms a separable 
electrical contact between a first circuit substrate 10 and a second 
circuit substrate 12 when the two substrates are compressed. That is, 
first circuit substrate 10 can be easily unplugged from second circuit 
substrate 12 after the two are mated. 
As shown in FIG. 2, a dendrite interposer 14 is disposed between first 
circuit substrate 10 and second circuit substrate 12. Dendrite interposer 
14 is typically on the order of several millimeters thick. The 
Flexiposer.TM. dendrite by International Business Machines Corporation is 
suitable. Dendrite interposer 14 includes: (a) a carrier 15 made of 
polyamide film or other similar type of circuit board material, and (b) 
pairs of electrical contact pads 16a and 16b attached to the carrier 15. 
Contact pads 16a and 16b are generally flat and are preferably made with 
gold or nickel alloy. A pair of contact pads includes: (a) a circuit 
contact pad 16b attached to dendrite interposer 14 and facing second 
circuit substrate 12; and (b) a solder contact pad 16a attached to 
dendrite interposer 14 and facing solder body 20. For a given pair, 
contact pads 16a and 16b are attached to the carrier 15 on opposing sides 
and electrically coupled to one another through vias 18 formed in the 
carrier 15. Solder pad 16a has a plurality of ridges 26, illustrated in 
FIG. 2, extending from dendrite interposer 14 toward solder body 20. 
Circuit contact pad 16b is similarly provided with a plurality of ridges 
26, illustrated in FIG. 2, extending from dendrite interposer 14 toward 
second circuit substrate 12. Ridges 26 are typically needle-like 
structures of palladium which are grown off circuit contact pads 16a and 
16b. Ridges 26 generally function to penetrate dust and debris in forming 
an electrical contact between mating surfaces of circuit substrates 10 and 
12. Ridges 26 are typically on the order of 0.025 to 0.051 mm (1 to 2 
mils) in height. 
A solder body 20 is disposed between first circuit substrate 10 and 
dendrite interposer 14. Solder body 20 has a mounting end 30 mounted on 
first circuit substrate 10. Solder body 20 has a contact end 32 facing 
dendrite interposer 14. Contact end 32 engages dendrite interposer 14 when 
compressive forces, in the direction of arrows A in FIG. 2, are exerted on 
first circuit substrate 10 and second circuit substrate 12. Unlike 
conventional systems, socket connector system 100 provides for all contact 
ends 32 to engage and form reliable electrical contacts, in areas 22, with 
all of the contact pads 16a. 
In one embodiment according to the present invention, solder body 20 
includes a solder column 20a, as illustrated in FIG. 3. In another 
embodiment according to the present invention, solder body 20 includes a 
solder column 20b, as illustrated in FIG. 5. In yet another embodiment 
according to the present invention, solder body 20 includes a solder ball 
20c, as illustrated in FIG. 7. 
Contact end 32 has a void 28 in area 22 engaging dendrite interposer 14. In 
one embodiment of the present invention, void 28 in contact end 32 is 
provided such that the area of contact end 32 engaging dendrite interposer 
14 is less than a predetermined area of mounting end 30 which engages 
first circuit substrate 10. As illustrated in FIGS. 2 and 3, in another 
embodiment of the present invention in which solder body 20 includes 
solder column 20a, void 28' is formed by a solder column extension 34 
protruding from contact end 32a of solder column 20a. 
As illustrated in FIG. 4A, one example of solder column extension 34 is in 
the form of a substantially rectangular tab 34a. Illustrated in FIG. 4B, 
solder column extension may also be in the form of a cylinder 34b. In both 
FIGS. 4A and 4B, a void 28 is still formed by solder column extensions 34a 
and 34b. Void 28 in these embodiments of the present invention is formed 
by the removal of material in the peripheral region of contact end 32a 
surrounding centrally disposed column extension 34. 
As illustrated in FIGS. 5, 6A, and 6B, in another embodiment of the present 
invention in which solder body 20 includes solder column 20b, void 28 is 
defined by a bore in contact end 32b of solder column 20b. In one example 
of this embodiment, in which solder body 20 includes solder column 20b, 
void 28 is defined by a trench formed in contact end 32b of solder column 
20b (see FIG. 6B). In the example illustrated in FIG. 6A, void 28 is 
defined by a centrally disposed hole formed in contact end 32b of solder 
column 20b. Void 28 in these embodiments of the present invention is 
formed by the removal of material in the central region of contact end 32b 
and is surrounded by column extension 34. 
FIGS. 7 and 8 show yet another embodiment of the present invention. As 
illustrated in these Figures, solder body 20 includes solder ball 20c and 
void 28 is defined by a bore in contact end 32c of solder ball 20c. 
According to socket connector system 100 of this invention, as illustrated 
in FIG. 2, ridges 26 of solder contact pads 16a engage contact ends 32 of 
solder bodies 20 when first circuit substrate 10 and second circuit 
substrate 12 are compressed, such that ridges 26 of solder contact pads 
16a and contact ends 32 of solder bodies 20 form separable and reliable 
electrical contacts throughout the system 100. In addition, circuit 
contact pads 16b engage second circuit substrate 12 when first circuit 
substrate 10 and second circuit substrate 12 are compressed, such that 
ridges 26 of circuit contact pads 16b and second circuit substrate 12 form 
separable and reliable electrical contacts throughout the system 100. 
As mentioned above, rarely do all of the solder bodies 20 form reliable 
electrical contacts with the contact pads 16a in the conventional socket 
connector system illustrated in FIG. 1. Often, solder body 20 makes only a 
poor or non-existent electrical contact with contact pad 16a (see area 24 
of FIG. 1). This problem results from inherent tolerance variations in the 
conventional socket connector system of FIG. 1. The present invention 
overcomes this problem. Void 28 renders contact end 32 of solder body 20 
more compliant and responsive to the compressive forces exerted on first 
circuit substrate 10 and second circuit substrate 12. Accordingly, when 
the first of contact ends 32 to engage contact pads 16a makes contact, 
that contact end 32 compresses (see the central contact end 32 of FIG. 2). 
Such compression allows the remaining contact ends 32 to engage their 
respective contact pads 16a (see the noncentral contact ends 32 of FIG. 
2). 
According to another embodiment of the present invention, the socket 
connector system of the present invention is used in a test-and-burn-in 
socket where, for example, it is desirable to test single chip and 
multi-chip modules before permanently soldering them to circuit boards 
used in production. The socket connector system according to the present 
invention can be used to form a separable electrical connection according 
to the following steps: (1) solder the chip modules to test equipment, (2) 
burn-in the chip modules, (3) test the chip modules, (4) unplug the chip 
modules, and (5) permanently solder the chip modules onto circuit boards 
for production. This provides the advantage of allowing one to do a test 
and burn-in on even high density chips. 
Socket connector system 100 according to the present invention provides the 
desired reliable electrical contacts over all of the contact pads in the 
system. The present invention overcomes the tolerance variations that 
exist throughout the conventional socket connector system. Moreover, the 
solder bodies according to the present socket connector system can deform 
under lower compressive forces to provide reliable electrical connections. 
Although illustrated and described herein with reference to certain 
specific embodiments, the present invention is nevertheless not intended 
to be limited to the details shown. Rather, various modifications may be 
made in the details within the scope and range of equivalents of the 
claims and without departing from the spirit of the invention.