Tandem loop contact for an electrical connector

An electrical contact comprises a generally flat contact body having a central member and a pair of compliant sections formed as tandem loops on opposite sides of the central member. Each of the compliant sections has a contact section at an end remote from the central member, and the body is resiliently compressible along an axis of deflection extending through the central member and the contact sections.

FIELD OF THE INVENTION 
The invention relates to an electrical contact having a tandem loop 
configuration for use in a connector between electrical interfaces. 
BACKGROUND OF THE INVENTION 
Electronic packages having leads arranged in a ball grid array (BGA) or a 
land grid array (LGA) are known. These packages have a relatively low 
height which is desirable for saving space in electronic assemblies. The 
packages may be surface mounted directly on a circuit board in a soldering 
process wherein the leads become solder bonded to a corresponding array of 
circuit pads on the board. However, solder bonding has the drawback that 
the package is not easily removable for replacement or upgrade. It is 
often desirable to provide a connector for mounting the electronic package 
on the circuit board in a separable fashion. Keeping the connector height 
low is a major consideration. 
Connectors are known for removably mounting a BGA or LGA package on a 
circuit board. Such connectors are disclosed in U.S. Pat. Nos. 4,511,197; 
4,513,353; 4,647,124; and 4,699,593. These connectors comprise a 
substantially flat dielectric housing which resides between the electronic 
package and the circuit board. The housing has an array of cavities in 
which are disposed electrical contacts arranged in correspondence with the 
array of leads of the electronic package. Each of the contacts has a pair 
of oppositely extending noses which project beyond external surfaces of 
the connector housing. When the package is mounted on the connector, each 
of the contacts has one nose engaged with a respective lead of the package 
and the other nose engaged with a respective pad on the board. A 
compressive force is applied to the package and the board to assure firm 
engagement of each nose with its respective lead or pad. Typically, the 
compressive force may be applied by pressure plates which are fastened 
together to sandwich the package, connector and board therebetween. 
Considering the circuit board to lie in a horizontal plane, the contacts 
must be compliant in the vertical direction in order to assure engagement 
with all of the leads and pads despite any coplanarity mismatch between 
the package and the board. Further, the contacts, which exert a spring 
force when compressed, should have a low spring rate to keep normal forces 
on the leads and pads within a desired range. High compliancy and a low 
spring rate can be accomplished by contacts which have a long spring 
length and a small cross-sectional area, but increases in spring length 
are associated with increases in inductance, and reductions in 
cross-sectional area are accompanied by increases in electrical 
resistance, both of which are detrimental to electrical performance. 
Accordingly, it is an object of the present invention to provide a contact 
for an electrical connector which offers high compliancy and a low spring 
rate along with low electrical resistance and a reduction in 
self-inductance effects. 
SUMMARY OF THE INVENTION 
An electrical contact for use in a connector between electrical interfaces 
comprises a generally flat contact body formed as a monolithic unit, the 
body having a central member and a pair of compliant sections on opposite 
sides of the central member. Each of the compliant sections has a contact 
section at an end remote from the central member, and the body is 
resiliently compressible along an axis of deflection extending through the 
central member and the contact sections. Each of the compliant sections 
extends along a looping course which originates at the central member and 
terminates at a respective distal end having a convex surface, and the 
central member has a pair of pockets each with a concave surface which is 
shaped complementary to the convex surface of a respective one of the 
distal ends, whereby the distal ends reside in their respective pockets 
and are held in the central member during deflection of the contact body.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
An electrical contact according to the invention is patterned on a 
kinematic linkage model 10 shown in Fig. 1. The linkage model is a simple 
arrangement of cross bars 11, 12 which are connected by a central pivot 
pin 20, and links 13, 14, 15, 16 which have ends attached to respective 
pivot pins 21 thru 26. Compression springs 18 and tension springs 19 
resiliently resist compression of the linkage model along its major axis A 
and maintain the linkage model in equilibrium during periods when it is 
undeflected by external forces. The springs 18 and 19 can be selected 
according to their spring characteristics in order to obtain a desired 
spring rate for the linkage model in the direction of the axis A. This 
linkage model possesses desirable characteristics for an electrical 
contact such as relatively high compliancy in a relatively short length, 
and a dual pathway between opposite ends. The linkage model is converted 
to a workable shape for a contact in order to retain the desirable 
characteristics. 
An electrical contact 30 patterned after the linkage model 10 is shown in 
FIGS. 2 and 3. The contact 30 is a generally flat body which is stamped 
from sheet material as a monolithic, i.e., one-piece unit. The contact 30 
has opposite side surfaces 31 and 32 which are formed by the side surfaces 
of the sheet material from which it is stamped. The contact body has a 
central member 33 and a pair of compliant sections 34, 35 on opposite 
sides of the central member. Each of the compliant sections 34, 35 has a 
respective contact section 38 at an end of the compliant section remote 
from the central member, and the contact sections 38 each have a 
pronounced tip 40 which is formed by the junction of two angled edges 41, 
42. 
The contact 30 is resiliently compressible along a deflection axis B which 
extends through the central member 33 and the pair of contact sections 38. 
Resiliency of the contact is enhanced by the compliant sections 34, 35 
each of which extends along a looping course which originates at the 
central member 33 and terminates at a respective distal end 38, 39 having 
a convex surface. Each of the compliant sections 34, 35 includes a pair of 
U-shaped sections 46, 47 and 48, 49, respectively. Each pair of U-shaped 
sections are open toward each other on opposite sides of the axis B. The 
central member 33 has a pair of pockets 44, 45 each with a concave surface 
which is shaped complementary to the convex surface of a respective one of 
the distal ends 38, 39. The distal ends 38, 39 reside in the pockets 44, 
45 and are held in the central member 33 during deflection of the contact 
body. In the preferred embodiment shown, the central member 33 extends 
diagonally across the deflection axis B, and the distal ends 38, 39 are 
disposed on respective opposite sides of the deflection axis B, 
The contact 30 is useful in a connector for electrically connecting a first 
electrical interface, such as leads of an electronic package, to a second 
electrical interface, such as circuit paths on a circuit board. There is 
shown in FIG. 4 a top view of a portion of such a connector which includes 
a connector body 60 made from plastic or other dielectric material having 
a plurality of contact cavities 62. The cavities 62 are arranged in an 
array corresponding to the array of leads of the electronic package to be 
mounted on the connector, Each of the cavities 62 is a rectangular-shaped 
slot which extends through the connector body 60 from top to bottom 
thereof. Each of the contacts 30 has a slight bow whereby the contacts 30 
are held in their respective cavities 62 by friction but are free to flex 
within their cavities. 
There is shown in FIG. 5 a cross-sectional view showing a representative 
contact in a connector which serves to electrically connect an electronic 
package 2 to a circuit board 8. The package 2 has electrical leads each in 
the form of a solder ball 4, the leads being disposed in a standardized 
array over a face of the package. Instead of solder balls 4, the package 2 
could have an array of contact pads similar to pad 6 which is a terminal 
for an electrical trace on circuit board 8, the present invention being 
engageable with either ball or pad type leads. The leads on the package 2 
may be urged into engagement with the array of contacts in the connector 
such as by pressure plates (not shown) arranged above the package 2 and 
below the circuit board 8 and secured together with threaded fasteners, 
thereby sandwiching the package 2, the connector and the circuit board 8 
therebetween. 
If the contact 30 is gold plated, it ordinarily picks up a small amount of 
tin from the solder ball 4 on the first engagement with the solder ball. 
Upon separation of the electronic package, this extremely thin layer of 
tin oxidizes, and may be fully converted to oxide, or some other film, 
depending on the presence of surrounding gas components. Therefore, the 
metallurgy must be such that the contact surface has either none or very 
poor solubility with tin. Materials such as nickel-boron and some 
stainless steels have poor affinity for tin and are therefore good 
candidates for the contact material. In addition, contact tips could be 
sputtered with materials such as platinum or titanium nitride or others, 
to prevent wetting by solder. 
A contact according to the invention has a number of advantages. Tandem 
compliant loop sections provide high compliancy and also provide two 
parallel electrical paths for low resistance and low self-inductance. The 
effective self-inductance of this contact is approximately one-half of the 
self-inductance of a straight wire having the same point-to-point length. 
Further, the contact tips are made from materials which resist formation 
of an oxide film. 
The invention having been disclosed, a number of variations will now become 
apparent to those skilled in the art. Whereas the invention is intended to 
encompass the foregoing preferred embodiments as well as a reasonable 
range of equivalents, reference should be made to the appended claims 
rather than the foregoing discussion of examples, in order to assess the 
scope of the invention in which exclusive rights are claimed.