Patent Publication Number: US-6220870-B1

Title: IC chip socket and method

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to achieving electrical interfaces between two conductive paths, and more particularly to the construction of sockets for holding the I/O contacts of an IC chip in electrical contact with the compliant tips of an array of conductive probes, typically pogo pin probes. The invention has particular application in contacting ball grid array (BGA) devices wherein electrical contact must be achieved within an array of densely packed solder balls. However, the socket and method of the invention can also be used in connection with IC chips having other contact or lead formations. 
     Sockets for holding IC chips are well known and widely used in a variety of industrial applications for achieving efficient electrical contact between the I/O contacts of an IC chip and the conductors of a connector, circuit board or the like. Such applications include test sockets for testing IC devices and contactor sockets for removably mounting an IC device to a printed circuit board. The basic function of the socket is to hold the IC chip in a fixed position such that its array of I/O contacts, eg. the solder balls of a BGA, can be contacted by the compliant tips of a corresponding pogo pin array to produce a desired electrical connection. Precise centering of the pogo pins relative to the chip&#39;s I/O contacts is critical to this function and generally to the electrical and mechanical performance of the socket. Any misalignment between the pogo pin tips and the I/O contacts can result in a poor electrical interface to the IC chip. Distortion of the I/O contacts can also result when misalignments produce off-center contacts. 
     In a conventional IC socket, an IC chip is held in a socket base directly over an array of pogo pins. In such socket designs, the chip&#39;s I/O contacts are typically centered by referencing the known lateral dimensions of the IC package to the sidewalls of the socket base in which the IC chip is held. The difficulty with this centering approach is that the true position of the I/O contacts within the socket are subject to usual variations that occur in the dimensions of the IC package. The resulting deviations from a true center detrimentally affects the electrical performance of the socket and increases undesirable deformations in the chip&#39;s contacts. The problem of contact deformation is particularly acute in BGA devices where deformations in the device&#39;s solder balls can detrimentally affect the inspection of the IC device as well as the assembly of the device onto a printed circuit board. Excessive deformation of the extremely small solder balls of micro-BGAs can, for example, cause such IC devices to be rejected during quality control procedures which typically employ laser or other inspection techniques. 
     Another drawback of conventional IC socket designs is the need to apply a modicum of force to disengage the IC chip from the pogo pins upon removal of the IC chip from the socket. Generally, the tips of the pogo pin probes will, to a certain degree, indent, displace or disturb the chip&#39;s I/O contacts, creating a binding force between the chip and pogo pins which must be overcome upon removal. The need therefore exists for an IC socket where the removal of the chip from the pogo pin tips can be facilitated. 
     The present invention provides an improved IC socket design and method that overcomes the aforementioned drawbacks of conventional IC sockets. The improved IC socket and method of the invention reduces chip positioning errors and more particularly permits extremely accurate centering of the chip&#39;s I/O contacts over the tips of the socket&#39;s pogo pins. The invention also facilitates the removal of the chip from the IC socket. 
     SUMMARY OF THE INVENTION 
     Briefly, the invention herein described and claimed involves a socket which holds an IC chip having lateral edges and a planar array of I/O contacts. The socket is comprised of a base portion having a defined z-axis, and a separate chip docking structure for receiving the IC chip and for fixing the position of the chip&#39;s planar array of I/O contacts in an x-y plane perpendicular to the z-axis of the base portion, without reference to the lateral edges of the IC chip. In the preferred embodiment, the docking structure of the socket is provided in the form of a docking platform having a chip-seating surface extending in the x-y plane. This seating surface has an array of locator openings for receiving the chip when the chip is seated on the seating surface with its I/O contacts engaged in the locator openings. The position of the chip&#39;s I/O contacts is therefore fixed in the x-y plane in reference to the I/O contacts themselves, without reference to the lateral edges of the chip. Furthermore, the seating plane of the chip docking structure, which is contacted by the underside of the chip package, provides a reference plane for the chip that has relatively true planarity as compared to the planarity of the seating plane of the I/O contacts. Thus, the IC chip will be held within the socket in the true x-y plane established by the chip docking structure. 
     The socket of the invention also provides for an array of conductive probes which are held in the socket base in opposition to the chip docking structure. The array of conductive probes are configured to correspond to a standardized configuration of I/O contacts of an IC chip. Specifically, the probes have compliant probe tips arrayed in an x-y plane so that the probe tips and the I/O contacts held on the chip docking structure are centered relative to each other in the direction of the z-axis of the socket base. In the preferred embodiment, the probe array is held in a fixed transverse bottom wall of the socket&#39;s base portion. However, it will be understood that the probe array can be provided beneath the docking structure by other means, such as a separate probe retention structure that moves within the socket base or that externally connects to the socket base. 
     The socket of the invention further includes z-axis closure means for operatively bringing the I/O contacts of the IC chip held on the docking structure into contact with the compliant probe tips of the conductive probe array opposed thereto by a relative movement of the contacts and probe tips along the z-axis of the base portion. It is particularly contemplated that the chip docking structure will be movable relative to the probe tip array along the z-axis by providing a docking structure that is depressible in the z-axis against spring supports in the socket base. In the illustrated embodiment of the invention, the docking structure is provided in the form of a rectangular docking platform which is depressibly captured in the socket base along its four corners by means of corner blocks. The corner retention of the docking platform will inhibit any rotational movement of the docking platform in the base portion which would cause positional errors of the outermost I/O contacts of the IC chip seated on the platform. 
     The invention also involves a method for bringing the I/O contacts of an IC chip into contact with the compliant tips of an interface probe array. The method is generally comprised of fixing the position of the planar array of I/O contacts of an IC chip in the x-y plane in direct reference to the I/O contacts, and providing an array of probes having probe tips centered to such known fixed reference position. The centered probe tips and the I/O contacts of the I/C chip are brought into contact along the z-axis, preferably by moving the IC chip relative to the probe tips. 
     Therefore, it can be seen that a primary object of the present invention is to provide an IC socket and a method for centering the I/O contacts of an IC chip over the compliant tips of a probe array with minimal positional error. It is a further object of the invention to provide a test socket and method which permits the IC chip to self-release from the probe array upon removal of the IC chip from the test socket. Other advantages of the invention will be apparent from the following specification claims, as well as the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded, top perspective view of an improved IC chip socket in accordance with the invention, showing an IC chip positioned over the docking platform of the socket. The pogo-pin probes in the socket base have been omitted in the figure for illustrative purposes. 
     FIG. 2 is a top plan view of the base portion of the socket shown in FIG. 1 with the docking platform retained therein. 
     FIG. 3 is a bottom plan view of the docking platform of the test socket shown in FIGS. 1 and 2. 
     FIG. 4A is a cross-sectional view of the base portion and docking platform shown in FIG. 1 taken along section lines  4 — 4  in FIG. 2, and showing the pogo pin probes in the socket base and the positioning of an IC chip above the docking platform. 
     FIG. 4B is another cross-sectional view thereof in side elevation, showing the IC chip placed on the docking platform. 
     FIG. 4C is another cross-sectional view thereof in side elevation, showing the IC chip and docking platform depressed toward the socket&#39;s pogo pin probe tips. 
     FIG. 5A is an enlarged fragmentary cross-sectional view of the socket base, docking platform, and IC chip shown in FIG. 4 A. 
     FIG. 5B is an enlarged fragmentary cross-sectional view of the socket base, docking platform, and IC chip shown in FIG.  4 B. 
     FIG. 5C is an enlarged fragmentary cross-sectional view of the socket base, docking platform, and IC chip shown in FIG.  4 C. 
     FIG. 6A is a side elevational view, in partial cross-section, of one of the pogo pin probes of the IC socket. 
     FIG. 6B is another side elevational view thereof showing the compliant probe tips of the pogo pin depressed against the pogo pin&#39;s internal compression spring. 
     FIG. 7 is a fragmentary cross-sectional view of the docking platform of the invention showing an alternative embodiment thereof. 
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
     Referring now to the drawings, FIGS. 1 and 2 illustrate an IC chip socket  11  in accordance with the invention for holding an IC chip such as a BGA  15  having a planar array of solder ball contacts  37 . The test socket includes an outer frame  12 , a base portion  14  held within the outer frame, and a docking structure in the form of docking platform  13 . The outer frame has right angle perimeter walls  19  generally defining a rectangular socket perimeter, and a cover portion  17 . The base portion, in turn, has a transverse bottom wall  23  extending in the x-y plane of the x-y-z coordinate system of the base portion shown in FIGS. 1 and 2. Corner blocks  25  are provided at the four corners of the base portion inside the interior corners  27  of the frame to form a generally rectangular central cavity region  31  over the transverse bottom wall  23 . This cavity region receives the similarly shaped docking platform  13  whose function it is to receive the BGA  15 , and, as hereinafter described, to fix the position of the BGA&#39;s solder ball array  37  in the test socket. 
     While illustrated and described as two separate parts, it will be appreciated that frame  17  and base portion  14  can be fabricated as a single base unit with a hinged or separate cover, all of which could suitably be molded plastic parts. It is also contemplated that the corner blocks  25  of base  14  can be provided as separate elements that are detachable from the base. This would allow different sized docking platforms to be used with the socket as herein described. 
     The docking platform is seen to include a transverse support wall  39  having a planar, rectangular IC chip seating surface  41  extending in the x-y plane, that is, in a plane perpendicular to the z-axis of socket base  14 . Raised corner guides  43  formed at the four corner positions of the docking platform provide a means for slidably retaining docking platform  13  within the center cavity region of the socket base. More specifically, it is seen that each of the raised corner guides of the docking platform includes right angle outer guide walls  45  and a curved bottom lip  49  which nest within the right angle inner guide walls  47  and curved edge  51  of the corner blocks. Curved retainer clip recesses  52  in the top of the corner blocks  25  receive correspondingly shaped corner retainer clips  53  which are removably fastened in the clip recesses by means of screw fasteners  55 . The bottom wall  57  of each clip recess has a thickness suitably greater than the curved bottom lip  49  of the platform&#39;s corner guides to allow the platform suitable freedom of travel along the z-axis when the retainer clips bottom out against the recessed bottom wall. The retainer clips slidably engage the shaped corner guide surfaces  59  on the platform&#39;s raised corner guides  43  to fix the position of the platform in the socket base. Such corner retention of the docking platform also inhibits rotational movements which would lead to positional errors. The positional integrity of the docking platform is further enhanced by the double curved shape of the corner retainer clips and guide surfaces  59  which maximize the contact area at the corners. 
     As best illustrated in FIG. 1, the docking platform is supported on the transverse bottom wall of the socket&#39;s base portion by means of a series of compression springs  61  provided at the four corners of the docking platform. Suitably, two compression springs are provided at each corner adjacent corner blocks  25  to provide evenly distributed support of the docking platform in the center cavity region  31  of the socket base. To hold the compression springs in place, the bottom ends of the compression springs are retained within recesses  63  in the interior surface of the bottom wall of the socket base, while the top ends of the compression springs are retained in corresponding opposed recesses  65  in the bottom of the docking platform (see FIG.  3 ). The compression spring supports will permit the docking platform to be depressed along the z-axis of the socket base within the base corner blocks  25 , thereby providing z-axis closure means for bringing the BGA seated on the docking platform into electrical contact with the socket probes as herein described. It will be appreciated that other forms of spring supports could be provided for the docking platform, for example, leaf spring supports. 
     With further reference to FIG. 1, it can be seen that the docking platform  13  is installed in the center cavity region  31  of the socket base  14  by slidably engaging the raised corner guides  43  of the docking platform within the base&#39;s respective corner block  25 , and then by screwing the corner retainer clips  53  down into the clip recesses  52  of the corner blocks. The retainer clips are screwed down into the corner block recesses to capture the bottom lip  49  of the platform&#39;s raised corner guides. Separate counter-bored screw holes  29  in the corner blocks provide a means for fastening the socket to a circuit board. 
     It should be noted that docking platforms having different design characteristics for different applications can readily be exchanged in the socket base  14  by removing the corner retaining clips  53  and replacing the docking platform. Further, if required, and as above-mentioned, the corner blocks of the socket base could themselves be designed to be removable so that they could be exchanged to meet different design requirements for the outer perimeter of the docking platform. However, it is contemplated that a universal docking platform could be provided for accommodating a variety of IC chip designs, such as BGAs having different solder ball densities. 
     To precisely position the solder balls of the BGA on the docking platform, the chip seating surface  41  includes an array of locator openings  67  sized to receive the solder balls  37  of the BGA chip  15  when the BGA chip is seated against this seating surface. Preferably, locator openings  67  are circular openings having a diameter somewhat larger than the mean diameter of the solder balls so as to accommodate all of the solder balls of the chip&#39;s solder ball array, but not so large as to permit undue lateral movement of the BGA on the seating surface of the platform. When seated on the platform seating surface, the position of the BGA will be fixed in reference to these openings, as opposed to referencing the BGA package itself. In this connection, it is noted that the right angle interior walls  69  of the platform&#39;s corner guides  43  should provide suitable lateral interior dimensions at the corner of the platform to accommodate the lateral edges  18  of the BGA package without interference, since contact with the edges of the package would adversely affect the ability of the BGA&#39;s solder balls to seat properly in the platform&#39;s locator openings. 
     In addition to the locator opening array  67 , a number of larger vent holes  71  (see FIG. 2) are suitably provided in the platform&#39;s support wall  39  to permit passage of air through the docking platform when it is depressed in the center cavity region of the socket base. Trapped air between the transverse bottom wall  23  of the socket base and the docking platform is also permitted to escape through a relief channel  73  in the perimeter of the socket base and outer frame  12 . 
     As best illustrated in FIGS. 4-6, the socket of the invention is provided with an array of conductive probes  77  which provide an array of compliant probe tips  79  in opposition to the docking platform  13  and the planar solder ball array  37  of the BGA  15  seated on the seating surface  41  of the docking platform. To accommodate the probe tip array, the transverse support wall  39  of the docking platform is provided with a corresponding array of guide holes  81  extending through the support platform to the locator openings  67 . The guide holes are sized to receive the probe tips so that the BGA solder balls  37  registered in the platform&#39;s locator openings are contacted by the probe tip array as the docking platform is depressed toward the probe tips. The individual pogo pins of the probe array are, in turn, positioned and held in their desired z-axis orientation by the transverse bottom wall  23  of the socket base. As best shown in FIGS. 5A-5C, the pogo pins of the pogo pin probe array are removably retained in the base&#39;s bottom wall by inserting them into suitably dimensioned probe retainer holes  83  from the outer side  85  of the bottom wall. When fully inserted, the pogo pin probe tips  79  will extend from the bottom wall into the bottom of the docking platform through apertures  87  at the end of retainer holes  83 . The pogo pins are held in place in the retainer holes by means of a retainer cap  89  suitably secured in place over the back ends of the retainer holes by pan head screws (not shown). The pogo pin retainer cap  89  is provided with apertures  91  to accommodate probe tips  93  extending from the opposite ends of the pogo pins. 
     The general construction of the pogo pins used in the pogo pin probe array of the invention is illustrated in FIGS. 6A and 6B. Each pogo pin probe  77  includes an elongated spring barrel  95 , a compression spring  97 , and opposed probe tips  79 ,  91 , which extend from piston ends  99 ,  101  captured inside the crimped ends  103 ,  105  of the spring barrel. The internal compression spring  97  is compressed between the piston ends of the probe tips to hold the probe tips in their normally extended position as shown in FIG.  6 A. When the probe tips are contacted, they will depress against the restoring force of the compression spring as shown in FIG. 6B, so that the probe tips operatively provide compliant spring contacts when the socket is used. 
     It will be understood that, while the pogo pins opposing the docking platform of the invention are shown as being stationary relative to the docking platform, it would be possible to supply the pogo pin array in a floating structure within the socket base, or to even provide for the attachment of a separate pogo pin array component to the end of the socket in proper opposition to the docking platform. In either case, the pogo pin array must be configured and positioned such that the extending probe tips  79  of the pogo pin array register with the array of guide holes  81  in the docking platform. 
     The use and operation of the improved socket of the invention, along with the method of the invention, is now described in reference to FIGS. 4 and 5 of the drawings. To operatively insert BGA  15  in socket  11 , the BGA chip is positioned over seating surface  41  of the docking platform with the chip&#39;s solder ball contacts  37  generally aligned with the locator openings  67  in the seating surface. As shown in FIGS. 4B and 5B, the BGA is then placed on the seating surface of the platform such that its solder balls are received in the locator openings, and such that the bottom surface  16  of the BGA seats against the platform&#39;s seating surface. The BGA will thus be precisely positioned on the docking platform in reference to its solder ball contacts. Also, thusly seated, the orientation of the BGA relative to the x-y seating plane will be determined by the seating surface of the platform instead of the bottoms of the solder balls. Once the BGA is properly seated on the docking platform, the BGA and docking platform are depressed as a unit along the z-axis of the socket base against the platform&#39;s support springs  61  as shown in FIGS. 4C and 5C. Referring to FIG. 1, the BGA and docking platform are depressed toward the probe tip array by closing the socket cover  17  over the unit&#39;s outer frame  12 . When the socket cover is closed, a depressor ring  107  formed on the inside of the cover contacts the top of the BGA and depresses the BGA and docking platform together as a unit. The depressor ring should be sized to standard BGA package dimensions to limit the z-axis travel of the docking platform and BGA. The degree of travel should be sufficient to permit the probe tips of the probe tip array to contact the solder balls, yet not so great as to cause the bottom of the docking platform to bottom out against the bottom wall of the socket base. 
     It should be noted that, when the socket is opened by opening the socket cover  17 , the support springs  61  for docking platform  13  will automatically return the docking platform to its non-depressed position. As this occurs, the mechanical contact between the array of probe tips  79  and the BGA solder ball contacts  37  will be broken permitting the chip to be easily lifted off the docking platform. Removal of the chip from the platform is further facilitated by providing access openings  109  (see FIG. 3) around the perimeter of the docking platform between the platform&#39;s raised corner guides. 
     FIG. 7 is an enlarged fragmentary view of a portion of the docking platform showing an alternative design for the platform&#39;s locator openings and guide holes. The design of FIG. 7 is particularly adapted for use with an improved probe tip design wherein the probe tips  111  are provided with perimeter point structures  113  having point ends  115  lying on a perimeter circle that exceeds the diameter of the solder balls  116  of BGA  118 . In this design, the seating surface  117  of the docking platform has locator openings  119  which are smaller in diameter than the probe tip guide holes  121  extending through the platform&#39;s bottom wall  123 . The guide holes are cylindrical to conform to the cylindrical shape of the probe tip  111  and terminate at annular shoulders  125  that form the locator openings. This platform design can thus accommodate the larger diameter probe tip while providing solder ball locator openings in the platform seating surface that are optimally sized to receive and to precisely fix the position of the solder ball contacts. The annular tapered extension  127  provided at the inside diameter of the annular shoulder  125  acts to extend the interior walls  129  of the locator opening while accommodating the point ends  115  of the probe tip when the probe tip and solder ball are brought into contact with each other. Specifically, it is seen that the interior contact edges  131  of the four probe tip point structures  113  (see the probe tip  79  shown in FIGS.  6 A and  6 B), are the only portions of the probe tip that contact the solder ball as the BGA and docking platform are depressed toward the probe tip as above described. As the probe tip comes into contact with the solder ball, the point ends of the probe tip are accommodated by the recesses  133  formed by the tapered extensions of the annular shoulder. Furthermore, the annular interior surface  135  provided on the annular shoulder to the outside of the tapered extension will provide a stop for the point ends of the probe tip, thereby preventing excess penetration of the probe tip into the solder ball. Thus, the docking platform design illustrated in FIG. 7 will also act to minimize undesirable deformations in the solder ball. 
     The socket base  14 , docking platform  13 , and socket cover  17  are all suitably fabricated of a high temperature plastic such as TORLON®, available from Amoco Polymers, Inc. The outer frame  12  is suitably fabricated of metal or plastic and as above mentioned can be incorporated as a part of the socket base. 
     Therefore, it has been seen that the present invention provides a socket for an IC chip wherein the ability to precisely center the I/O contacts of the IC chip with an array of compliant probe tips associated with the socket is greatly increased over conventional socket designs. The invention also permits the use of a IC chip docking structure that can be adapted to IC chips having different contact densities. Still further, the socket of the invention provides a means for automatically releasing the IC chip from the probe tip array when the socket is opened. While the improved socket and method of the invention has been described in considerable detail in the foregoing specification, as well as in the accompanying drawings, it shall be understood that it is not intended that the invention be limited to such detail, except as necessitated by the following claims.