Abstract:
An electrical contactor for establishing an electrical contact between a chip and a printed circuit board for testing chips comprising a conductive layer and a ground contact pedestal. The ground contact pedestal is preferably press fit and soldered onto a ground layer of the printed circuit board for establishing good ground contact with the chip. In addition, the contactor preferably has a thin profile that does not electrically interfere with sensitive electronics contained within the chip. The contactor also does not use a plastic frame for mounting purposes. Such plastic frames reduce accessibility to the printed circuit board, making fine tuning electronics in the printed circuit board more difficult. Rather, the contactor preferably includes two holes in its conductive layer that mount on dowels in the printed circuit board and is secured to the dowels by two O-rings.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to an electrical contactor for conducting electrical signals between a chip being tested and a printed circuit board for testing chips. 
     BACKGROUND OF THE INVENTION 
     Currently available electrical contactors that interface chips and printed circuit boards that test the chips typically use conductive wires or spheres to conduct electrical signals between the chip and the printed circuit board and to ground the chip on the printed circuit board. Although these wires and spheres may be adequate for conducting electrical signals, they often do not do a good job of grounding the chip under test, especially grounding radio frequency (RF) signals. 
     Additionally, currently available electrical contactors typically use plastic guide frames that require screws for mounting. The plastic guide frames reduce accessibility to the printed circuit board, making tuning of the electronic components on the printed circuit board difficult. Furthermore, mounting and dismounting electrical contactors having plastic frames and screws is time consuming. 
     Currently available electrical contactors also typically have large profiles that can electrically interfere with sensitive electronic components contained in the chip and, therefore, can affect the results of chip testing. 
     There is a need, therefore, for an electrical contactor that can establish a good direct current ground as well as an RF signal ground, does not require a frame that can decrease accessibility to the printed circuit board, mounts and dismounts easily and quickly from the printed circuit board, and has a thin profile to avoid electrically interfering with electronics contained in the chip. 
     SUMMARY OF THE INVENTION 
     In accordance with a preferred embodiment of the present invention, the foregoing and other objects and advantages are attained by an electrical contactor for establishing electrical contact between a chip and a printed circuit board for testing chips comprising at least one interconnect that electrically connects at least one chip lead to the printed circuit board and a separate ground contact that grounds the chip through a ground located on the bottom of the chip. The ground contact preferably comprises a block of metal that contacts is shaped to fit in an indentation in a ground layer located beneath the printed circuit board. In addition, the ground contact may further comprise solder applied on top of the block of metal for establishing a good contact with the ground on the chip. Moreover, the ground contact preferably also functions as a solid stop to prevent a chip being plunged on the electrical contactor during testing from damaging the electrical contactor. 
     The electrical contactor preferably further comprises an interposer, wherein at least one interconnect is fabricated on the interposer. The at least one interconnect preferably comprises an abrasive material that scratches open the surface of a chip lead, exposing conductive material to establish good electrical contact. 
     The electrical contactor preferably further comprises a conductive elastomer bonded to and below the interposer. The conductive elastomer preferably comprises an array of wires embedded in an elastomer wherein the wires carry electrical signals between the at least one interconnect and the printed circuit board. The elastomer absorbs the compressional forces imparted when the chip is plunged on the electrical contactor, thus protecting the printed circuit board. Preferably, the wires are parallel to one another and oriented at a slanted angle. 
     The bonded interposer and conductive elastomer preferably contain at least one hole for mounting the contactor on at least one dowel on the printed circuit board. An O-ring preferably secures the bonded interposer and conductive elastomer layer to the at least one dowel on the printed circuit board. In addition, the bonded interposer and conductive elastomer preferably have a thin profile to avoid electrically interfering with electronic components contained within the chip being tested. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is made to the following Detailed Description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a side view of an electrical contactor in accordance with a preferred embodiment of the present invention that is mounted on dowels in a printed circuit board; 
     FIG. 2 is a top view of the electrical contactor of FIG. 1; 
     FIG. 3 is a top view of the electrical contactor of FIG. 1 mounted on a printed circuit board for testing chips; and 
     FIG. 4 is a side view of a ground contact pedestal mounted within the electrical contactor of FIG. 1, illustrating a chip being plunged on the electrical contactor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-4 illustrate a preferred embodiment of an electrical contactor according to the present invention. As shown in FIGS. 1 and 4, electrical contactor  10  preferably comprises a conductive elastomer  100 , an interposer  200 , and a ground contact pedestal  300 . 
     As shown in FIG. 1, conductive elastomer  100  preferably comprises elastomer  104  and an array of wires  102  embedded in elastomer  104 . Wires  102  are preferably parallel to one another and conduct electrical signal across the thickness of conductive elastomer  100 . Elastomer  104  makes conductive elastomer  100  compressible, which is important for protecting printed circuit board  400 , shown in FIG. 3, as described in further detail below. The preferred conductive elastomer is the Shin-etsu type MT conductive elastomer, which includes parallel gold plated brass wires embedded in silicon rubber. 
     Wires  102  are preferably oriented at a slant, as illustrated in FIG.  1 . The preferred angle of the slant is 65° from the horizontal. When conductive elastomer  100  is compressed, wires  102  are pressed on their sides towards one another while elastomer  104  absorbs most of the compressional forces. Orienting wires at a slant prolongs the life of wires  102  since wires  102  experience most of the compressional forces distributed along their sides. If wires  102  are oriented perpendicular to the horizontal, wires  102  would receive most of the compressional forces that are concentrated at particular points in the wires, and significantly decreasing the compressibility of conductive elastomer  100 . 
     Conductive elastomer  100  may include two strips of conductive elastomer placed together in the fashion shown in FIG. 1 to prevent wires  102  from inadvertently grounding chip leads  602  (FIG. 4) when chip  600  is mounted on electrical contactor  10 . In particular, when the wires are in this configuration, they connect ground on printed circuit board  400  to the center of chip  600  and direct ground away from interconnects  202 . 
     Turning now to FIGS. 1 and 2, the preferred thickness of conductive elastomer  100  is 30 mils. Three holes  502 ,  504  are cut into the conductive elastomer  100 . Two holes  502  are cut at opposite ends of the conductive elastomer  100  for mounting electrical contactor  10  onto dowels  402  on the printed circuit board, and one center hole  504  is cut to accommodate ground contact pedestal  300  (shown in FIG.  4 ). 
     Interposer  200  preferably includes a circuit printed on a polyimide film substrate  204 . The circuit preferably includes a plurality of interconnects  202  which are plated through the thickness of polyimide film substrate  204  for electrically connecting leads  602  of chip  600  (FIG. 4) to wires  102  of conductive elastomer  100  when chip  600  (FIG. 4) is mounted on electrical contactor  10 . Interconnects  202  are preferably particle interconnects that include small diamonds embedded in nickel that is plated with gold. The preferred particle interconnect  202  is one made by Exatron. The embedded diamonds scratch the surface of chip leads  602  as chip  600  (FIG. 4) is plunged on electrical contactor  10  to expose fresh conductive material in leads  602 , establishing good electrical conduction between leads  602  and interconnects  202  during chip testing. In addition, the preferred polyimide film substrate  204  material is Kapton, although other suitable interposer materials may be used such as FR4, Upilex S, R Flex Crystal 7200, and R Flex 1100 manufactured by Rogers Corporation. 
     Like conductive elastomer  100 , interposer  200  preferably has a thin profile, preferably 2-3 mils thick. Also, like conductive elastomer  100 , three holes  502 ,  504  are cut in interposer  200 . Two holes  502  are cut at opposite ends of interposer  200  for mounting electrical contactor  10  onto dowels in printed circuit board and one center hole  504  is cut to accommodate ground contact pedestal  300 . 
     Conductive elastomer  100  is preferably bonded to interposer  200  using an adhesive, such as Loctite  454 . The three holes cut in both layers  100 ,  200  are aligned and the adhesive is applied such that it does not interfere with the conductivity between interconnects  202  of interposer  200  and wires  102  of conductive elastomer  100 . The combined layers  100 ,  200  preferably have a thickness of about 32-33 mils. Due to the thin profile of layers  100 ,  200 , holes  502  cut at the ends of the contactor are preferably strengthened by metallization, which prevents tearing around the holes when the electrical contactor  10  is mounted on the printed circuit board. The metallization preferably comprises copper with gold plating over nickel. 
     Turning now to FIGS. 3 and 4, ground contact pedestal  300  preferably comprises a block of brass press fitted and soldered to ground layer  406  of printed circuit board  400 . The press fitting and soldering establish a good ground contact between ground contact pedestal  300  and printed circuit board ground layer  406 . The printed circuit board ground layer  406  comprises a block of metal located at the bottom of the printed circuit board. An indentation is machined in the block of metal and a hole is cut in printed circuit board  400  over the indentation such that ground contact pedestal  300  can be press fitted into the indentation through the hole. Solder is applied to the top of ground contact pedestal  300  for establishing good ground contact with ground of the chip being tested. Solder should be reapplied regularly to maintain good electrical contact characteristics. The thickness of ground contact pedestal  300  should be such that when chip  600  is plunged down on electrical contactor  10 , ground contact pedestal  300  acts as a solid stop to prevent chip  600  from excessively compressing contactor  10 , which may bend chip leads  602  and damage contactor  10 . 
     As shown in FIGS. 1,  2 , and  3 , electrical contactor  10  is preferably mounted onto printed circuit board  400  by sliding metallized holes  502  over dowels  402 . Center hole  504  of electrical contactor  10  preferably slides over ground contact pedestal  300  press fitted and soldered to printed circuit board ground layer  406 . Electrical contactor  10  is preferably secured to printed circuit board  400  by sliding O-rings  404  down dowels  402 . This method of mounting electrical contactor  10  onto printed circuit board  400  permits easy mounting and removal of electrical contactor  10  during changeover or initial set-up, resulting in reduced down time during chip testing. In addition, because bulky mounting mechanisms such as plastic mounting frames are not used, printed circuit board  400  is more accessible to chip testors for fine tuning electronic components on printed circuit board  400 . 
     In operation, chip  600  is plunged down on electrical contactor  10  for chip testing. Chip leads  602  make electrical contact with interconnects  202  while the ground located at the bottom of chip  600  makes contact with ground contact pedestal  300 . Preferably, as described above, small diamonds within interconnects  202  scratch open surfaces of chip leads  602  to expose fresh conductive material, establishing a good electrical connection between chip leads  602  and interconnects  202 . In addition, malleable solder on top of ground contact pedestal  300  conforms to the surface of the ground of chip  600  to ensure good ground contact. 
     Besides grounding chip  600 , ground contact pedestal  300  also serves as a solid stop to prevent chip  600  from plunging too close to electrical contactor  10 , which can cause chip leads to bend and damage electrical contactor  10 . Preferably, ground contact pedestal  300  allows some compression of electrical contactor  10 , preferably about 4 mils, to ensure solid lead  602  contact with interconnects  202 . Elastomer  104  of conductive elastomer  100  absorbs residual compressional forces caused by chip  600 , protecting printed circuit board  400 . Interposer  200  prevents debris from contaminating conductive elastomer  100 , prolonging electrical contactor  10 &#39;s life. 
     After establishing electrical and ground contact with printed circuit board  400  through electrical contactor  10 , chip testing commences. Electrical signals are exchanged between printed circuit board  400  and chip  600  through electrical contactor  10 . Ground contact pedestal  300  provides DC ground as well as RF signal ground for chip  600 , if chip  600  is an RF device. Since chip  600  can contain finely-tuned and sensitive electronic components, as radio frequency devices often do, it is desirable for contactor  10  to cause minimal electrical interference with chip  600 . Contactor  10  achieves this result due to its thin profile which does not significantly interfere with incoming radio frequency signals and does not contain much signal-carrying metallic material that can interfere with fine tuned electronics within the chip being tested. 
     It should be noted that the disclosed embodiments can be modified by a person skilled in the art without deviating from the scope of the present invention. For example, conductive elastomer  100  may include just one piece of wire-embedded elastomer material, as long as chip leads  602  are not inadvertently grounded by the ground of printed circuit board  400 . In addition, wires  102  may be oriented at various angles as long as they conduct electricity across the thickness of conductive elastomer  100 . Also, abrasive materials other than diamonds may be embedded in interconnects  202 . Furthermore, although brass is the preferred metal for ground contact pedestal  300  because it is easy to machine, other metals such as aluminum, copper, and gold may also be used. 
     While the invention has been described in conjunction with specific embodiments, it is evident that numerous alternatives, modifications, and variations will be apparent to those skilled in the art in light of the forgoing descriptions. The scope of this invention is defined only by the following claims.