Abstract:
An integrated circuit (IC) package testing device using a selectable number of leaf springs to provide a resilient and consistent normal force to the IC package and the method of operating the device. The leaf springs are shaped to provide the proper compliance and resilient force and are shaped to fit side-by-side within the lid of the device. The springs can be easily changed for differently sized IC packages.

Description:
FIELD OF THE INVENTION 
   This invention relates to integrated circuit testing sockets and more particularly to burn-in testing sockets. 
   BACKGROUND OF THE INVENTION 
   Integrated circuit (IC) packages must be tested after their manufacture, normally at elevated temperatures, which is the burn-in process. The integrated circuits are temporarily installed on a circuit board, tested, and then removed from the circuit board and shipped. Accordingly, test sockets are necessary to install the IC packages on the printed circuit board for testing. These test sockets include multiple contacts to connect each of the terminals of the IC package to corresponding conductors on the printed circuit board. Since the test sockets are used repeatedly in high volume IC package manufacture, it is desirable that the sockets be durable and capable of reliable, repeated operation. 
   The test sockets are positioned on a burn-in board where the sockets are arranged in a relatively dense array to allow for as many IC packages as possible to be tested at once. These sockets are therefore arranged in a relatively close side-by-side and end-to-end spacing. 
   It is desirable that the test sockets be capable of conforming to a large tolerance of package thicknesses. IC packages are manufactured with a metal “heat spreader” attached to the back of the package to help more evenly distribute the heat generated by the silicon die that is generally in or on the back of the substrate. A tolerance stack-up builds up because of the thickness tolerances of the IC package, the adhesive joint between the substrate and the “heat spreader,” and the “heat spreader” itself. For example, an IC package can end up with an overall thickness tolerance of + or −0.013 inches. 
   One of the test socket types that performs the burn-in function includes a base portion and a lid which rotates about one side by way of a hinge, and a latch which holds the lid and base together, where the latch is opposite the hinge. Unless the tolerance of package thickness is accounted for in the design and manufacture of this type of socket, there can be a great disparity in contact pressure between the contacts of the socket and the contact sections of the package. Some prior art examples of this type of test socket are Wells-CTI socket numbers  654 ,  692  and  693  shown in  FIGS. 1–3 , respectively. 
   The prior art Wells-CTI  654  socket  20 , shown in  FIGS. 1A and 1B , accommodates an IC with length and width dimensions of 8 mm×8 mm. The  654  socket  20  accounts for thickness tolerances by allowing its pressure pad  30  to rock around a center pivot pin  22  mounted parallel to the hinge  24  and by providing compliance via coil springs  26  mounted in its lid  28  on either side of the center pivot pin  22  applying balanced force to the back of the pressure pad  30  and by a coil spring  32  positioned beneath the IC receiver pocket  34 . 
   The prior art Wells-CTI  692  socket  36 , shown in  FIGS. 2A and 2B , accommodates IC packages with dimensions of approximately 31 mm×31 mm. The  692  socket  36  also accounts for thickness tolerances by allowing its pressure pad  38  to rock around a center pivot pin  40  mounted parallel to the hinge  42 . The  692  socket  36  provides compliance by means of coil springs  44  positioned within the corner posts  46  with the lid  48  and latch  50  connected to those corner posts by bars  52  positioned parallel to the hinge  42 . The force of the coil springs  44  may be slightly adjusted by adjusting the threaded engagement  54  of the corner posts  46 . The adjustment is limited by the properties of the individual coil springs  44  and any wider adjustment would require a complete disassembly of the socket and replacement of the springs. 
   The prior art Wells-CTI  693  socket  56 , shown in  FIGS. 3A and 3B , accommodates even larger IC packages  58  with dimensions of up to 42.5 mm×42.5 mm. The  693  socket  56  applies evenly distributed pressure to an IC package  58  via side pads  60  that are linked through symmetrically mechanical rockers  62  to torsional coil springs  64  positioned on the hinge  66  and latch  68  ends of the lid  70 . The compliance force of the torsional coil springs  64  can only be adjusted by replacing the torsional coil springs  64 , and that requires disassembly of the lid  70 . 
   Therefore, it would be advantageous to have a test socket that accommodates IC packages with a wide range of thickness tolerances by allowing for flexible and more easily adjustable compliance. 
   The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention which proceeds with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a top view of a prior art IC socket testing device shown in a closed and latched position. 
       FIG. 1B  is a cross-sectional view taken of the prior art IC socket testing device of  FIG. 1A  taken along line  1 — 1 . 
       FIG. 2A  is a perspective view of another prior art IC socket testing device shown in a closed and latched position. 
       FIG. 2B  is a cross-sectional view of the prior art IC socket testing device of  FIG. 2A  taken along line  2 — 2  and shown in an open position. 
       FIG. 3A  is a perspective view of another prior art IC socket testing device shown in a closed and latched position. 
       FIG. 3B  is a perspective view of the prior art IC socket testing device of  FIG. 3A  shown in an open position. 
       FIG. 4  is a perspective view of an IC socket testing device according to the invention show in an open position. 
       FIG. 5  is a perspective view of the IC socket testing device of  FIG. 4  shown in a closed and latched position. 
       FIG. 6  is a top plan view of the IC testing device of  FIG. 4  shown in a closed and latched position. 
       FIG. 7A  is a cross-sectional view of the IC testing device of  FIG. 4  taken along line  7 — 7  of  FIG. 6  showing compliance springs with the device in a partially closed position. 
       FIG. 7B  is a cross-sectional view of the IC testing device of  FIG. 4  taken along line  7 — 7  of  FIG. 6  showing compliance springs with the device in a closed but unlatched position. 
       FIG. 7C  is a cross-sectional view of the IC testing device of  FIG. 4  taken along line  7 — 7  of  FIG. 6  showing compliance springs with the device in a closed and latched position. 
       FIG. 8A  is a cross-sectional view of the IC testing device of  FIG. 4  taken along line  8 — 8  of  FIG. 6  showing the pressure pad with the device in a partially closed position. 
       FIG. 8B  is a cross-sectional view of the IC testing device of  FIG. 4  taken along line  8 — 8  of  FIG. 6  showing the pressure pad with the device in a closed but unlatched position. 
       FIG. 8C  is a cross-sectional view of the IC testing device of  FIG. 4  taken along line  8 — 8  of  FIG. 6  showing the pressure pad with the device in a closed and latched position. 
       FIG. 9  is a side elevation view of the leaf springs according to an embodiment of the invention. 
       FIG. 10A  is a perspective view of the IC testing device of  FIG. 4  using 20 leaf springs. 
       FIG. 10B  is a perspective view of the IC testing device of  FIG. 4  using 10 leaf springs. 
       FIG. 10C  is a perspective view of the IC testing device of  FIG. 4  using 4 leaf springs. 
   

   DETAILED DESCRIPTION 
     FIGS. 4 and 5  show an IC package testing device  72  according the invention in an open position. An IC package receiver  74  is contained within the base  76  of device  72  for receiving an IC package  88  (See  FIGS. 7A–7C  and  8 A– 8 C) for testing such as burn-in testing. The pressure pad  78  is positioned within the lid assembly  80 . When the device  72  is in a closed position, as shown in  FIG. 5 , the latch cam  82 , positioned on lid  80 , is engaged by the latch  84 . A normal force is applied to the IC package  88  through the pressure pad  78  by the leaf springs  86 . Referring to  FIGS. 4 and 5 , twenty leaf springs  86  are shown, with ten leaf springs stacked side-by-side on either side of the pressure pad  78  for a total of 20 leaf springs. It is preferable that at least two leaf springs  86  are used with at least one leaf spring positioned on either side of the pressure pad  78  to create a symmetrically balanced normal force in the pressure pad  78 . A single such leaf spring  86  is shown in  FIG. 9  and further described below. 
     FIG. 6  shows a top view of the IC package testing device according to the invention in a closed and latched position. The pressure pad  78  is positioned to overlie an IC package. Multiple leaf springs  86  are stacked in equal number on either side of the pressure pad  78  with the lid  80  and pressure pad  78  sized to accommodate multiple leaf springs  86 . The leaf springs  86  are laterally thin such that from one to a dozen or more can be stacked side-by-side on both sides of the pressure pad  78  between the pressure pad  78  and the lid  80 . 
     FIG. 7A  shows a cross-sectional view of the IC package testing device according to the invention taken at line  7 — 7  in  FIG. 6 . The pressure pad  78  is positioned to overlie an IC package  88  that is received in a recess  89  sized and shaped to receive an IC package  88  that is located on an IC package receiver  90 . The lid  80  is attached to the base  76  by a hinge  92 . A closure mechanism  94  is positioned opposite the hinge  92 . The closure mechanism preferably comprises a latch  84  positioned on the base  76  and a latch cam  82  positioned on the lid  80  to receive and engage the latch  84 . 
   A leaf spring  86  is coupled by a center pivot pin  96  to the pressure pad  78 . The leaf spring  86  is also pivotably coupled to the lid  80  at two distal pins  98 .  FIG. 7A  shows the pressure pad  78  rocking about the center pivot pin  96  to lie flat upon the IC package  88  when the lid  80  is in a partially closed position. 
     FIG. 7B  shows the lid  80  in a partially closed position with the latch cam  82  preliminarily engaging the latch  84 .  FIG. 7C  shows the lid  80  in a fully closed position with the latch cam  82  rotated over to fully engage the latch  84 . In the fully closed and latched position, the lid  80  forces the leaf spring  86  through the two distal pins  98  to press the pressure pad  78  onto the IC package  88  through the symmetrically located center pivot pin  96 . Because the pressure pad  78  is pressed down onto the IC package  88  by the center pivot pin  96  and the symmetrically shaped leaf spring  86 , the normal force is equally distributed across the whole IC package  88  ensuring a reliable connection in repeated uses of the IC packaged testing device  72 . 
   The leaf spring  86  shown in  FIGS. 7A–7C  is preferably formed such that its effective beam length is longer than the linear distance between the two distal pins  98 . This may be achieved, as shown in  FIG. 9 , by shaping the leaf spring  86  with a center pivot attachment hole  100  and spring material extending symmetrically to distal end portions  102 . Both distal end portions  102  terminate with fixed attachment holes  104 A and  104 B that are spaced equidistant from the center pivot attachment hole  100 . Both distal end portions  102  curve proximally back toward the center pivot attachment hole  100  such that the curvilinear length along the leaf spring from fixed attachment hole  104 A to fixed attachment hole  104 B is greater than the linear distance between the fixed attachment holes  104 A and  104 B. 
   For certain IC packages, the preferred compliance qualities of the leaf spring  86  are achieved by materials with a modulus of elasticity within a range of 18×106 psi to 22×106 psi. The same Beryllium-Copper alloy that can be used in the manufacture of the IC packages  88 , has a modulus of elasticity within the above stated range making a preferred leaf spring material for certain IC packages. A corrosion-resistant plating, such as Nickel plating, may be added to the leaf spring. 
   For other IC packages, the preferred compliance qualities of the leaf spring  86  are achieved by materials with a modulus of elasticity within a range of 27×106 psi to 30×106 psi. Commonly available stainless steel has a modulus of elasticity within this range and can withstand higher temperatures than the Nickel-plated Beryllium-Copper alloy. Thus, the stainless steel leaf springs can be used for IC packages that require higher burn-in test temperatures than the Nickel-plated Beryllium-Copper can withstand. 
   The proper amount of spring deflection and force is needed to allow for the variance in IC package thickness due to thickness tolerances. For example, a pair of leaf springs  86  can provide a normal force of 10 pounds on the IC package  88  through the pressure pad when the leaf springs are deflected 0.05 inches. This amount of deflection takes into account the overall thickness tolerance of the IC package as well as any tolerances within the individual components of the IC package testing device  72 . 
     FIG. 10A  is a perspective view of the IC package testing device  72  according to the invention, shown with twenty leaf springs  86  positioned in the device. The leaf springs  86  have a thickness that allows them to be positioned side-by-side within the device. Ten leaf springs  86  are positioned on either side of the pressure pad  78  to supply a balanced force to the pressure pad  78  when the device is closed and latched. 
     FIG. 10B  is a perspective view of the IC package testing device  72  according to the invention, shown with ten leaf springs  86  in the device. The lid  80  and the pressure pad  78  are sized to allow for a variable number of leaf springs  86  to be used in the device  72 . Plastic spacers  106  fill the gap on the two distal pins  98  created due to the reduced number of leaf springs  86  in the device. 
     FIG. 10C  is a perspective view of the IC package testing device  72  according the invention with four leaf springs  86  in the device. Clips  108  allow for easy disassembly and assembly of the distal pins  98  and the center pivot pin  96  from the lid  80  for changing the number of leaf springs  86 . 
     FIGS. 8A–8C  show a cross-sectional view of the IC package testing arrangement taken along line  8 — 8  in  FIG. 6 . The IC package  88  has predetermined lateral width and length dimensions and the IC package receiver  74  is sized to receive the IC package  88 . The IC package  88  has a predetermined thickness range. The range of thickness is due to the thickness tolerances of the various components of the IC package  88 . The number of leaf springs  86  can be selected to apply a resilient normal force to the IC package  88  through the pressure pad  78 . The number of leaf springs  86  selected correlates to a variety of factors including the length and width of the IC package  88 , the number of leads on the IC package  88  and the geometry of the leads on the IC package  88 . 
   OPERATION OF THE IC PACKAGE TESTING DEVICE 
   Operation of the IC package testing device according to the invention includes placing an IC package  88  into the IC package receiver  74 , clamping the IC package  88  into the receiver by closing the pressure pad  78  down onto the IC package  88  and applying a normal force to the IC package  88  through the pressure pad  78  by closing the latch. The normal force applied to the IC package  88  has variable resilience. This variable resilience is applied with a plurality of leaf springs  86 . 
   The force is varied by varying the number of leaf springs wherein differently sized IC packages  88  are accommodated by changing the number of leaf springs  86 . The number of leaf springs is easily changed, first by removing the clips  108  from the two distal pins  98  and the center pivot pin  96  and then by installing or removing the desired number of leaf springs  86 . 
   The leaf springs are engaged by a latch using a latch cam that presses the lid  80  down to apply force through the leaf springs  86  to the pressure pad  78  and normally onto the IC package  88 . 
   Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention can be modified in arrangement and detail without departing from such principles. I claim all modifications and variations coming within the spirit and scope of the following claims.