Patent Publication Number: US-2007113394-A1

Title: Air socket for testing integrated circuits

Description:
CROSS-REFERENCE TO RELATED CO-PENDING APPLICATION  
      This application is a divisional of U.S. patent application Ser. No. 11/435,081, filed May 15, 2006, which is a continuation of U.S. patent application Ser. No. 10/741,100, filed Dec. 19, 2003, now U.S. Pat. No. 7,069,638, issued on Jul. 4, 2006, which is a divisional of U.S. patent application Ser. No. 09/653,111, filed Aug. 31, 2000, entitled “AIR SOCKET FOR TESTING INTEGRATED CIRCUITS,” now U.S. Pat. No. 6,690,184, issued Feb. 10, 2004, which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to the testing of semiconductor devices. More specifically, the invention relates to a method and apparatus for improving the electrical connections during a testing sequence.  
      2. Description of the Related Art  
      Flip-chip and bumped die technology is well known in the art. A flip-chip or bumped die is a semiconductor chip having contact bumps, typically in the form of spherical solder balls or controlled collapse chip connector (C4s) balls, which are electrically connected to the I/O contacts, or contact pads, formed on the active circuit or front side thereof The I/O contacts provide signal, power, and ground contacts for the chip. The contact bumps are used as electrical and mechanical connectors between the contact pads on the flip-chip and a substrate such as a chip carrier, printed circuit board, or other surface mount device. In some cases the bond pads may be located too close to one another to allow the placement of contact bumps directly on each bond pad without unintentionally electrically connecting every contact bump together. One common solution to this problem is to create a Ball Grid Array (BGA) using commonly known passivation and tracing techniques to place the contact bumps away from the bond pads, yet still retain the electrical connection between the contact bumps and the bond pads.  
      Another common semiconductor configuration is the board-on-chip (BOC), which comprises a chip permanently attached to a circuit board. In contrast to a flip-chip, the inactive circuit or backside of the chip is attached to the circuit board. The exposed bond pads on the side away from the circuit board are connected to the circuit board with the use of curved wire bonds. A glob top protective resin is subsequently applied over the chip and wire bonds. The contact bumps, which allow further connection to an electrical device or substrate, are then attached directly to the circuit board rather than the chip.  
      Several materials are typically used to form the contact bumps on the die or the board, such as, for example, conductive polymers, conductive resins, and solder (e.g. alloys of lead and tin). The specific constituents of solder, if used, are dependent on the desired melting temperature as well as the thermal characteristics of the mating surfaces. When the device is permanently attached, the contact bumps are reflowed to form a solder joint between the flip-chip and the substrate, forming both electrical and mechanical connections between the flip-chip and substrate. Due to the presence of the contact bumps on the flip-chip, a standoff exists between the substrate to which the flip-chip is attached or bonded and the bottom surface of the flip-chip.  
      Before the flip-chip is permanently attached to a substrate, it is typically tested to ensure proper performance. The flip-chip is commonly tested by temporarily connecting it to a socket made of a rigid non-conductive material by which multiple contact members are attached. The use of a non-conductive material prevents interference and allows for electrical isolation of each contact bump of the flip-chip. The electrical connection is made by physically laying the flip-chip onto the socket in a manner that lines up the contact bumps of the device with the appropriate contact members of the socket. The contact members of the socket are electrically connected to testing equipment, which provides the flip-chip with the necessary power and input signals to test the functions of the flip-chip.  
      A subtle problem often associated with the temporary interconnection of electrical components for testing is that the terminals of an electronic component are not co-planar. With the testing of a chip, the test socket&#39;s upper surface is generally flat to allow for the temporary connection between the contact bumps and its contact members. The flip-chip&#39;s lower surface, or the board surface in a BOC configuration, is also generally flat. The contact bumps on the flip-chip are not usually a uniform size because of the imprecise solidifying characteristics of the conductive materials used in creating the contact bumps. For example, when the contact bump is originally liquified to attach to the flip-chip or BOC, it may solidify into a slightly different shape than any of the other contact bumps. Accordingly, since the contact bumps may not be of a uniform shape, it may not be possible to temporarily electrically connect each of them to the socket during a test sequence. Nevertheless, the bumps on the flip-chip are not reflowed to achieve a connection to the socket during testing, as this connection is only temporary and would lead to the loss of the contact bumps or at least their significant degradation. Furthermore, the process of reflowing contact bumps requires the addition of thermal energy to the coupled electrical components which can adversely affect not only the integrated circuit contained in the component, but also the test socket. Thus, the inability to achieve an electrical connection between all of the solder bumps and the socket prevents an accurate test from being conducted, and may lead to the discarding of an otherwise usable flip-chip.  
      Thus, it will be appreciated that there is a need in the technology for a system for providing a reliable electrical connection between a semiconductor device and a socket in a test environment. The present invention provides an apparatus and method for improving the number of temporary electrical connections between a semiconductor device and a test socket.  
     SUMMARY OF THE INVENTION  
      The invention improves the electrical connections that are made between a flip-chip or BOC and a socket during the testing phase of fabrication. The invention can increase manufacturing efficiency and quality control by enabling the testing of flip-chips or BOCs without regard to whether its contact bumps are co-planar or have varying heights or diameters. Rather than using a solid base upon which to attach the contact members, the invention employs a flexible membrane for their attachment to allow relative motion between the contact members. This relative motion, derived from the resiliency of the flexible membrane, allows the contact members on the flexible membrane to adapt to the height variations of the contact bumps so as to form an electrical connection, and thereby improve the probability of a successful test of the chip. These variations may be on the same chip or between multiple chips. The movement of the flexible membrane is permitted by its attachment to a housing with a recess. The recess is covered by the flexible membrane to form a chamber in the housing. The chamber is filled with a fluid material, herein defined as a liquid or gas, which may form a delta between the chamber pressure and ambient pressure at a steady state (i.e. pressurized), be sealed at ambient pressure at a steady state, or be open to ambient pressure during the test sequence. The contact members are electrically connected by way of electrical contacts to the test equipment to maintain the flexibility of the flexible membrane.  
      The fluid material within the chamber will create an upward force upon the lower surface of the flexible membrane. This upward force, which is coupled with the resiliency of the flexible membrane, may help each unconnected contact member rise up towards the appropriate contact bump when the other connected contact members are pushed down toward the chamber. This will occur when the pressure in the chamber is dynamically increased by a reduction in the volume of the chamber caused by the contacting contact members. The fluid will try to equalize the pressure between the chamber and ambient pressure by bulging out in other areas of the flexible membrane. This bulging out helps instigate contact between the unconnected contact members and contact bumps. In this way, all of the contact members are enabled to form electrical connections with the contact bumps on the flip-chip or BOC and to enable the test equipment to perform an accurate test. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and other features and advantages of the invention will become more apparent upon reading the following detailed description and upon reference to the accompanying drawings, in which;  
       FIG. 1  is a perspective view of a socket of the invention having pads thereon configured in a conventional manner;  
       FIG. 2  is a side elevation view of the socket illustrated in  FIG. 1 , taken along lines  2 - 2  of  FIG. 1 , and illustrating a flip-chip positioned above the socket;  
       FIG. 3  is a cross-sectional view of the socket illustrated in  FIG. 1 , taken along lines  3 - 3  of  FIG. 1 , and illustrating a flip-chip positioned above the socket;  
       FIG. 4  is a cross-sectional view of the socket illustrated in  FIG. 1  taken along lines  3 - 3  of  FIG. 1 , and illustrating a flip-chip positioned above the socket as occurs during a testing sequence;  
       FIG. 5  is a cross-sectional view of another embodiment of the socket illustrated in  FIG. 1 , taken along lines  3 - 3  of  FIG. 1 , and illustrating a flip-chip positioned above the socket.  
       FIG. 6  is a cross-sectional view of another embodiment of the socket illustrated in  FIG. 1  taken along lines  3 - 3  of  FIG. 1 , and illustrating a flip-chip positioned above the socket as occurs during a testing sequence;  
       FIG. 7  is a cross-sectional view of an alternative embodiment of a socket of the invention with a flip-chip located above the socket;  
       FIG. 8  is a cross-sectional view of an alternative embodiment of the socket of the invention with a wafer in a BOC configuration positioned above the socket;  
       FIG. 9  is a perspective view of a grid used in one embodiment of the invention;  
       FIG. 10  is a cross-sectional view of a socket of the invention secured in contact with a flip-chip by a cartridge.  
       FIG. 11  is a flow chart illustrating the test process in accordance with the invention.  
       FIG. 12  is a flow chart illustrating another embodiment of the test process in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Broadly stated,  FIG. 1  shows an overview of an embodiment of an adjustable membrane test device for a semiconductor.  FIG. 1  is a perspective view of a socket  10  of a preferred embodiment of the invention. The socket  10  consists of a housing  12  with a recess (not shown) covered on at least one side with a flexible membrane  14  to form a chamber (not shown) in the housing  12 . The housing  12  is rigidly designed to provide the support for the attached flexible membrane  14 . The flexible membrane  14  is made from a non-conductive material and creates an adjustable surface.  
      A plurality of contact members  16  are attached to that surface of the flexible membrane  14  which faces the electrical device under test. The plurality of contact members  16  may take the form of individual planar surfaces such as those shown or, alternatively, concave surfaces, convex surfaces, linear protrusions such as piercing contacts, dendrite protrusions or other geometric structures configured to make individual electrical contact with the semiconductor device. As such, the plurality of contact members  16  are electrically conductive and configured to temporarily connect to contact bumps on a flip-chip, BOC, or other semiconductor device during a test sequence. The plurality of contact members  16  are further electrically connected, by way of a plurality of electrical connectors  18 , to a testing device so as to maintain the flexibility of the flexible membrane  14 .  
      The testing device may be a computer, resistor, or other device used for originating or processing of electrical signals. Although  FIG. 1  illustrates the plurality of contact members  16  arranged in a two dimensional array for electrical connection with a BGA or other electrical device in the flip-chip or BOC configuration, it will be appreciated that the plurality of contact members  16  may be configured in any desired arrangement without adversely affecting the operation thereof  
      Referring now to  FIG. 2 , the mating surfaces on the flip-chip  20  and socket  10  can be described. Fixedly attached to the flip-chip  20  on its lower surface is a plurality of contact bumps  22 , which are generally spherical, hemispherical, or dome shaped. The height of each of the plurality of contact bumps  22  often varies, thus causing a non-planar contact surface across the flip-chip  20 . The plurality of contact bumps  22  are electrically connected to the I/O contacts, or contact pads, formed on the active circuit or front side thereof (not shown). The I/O contacts provide signal, power, and ground contacts for the flip-chip  20 . The plurality of contact bumps  22  are used as electrical and mechanical connectors between the contact pads on the flip-chip  20  and a substrate such as a chip carrier, printed circuit board, or other surface mount device. The plurality of contact bumps  22  are made of materials that are electrically conductive and have special liquefying properties such as conductive polymers, conductive resins, and solder (e.g. alloys of lead and tin). The specific constituents of solder, if used, are dependent on the desired melting temperature, as well as the thermal characteristics of the mating surfaces.  
      As shown in  FIG. 2 , the flip-chip  20  is geometrically aligned with the socket  10  in preparation for a testing sequence. Geometrical alignment involves horizontally aligning the plurality of contact bumps  22  on the flip-chip  20  with the corresponding plurality of contact members  16  attached to the flexible membrane  14  on the socket  10 .  
      Upon inspection of  FIG. 3 , it is seen that the flexible membrane  14  completes the chamber  24  by covering the recess of the housing  12 . The depth of the chamber  24  is configured so as to permit sufficient relative movement of the plurality of contact members  16  to accommodate the variations in height between the plurality of contact bumps  22 . These variations, in the plurality of contact bump  22  heights, may be measured by any simple means commonly know in the art. As these variations may be small, only a small gap between the flexible membrane  14  and the four sides and bottom of the housing  12  may be required to form the chamber  24 . It will be appreciated that the shape of the chamber  24  is not confined to the embodiment shown in  FIG. 3 .  
      The support for the flexible membrane  14  is provided around its periphery where it is attached to the four sidewalls of the housing  12 . The four sidewalls form a frame around the recess in the housing  12 . Alternately, the flexible membrane  14  may be attached to a lip formed around the edge of the housing  12 . The attachment of the flexible membrane  14  may be achieved with the use of an adhesive, circumferential strap or band, or another method known in the art. In another embodiment, the four sidewalls of the housing  12  are tapered so as to allow a mating frame to snugly fit around the four sidewalls and the flexible membrane  14  to form a seal. In still another embodiment, the flexible membrane  14  is preformed whereby the flexible membrane  14  covers the recess in the housing  12  and downwardly extends across the four sidewalls until it forms a sealing lip over the bottom wall of the housing  12 . In one preferred embodiment, the chamber  24  is sealed at the interface between the flexible membrane  14  and the four sidewalls of the housing  12 . In other embodiments, where pressurization of the chamber is not required, the flexible membrane  14  does not form a seal with the housing  12 .  
      As is also shown in  FIG. 3 , the flip-chip  20  is geometrically aligned with the socket  10  in preparation for its temporary connection to the socket  10  during a testing sequence. The flip-chip  20  must be placed in contact with the socket  10  so as to allow the plurality of contact bumps  22  to contact the plurality of contact members  16  before a test can occur. The flip-chip  20  may be lowered or the socket  10  raised so as to achieve contact. In the preferred embodiment, the chamber  24  is filled with a fluid material, herein defined as a liquid or gas such as air, which may form a delta between the chamber pressure and ambient pressure at a steady state (i.e. pressurized), be sealed at ambient pressure at a steady state, or be open to ambient pressure during the test sequence. The pressure of the fluid material contained within the chamber  24  is controlled by adding or removing fluid material through a pipe  26  configured as a conduit between the chamber  24  and a device or the ambient environment.  FIG. 3  illustrates the location of the pipe  26  as extending from the bottom surface of the chamber  24  in the housing  12  and continuing through to the external surface of the housing  12  to thereby provide a conduit to the chamber  24 . It will be appreciated that the location of the pipe  28  is not confined to the embodiment shown in  FIG. 3 , as multiple locations of the pipe  26  may be used without deviating from the scope of the invention.  
      The pressure of the fluid material may be increased by adding fluid material through the pipe  26  into the chamber  24  in the direction of an arrow  28 . The pipe  26  may contain a releasable one-way valve (not shown) that prevents fluid from travelling in the opposite direction to the arrow  28  unless the valve is released, thereby allowing the fluid material&#39;s pressure to increase as more fluid material is added. Each pipe  26  may contain a fitting (not shown) that will allow an operator to attach a fluid material source such as an electric pump, hand-held pump, etc.  
      The operation of the socket  10  may be understood by reference to  FIG. 4  which shows the socket  10  adapting to the non-planar surface of the flip-chip  20  socket  10  during a testing sequence. As the flip-chip  20  is lowered, or socket  10  raised, at least one of the plurality of contact members  16  and at least one of the plurality of contact bumps  22  will come in contact with each other. Some of the plurality of contact bumps  22  and their associated contact members  16  will not form an electrical contact because of the non-uniform or non-planar shape of the plurality of contact bumps  22 . These variations may be on the same chip or between multiple chips.  
      The flexible membrane  14 , which permits the relative motion of each of the plurality of contact members  16 , will allow additional contact bumps  22  to form an electrical connection with the contact members  16 . As the flip-chip  20  continues to lower and contacts the socket  10 , the fluid material within the chamber  24  will create an upward force upon the surface of the flexible membrane  14  opposite to the surface facing the device to be tested. This upward force, which is coupled with the resiliency of the flexible membrane  14 , will help the unconnected contact members  16  rise up towards the appropriate contact bumps  22  when the other connected contact members  16  are pushed down toward the chamber  24 . This will occur when the pressure in the chamber  24  is dynamically increased by a reduction in the volume of the chamber  24  caused by the contacting contact members  16  as they are pushed into housing  12 . The fluid will try to equalize the pressure between the chamber  24  and ambient by bulging out in other areas of the flexible membrane  14 . This bulging out may help instigate contact between the unconnected contact members  16  and the associated contact bumps  22 , thus increasing the number of successful electrical connections between the plurality of contact members  16  and the plurality of contact bumps  22  on the flip-chip  20  or BOC, thereby improving the probability of a successful test of the chip  20 . The bendable nature of the flexible membrane  14  along with such factors as, for example, the initial pressure of the fluid material in the chamber  24 , whether the chamber  24  is sealed, and whether the fluid in the chamber  24  is compressible or incompressible, allows the plurality of contact members  16  to move relative to one another. Upon contact between the plurality of contact members  16  and the plurality of contact bumps  22 , the electrical signals are then conducted by the electrical connections  18  to the electrical test equipment (not shown).  
      The pressure of the fluid material inside the chamber  24  may be adjustable so that an operator can increase or decrease the pressure when the operator wants to change the characteristics of the socket  10 . In one embodiment, the pressure in the chamber  24  may be increased prior to contact between the flip-chip  20  and the socket  10 . In this or another embodiment, once an electrical connection is made between some of the plurality of contact members  16  and the associated plurality of contact bumps  22 , the pressure may then be increased to move the unconnected plurality of contact members  16  towards the unconnected plurality of contact bumps  22 . It may not be desirable to keep the pressure in the chamber  24  at a high level unless the particular test sequence requires it to be at a high level. Thus, after its use in a test, the operator may then desire to open a release valve which is in flow communication with the pipe  26  to lower the pressure of the fluid material back down to a nominal or selected pressure. Alternatively, when initially positioning the flip-chip  20  and socket  10 , the user may choose to lower the flip-chip  20  further towards the socket  10  rather than increasing the pressure of the fluid material. It may also be desirable to include guards (not shown) which prevent the flip-chip  20 , or BOC, from being lowered too far down toward the socket  10 , and thereby accidentally tearing the flexible membrane  14  or destroying its seal with the housing  12 .  
       FIG. 5  illustrates another embodiment of the socket  10 . This embodiment is substantially the same as the socket  10  of  FIG. 3 , except that several elastomer members  17  have been added. The elastomer members  17  are attached to the lower portion of each of the contract members  16  so as to each extend downward in substantially vertical direction from the associated contact member  16 . Each of the elastomer members  17  extend at least partially across the chamber  24 .  
      By reference to  FIG. 6 , it is seen that when the larger contact bumps  22  press downward on the contact members  16 , those contact members  16  which are displaced by the larger contact bumps  22  move downward across the chamber  24 . As the larger contact bumps  22  attempt to continue their downward travel, their associated elastomer members  17  contact an upper surface  11  of the base of the housing  12 , such as is shown at  19 . This surface  11  prevents further downward travel of the elastomer members  17  and their associated contact bumps  22  and provides support therefor. This prevents undesirably large travel distances for the flexible membrane  14  while still providing some compressibility so that contact between the other contact bumps  22  and their associated contact members  16  can occur. Thus, the embodiment of  FIGS. 5 and 6  prevents the possible tearing of the flexible membrane  14 , and destruction of the seal within the housing  12 .  
      Referring now to  FIG. 7 , an embodiment of a bumped socket  32  is shown which is substantially like that of  FIG. 3 . The two differ primarily on which of the mating surfaces includes the plurality of contact bumps  22 . A bumped socket  30  is shown with a device to be tested  32 , for example a non-bumped semiconductor chip  32  located above the bumped socket  30 . In this embodiment, the plurality of contact bumps  22  are attached to the plurality of contact members  16  on the bumped socket  30 , rather than on the device to be tested  32 . This configuration is operated in substantially the same manner as the embodiment described in  FIG. 4 . When the device to be tested  32  is lowered onto the bumped socket  30 , the flexible membrane  14  and pressure of the fluid material in the chamber  24  will allow the plurality of contact bumps  22  and the attached plurality of contact members  16  to raise or lower depending on their relative differences in height. The other components will function in a similar manner to the embodiment described in  FIG. 4 . This embodiment may allow a test to be conducted on a flip-chip, BOC, or other electrical device that does not have contact bumps  22  attached.  
       FIG. 8  is a cross-sectional view of another embodiment of the invention which allows the simultaneous testing of multiple semiconductor devices. An extended socket  34 , with a semiconductor wafer  36  in a BOC configuration positioned above the extended socket  34 , is shown. A semiconductor wafer is comprised of a plurality of semiconductor devices (not shown) prior to the devices being singulated from the wafer. This embodiment displays the extended socket  34  as configured to test an array of devices, for example, the semiconductor wafer  36 , which is shown mounted in a BOC configuration in which each individual semiconductor device on the semiconductor wafer  36  is mounted to a substrate  38 , such as a chip carrier, printed circuit board, or other surface mount device. In contrast to a flip-chip, the inactive circuit or backside of the semiconductor wafer  36  is attached to the substrate  38 . The exposed contact pads (not shown) on the side away from the substrate  38  are subsequently connected to the substrate  38  with the use of curved wire bonds (not shown). A glob top protective resin (not shown) is subsequently applied over each individual semiconductor device and the wire bonds.  
       FIG. 8  further shows the plurality of contact bumps  22  attached to the surface of the substrate  38  facing the contact members  16  on the flexible membrane  14  of the extended socket  34 . In this embodiment, there are a plurality of chambers  40  in a housing  42 , each of which are separated from the others by a grid of walls  44 . To create the plurality of chambers  40 , the flexible membrane  14  is attached to the upper surface of the grid of walls  44  and the four sidewalls of the housing  42 . In this embodiment, the flexible membrane  14  covers the plurality of chambers  40 . Alternately, the flexible membrane  14  may consist of a plurality of smaller flexible membranes (not shown) with each covering a portion of the grid of walls  44 . The lower surface of each of the plurality of chambers  40  is created by the bottom surface of the housing  42 .  
      The plurality of chambers  40  have individually controlled fluid material pressures with each being fed individually by a pipe  46  that extends through the housing  42  so as to form a conduit between the associated chamber  40  and the ambient environment. Alternatively, the plurality of chambers  40  may be fed from the same pipe  46 . Numerous other piping methods and designs known in the art may be employed to pump fluid into the plurality of chambers  40 . Of course, it will be appreciated that the flip-chip configuration can be readily substituted for the BOC configuration in this embodiment, such that the contact bumps on the flip-chip are aligned with the contact members  16  in the manner illustrated in  FIG. 6 .  
       FIG. 9  is a perspective view of the grid of walls  44  used in one embodiment of the invention described in  FIG. 8 . The grid of walls  44  forms the lateral walls of each of the plurality of chambers  40  in the embodiment shown in  FIG. 8 . Alternatively, the grid of walls  44  may be manufactured as part of the housing  42  rather than being a separate object.  
      The embodiment illustrated in  FIG. 8  may be used for the testing of an entire semiconductor wafer such as wafer level packages including flip-chip or selected groups of semiconductor devices in the wafer depending on such factors as, for example, the density of the semiconductor devices, the number of contact members  16 , and the availability of electrical connectors  18  or test equipment to process the test data. The plurality of chambers  40  associated with the semiconductor devices to be tested could be selectively pressurized. This entire semiconductor wafer level testing can also be employed for wafer level burn-in (WLBI). Here the entire wafer and socketing assembly are placed in a cartridge, such as that illustrated at  31  in  FIG. 10 , whose temperature can be controlled. Burn-in is typically done at 125° C. to 150° C. and so the wafer is subjected to these temperatures while secured in position within the cartridge  31 .  
      If desired, the illustrated clam shell configuration of the cartridge  31  permits application of pressure across the entire surface of the substrate  38  and socket  34 . Although two securing members such as clamps are shown defining the cartridge  3   1 , it will be appreciated that additional ones of these can be added to secure additional portions of the substrate  38  and socket  34  together. It will also be appreciated that the cartridge  31  can comprise a single member surrounding and sandwiching the substrate  38  and socket  34  together  
      Operation of the test procedure in accordance with one embodiment of the invention can be described with reference to  FIG. 11 . In particular, flow begins in start block  46 . Flow proceeds to block  48  where the chamber inside the housing is filled with a fluid. Flow continues to block  50  where the device to be tested is lowered onto flexible membrane covering the housing. Continuing to block  52 , where as some contact bumps contact the flexible surface and force the surface into the chamber, the resiliency of the flexible membrane coupled with the chamber pressure facilitates contact with the unconnected contact bumps. Flow proceeds to block  54  where the test on the device is performed. Finally, in block  56 , the device is removed from the test apparatus.  
       FIG. 12  illustrates an alternate embodiment of the method of  FIG. 11 . The embodiment of  FIG. 12  is substantially the same as the embodiment of  FIG. 11 , except that after completion of the acts of block  52 , the method moves to a block  53 A wherein it is determined if contact has been achieved between all contact bumps and the flexible surface. If such complete contact has not been achieved, the method moves to a block  53 B wherein pressure in the chamber is increased to bring the remaining contact bumps into contact with the flexible surface. From block  53 B, or from block  53 A if all contacts have been made, the method proceeds to block  54  and functions as defined with reference to  FIG. 11 . After completion of the acts of block  54 , the method moves to a block  55  wherein pressure in the chamber is reduced to ambient. Following completion of the acts of block  55 , the method moves to block  56  and functions as described previously with reference to  FIG. 11 .  
      Numerous variations and modifications of the invention will become readily apparent to those skilled in the art. Accordingly, the invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The detailed embodiment is to be considered in all respects only as illustrative and not restrictive and the scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.