Abstract:
A method for enhancing temporary solder ball connection comprises the application of thermal energy to the solder balls, heating them to a submelting “softening” temperature, whereby the compression force required to connect all balls in a BGA is achieved at much reduced force, avoiding damage to the package, insert, substrate and support apparatus. Several forms of heating apparatus, and temperature measuring apparatus are disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of application Ser. No. 10/651,664, filed Aug. 29, 2003, pending, which is a continuation of application Ser. No. 10/196,396, filed Jul. 15, 2002, now U.S. Pat. No. 6,614,003, issued Sep. 2, 2003, which is a continuation of application Ser. No. 09/892,156, filed Jun. 26, 2001, now U.S. Pat. No. 6,420,681, issued Jul. 16, 2002, which is a continuation of application Ser. No. 09/618,885, filed Jul. 18, 2000, now U.S. Pat. No. 6,329,637, issued Dec. 11, 2001, which is a continuation of application Ser. No. 09/145,832, filed Sep. 2, 1998, now U.S. Pat. No. 6,121,576, issued Sep. 19, 2000. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates generally to semiconductor chip packages. More particularly, the present invention pertains to methods for electrical contact of an array of solder balls with a noncompliant surface.  
         [0004]     2. State of the Art  
         [0005]     The testing of packaged semiconductor devices has always presented problems to device manufacturers. Various types of tests may be conducted at different stages of manufacture. In the current state of the art, “wafer sort” electrical tests may be conducted prior to packaging to determine nonworking dies. Following packaging, various tests including environmental tests as well as parametric and functional electrical tests may be performed. A final test which is known as “burn-in” may optionally be conducted. The test includes temperature cycling over an extended period of time. Essential to the testing of individual dies is reliable electrical connection of all die leads to the test board, without incurring damage to the die or testing apparatus, and easy disassembly from the testing apparatus. While “permanent” wire connections are widely used, wirebonding is time consuming and expensive, and also makes the matching of device impedance to the substrate impedance very difficult to achieve. Much effort is being spent on developing alternative methods to reduce the time and expense of using wire bonds. The replacement of wire bonds with ball grid array (BGA) connections is becoming more common. Temporary conductive attachment of solder balls to e.g., a test board is less than satisfactory.  
         [0006]     Temporary connection of device circuits to a test apparatus is known to present a variety of problems. The insert member into which a semiconductor die is placed for testing is typically noncompliant, i.e., ceramic or silicon, for example.  
         [0007]     The current method for joining a ball grid array (BGA) to a noncompliant, i.e., rigid surface such as a silicon micromachined pocket interconnect or insert, is to apply, at ambient temperature, a relatively high compression force of about 22-30 grams-force per solder ball. Theoretically, all balls of the array should be pressed into mechanical and electrical contact with the insert pocket. The use of compressive forces lower than the above results in a further increased frequency of unsatisfactory electrical connections.  
         [0008]     The presence of such unconnected solder balls in a BGA attachment formed under ambient conditions is believed to be due to a significant variability in ball diameter and “height” which the industry has been unable to eliminate. As a result, the applied force of about 22-30 grams-force or even more per ball is, in practice, insufficient to ensure the required contact of all balls of the array. Furthermore, the use of compression forces in excess of about 30 grams-force tends to damage the underlying material of the die, insert, and/or substrate. For example, effective connection of a 48 ball BGA array using solder balls of a nominal diameter may require in excess of about 1.5 kg-force. Such pressures exerted on a die for connection to a ceramic insert may damage the die and/or insert and/or substrate below the insert. The total force required for connection of larger arrays will be even more. In addition, the use of larger balls not only increases the absolute variation in ball diameter but the force required to sufficiently deform each ball for establishing the required temporary electrical connection. The problem also exists with smaller solder balls such as comprise a fine ball-grid-array (FBGA) of 0.0125 inches (0.325 mm) diameter balls, for example. With the smaller diameter solder balls, variation in ball placement location may have a greater effect than nonuniform ball diameters.  
         [0009]     To date, the industry has continued to use relatively high compressive forces and necessarily accepted the increased occurrence of electrical connection failures of a BGA and/or damage to the die, insert or substrate.  
         [0010]     Ball grid arrays are used in a variety of semiconductor devices. Illustrative of such prior art are U.S. Pat. No. 5,642,261 of Bond et al., U.S. Pat. No. 5,639,695 of Jones et al., U.S. Pat. No. 5,616,958 of Laine et al., U.S. Pat. No. 5,239,447 of Cotues et al., U.S. Pat. No. 5,373,189 of Massit et al., and U.S. Pat. No. 5,639,696 of Liang et al.  
         [0011]     Semiconductor devices having dual sets of outer “leads,” e.g., twin BGA surfaces or a combination of e.g., J-leads and solder bumps, are shown in U.S. Pat. No. 5,648,679 of Chillara et al., U.S. Pat. No. 5,677,566 of King et al., and U.S. Pat. No. 5,668,405 of Yamashita.  
         [0012]     Chip carriers of several configurations are described in U.S. Pat. No. 4,371,912 of Guzik, U.S. Pat. No. 4,638,348 of Brown et al., and Japanese publication 60-194548 (1985).  
         [0013]     Semiconductor devices joined in stacks are disclosed in U.S. Pat. No. 4,868,712 of Woodman, U.S. Pat. No. 4,841,355 of Parks, U.S. Pat. No. 5,313,096 of Eide, U.S. Pat. No. 5,311,401 of Gates, Jr. et al., U.S. Pat. No. 5,128,831 of Fox, III et al., U.S. Pat. No. 5,231,304 of Solomon, and U.S. Pat. No. 4,956,694 of Eide.  
         [0014]     U.S. Pat. No. 5,637,536 of Val discloses a chip stacking configuration with solder ball connections.  
         [0015]     U.S. Pat. No. 5,012,323 of Farnworth discloses a dual-die package having wire interconnections.  
         [0016]     U.S. Pat. No. 4,761,681 of Reid discloses a multi-chip device having elevated (conductor covered mesa) interconnections.  
         [0017]     Despite the advanced state of the art in lead interconnection, device packaging and testing, the temporary connection of semiconductor devices to testing apparatus and burn-in boards remains an area which needs improvement.  
       BRIEF SUMMARY OF THE INVENTION  
       [0018]     The present invention pertains to methods for electrical contact of an array of solder balls with a noncompliant surface, that is, the mechanical and electrical contact of a ball grid array (BGA) to a relatively noncompliant contact set such as a silicon micromachined pocket interconnect (i.e., “insert”) for a test pad or burn-in board (BIB).  
         [0019]     The present invention further provides a reliable BGA connection method and apparatus whereby the required pressure is much reduced to eliminate or significantly reduce compression-caused damage to the die, insert and/or substrate.  
         [0020]     The present invention comprises methods and apparatus for softening solder bumps or balls so that all of the bumps/balls in an array readily conform to a matching array of conductive contact pockets or pads in another body. The array of solder bumps/balls is heated to a softening temperature lower than the melting point of the solder and quickly placed in slightly compressed engagement with the contact pockets or pads of a substrate. As compared to joining the arrays at ambient temperature, all bumps/balls of the BGA are reliably connected, and the connection is achieved at a much reduced pressure, avoiding damage to the die and/or substrate. In addition, much less stress is placed on the apparatus holding the packaged die, the insert and test board.  
         [0021]     The softening temperature to which the solder is heated is below the melting temperature of the solder alloy.  
         [0022]     A variety of heating apparatus and methods is disclosed, including direct heating of the bumps/balls, heating of the entire assembly, heating of a chuck holding the IC, heating of a chuck holding the insert, direct heating of the insert or substrate, etc. A temperature sensing circuit may also be incorporated into the insert, substrate, or substrate retaining socket for the purpose of measuring and controlling the temperature to which the bumps/balls are heated.  
         [0023]     While electrical contact is readily maintained during electrical tests or burn-in by maintaining a small compressive force, ball contact is easily removed by discontinuing the compressive force and lifting the BGA from the insert or substrate to which it was electrically connected.  
         [0024]     The invention is applicable to a wide variety of solder compositions, solder bump designs and ball diameters.  
         [0025]     Other features of the invention will become clear from study of the following description and related figures. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0026]     The invention is illustrated in the following figures, wherein the elements are not necessarily shown to scale:  
         [0027]      FIG. 1  is a perspective view of an insert assembly for the electrical testing of a typical flip-chip semiconductor package with BGA, wherein heat enhancement of the BGA connection in accordance with the invention is shown;  
         [0028]      FIG. 1A  is an edge view of a ball grid array on a semiconductor chip;  
         [0029]      FIG. 2  is a perspective view of an insert assembly for the electrical testing of a typical flip-chip semiconductor package with BGA, showing the package in compressive engagement with the insert assembly for heating enhancement of the BGA connection in accordance with the invention;  
         [0030]      FIG. 3  is a plan view of a substrate member of the invention;  
         [0031]      FIG. 4  is a bottom view of a substrate member of the invention;  
         [0032]      FIG. 5  is a cross-sectional view of a portion of a heating assembly of the invention; and  
         [0033]      FIG. 6  is a cross-sectional view of various solder ball contact sites to which the invention may be applied. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     The present invention relates to method and apparatus embodiments for the uniform temporary electrical connection of solder bumps, e.g., solder balls, of a semiconductor device to another body. Rapid thermal softening of the solder bumps may be achieved by a variety of specific methods and apparatus, as described herein. The methods are particularly useful for attachment of solder bumps to the surface of a noncompliant body such as formed of silicon, ceramic, etc.  
         [0035]     As shown in drawing  FIG. 1 , a semiconductor package  10  is exemplified by a flip-chip package (FCP) with a ball grid array (BGA)  30  of a plurality of solder bumps or balls  12  on one surface  14  of the semiconductor package  10 .  
         [0036]     A test apparatus for evaluating circuit performance of the semiconductor package  10  is shown as including an insert  16  and a substrate member  18 . The insert  16  is noncompliant and is typically formed of ceramic or silicon with a pattern of electrical contact sites  20  micromachined on its upper surface  22 . The contact sites  20  may comprise simple planar pads, or contact pockets of any configuration, as explained infra. The contact sites  20  are connected by conductive traces, not visible, to bond pads  24 , the latter being connected by wire bonds  26  to conductive traces  28  on the substrate member  18 . The wire bonds  26  and conductive traces  28  on the insert  16  and substrate member  18  may be encapsulated in resin for protection. Other means for connecting the contact sites  20  to a controller for conducting a test, burn-in, etc., may be used, as known in the art.  
         [0037]     The substrate member  18  and attached insert  16  are typically inserted into a socket on a test fixture or a burn-in board (BIB), neither shown in drawing  FIG. 1 .  
         [0038]     In accordance with the invention, the ball grid array  30  of solder bumps/balls  12  is heated and compressed under a slight pressure into the contact sites  20 , shown here as indentations or pockets. The solder bumps/balls  12  are heated to a submelting softening temperature T s  and are uniformly contactable to the contact sites  20  by an increased deformation under the slight compression force.  
         [0039]     In one simple embodiment, an external heater  40  emitting infrared radiation or heated air  42  is positioned to heat the semiconductor package  10  including the solder bumps/balls  12  to the desired softening temperature, and the BGA  30  is quickly inserted and compressed by force  38  into engagement with the contact sites  20  at a relatively low pressure such as about 2-10 g-force per solder bump/ball  12 . Referring to drawing  FIG. 1A , of course, the required force per solder bump/ball  12  will vary, depending upon the softening characteristics of the particular solder composition used, the temperature to which the solder bumps/balls  12  are heated, the nominal ball diameter  32 , the maximum variation in ball diameter  32  and the variation in drop distance  34  between ball centers  34 A and the surface  14  of semiconductor package  10 . Typically, the required compression force  38  at the softening temperature T s  to achieve complete ball connection is about 8-25 percent of the force at ambient temperature.  
         [0040]     Instead of directly heating the semiconductor package  10  to soften the solder bumps/balls  12 , heat may be applied to the insert  16  or substrate member  18  before connecting the BGA  30  to the contact sites  20 . Also, the semiconductor package  10  may be indirectly heated by applying thermal energy to a chuck, not shown, which holds the package.  
         [0041]     As shown in drawing  FIG. 2 , a semiconductor package  10  with an array of solder bumps/balls  12  is placed on an insert  16 , and placed under a compression force  38 . Thermal energy is applied either to the back side  36  (as shown in  FIG. 1 ) of the semiconductor package  10 , to the insert  16 , to the substrate member  18  (as shown in  FIGS. 4 and 5 ), to a compression member, not shown, compressing the back side of the semiconductor package  10  with compression force  38 , or to a socket, not shown, which surrounds the substrate.  
         [0042]     Alternatively, the assembly of semiconductor package  10 , insert  16  and substrate member  18 , together with compression and support apparatus, may be placed in a temperature controlled oven and rapidly heated to the desired softening temperature T s .  
         [0043]     Thus, the solder bumps/balls  12  may be heated by conduction, convection or radiation, or any combination thereof. For example, an external heater  40  ( FIG. 1 ) may heat the semiconductor package  10 , insert  16 , substrate member  18 , or a socket  66  into which the substrate member  18  fits by radiation or heated air  42 .  
         [0044]     The solder bumps/balls  12  may be of any diameter  32 , including those of a fine ball grid array (FBGA), where the balls have a pitch of less than one (1) mm.  
         [0045]     The solder bumps/balls  12  may be formed of various solder compositions, including tin-lead solders having a lead content of about 30 to 98 percent. Solder compositions having the higher lead concentrations often have a higher melting point.  
         [0046]     A softening temperature Ts of about 130° C. to about 180° C. has been found useful for reducing the compression force  38  to a relatively low value and simultaneously ensuring electrical contact of all solder bumps/balls  12 .  
         [0047]     As shown in drawing  FIG. 3 , resistive heating elements  44  may be applied to the top surface  48  of the substrate member  18 , preferably under the insert  16  and substantially beneath the semiconductor package  10 . The heating elements  44  are shown as having heater power leads  54 ,  56  for providing sufficient power to quickly heat the insert  16  including the electrical contact sites  20 , not shown, and the solder bumps/balls  12 , not shown, which are in engagement with the contact sites  20 .  
         [0048]     All of the conductive traces on substrate member  18 , including conductive traces  28 , heater power leads  54 ,  56 , and heating elements  44  may be formed simultaneously by screening a thick film of conductive material onto the substrate member. This method of forming conductive traces on a surface is well known in the art.  
         [0049]     A thermocouple junction  50  or other temperature detecting device may be installed in or on the insert  16  or substrate member  18  for obtaining temperature feedback and controlling the bump/ball temperature to attain a maximum desired softening temperature T s . Thus, for example, as shown in drawing  FIG. 3 , a temperature sensor  50  (such as a thermocouple junction) may be fixed on the top surface  48  of the substrate member  18  or back side  52  ( FIGS. 1 and 2 ) of the insert  16 , and have thermocouple leads  58  connected through otherwise unused conductive traces  28 A,  28 B to measurement/control instrumentation, not shown. In use, a heater controller, not shown, determines the measured temperature and shuts off (or reduces) power to the heating elements  44  upon sensing a predetermined temperature. A recorder, not shown, may be used to calibrate the measurements such that a desired softening temperature may be precisely attained.  
         [0050]     A short heating time is preferred, extending only several seconds or less. Most preferably, the heating time is less than one second. Thus, the heater power leads  54 ,  56  to the heating elements  44  must be sufficiently large to carry the necessary electrical load. In general, installation of the heating elements  44  on the insert  16  will require separate heater power leads  54 ,  56 . Normally, wire bonds  26  ( FIG. 1 ) are incapable of carrying the necessary load.  
         [0051]     Another form of heating apparatus which may be used in the invention is illustrated in drawing  FIGS. 4 and 5 . The substrate member  18  has on its back side (underside)  46 , as shown in  FIG. 2 , a pattern of heating elements  44  with junctions  62 ,  64 . The junctions  62 ,  64  may be planar pads or conductively surfaced indentations in the back side  46 .  
         [0052]     As shown in drawing  FIG. 5 , a semiconductor package  10 , insert  16 , and substrate member  18  are positioned in a socket  66  on a test board  70 . Test board  70  may be a board for an electrical test, for burn-in, or other purpose. The socket  66  is typically formed with walls  68  and base  72 , and many sockets  66  may be mounted on a single test board  70  to enable simultaneous testing or burn-in of many semiconductor packages  10 .  
         [0053]     A pair of through-holes  74 ,  76  is formed in the test board  70  along axes  84 ,  86 , and the axes which pass through junctions  62 ,  64 , respectively. Two metal spring-loaded compression pins  80 , also known as “pogo pins,” are mounted in the test board  70  or in another substrate  90  underlying the test board  70 . Substrate  90 , having a plurality of pogo pins  80  projecting therefrom, is known as a bed-of-nails (BON). The pogo pins  80  have a base  78  and a spring-loaded pin  82  which is axially movable relative to the base  78 . The spring-loaded pins  82  are shown passing through-holes  74 ,  76  to electrically contact the junctions  62 ,  64  when in compression, power leads  92 ,  94  from the two pogo pins  80  providing sufficient electric power to the heating elements  44  for rapidly heating the solder bumps/balls  12 . Following testing, the spring-loaded pogo pins  80  will push the substrate member  18  from the socket  66  with a short stroke.  
         [0054]     In drawing  FIG. 6 , several types of BGA contact sites  20  are shown as examples illustrating the wide variety of solder bumps/balls  12  and contact sites  20  combinations whose temporary connection is enhanced by use of an elevated submelting softening temperature T s . Each solder bump/ball  12  attached to semiconductor package  10  is configured to be in compressive conductive contact with a contact site  20 .  
         [0055]     Contact site  20 A comprises a flat pad or surface of the insert  16 .  
         [0056]     Contact site  20 B is a spherical indentation in the insert  16 .  
         [0057]     Contact site  20 C is a shallow spherical indentation.  
         [0058]     Contact site  20 D is a spherical indentation having a central axially directed projection  96  which punctures and enters the softened solder bump/ball  12 . Preferably, the projection  96  is pyramidal in shape.  
         [0059]     Contact site  20 E is a spherical indentation having several, typically four, peripheral projections  98  which contact and are forced into the circumferential surface of the solder bump/ball  12 .  
         [0060]     The illustrated contact sites  20  to which the invention may be applied are exemplary only and not exhaustive.  
         [0061]     It is clear that a wide variety of apparatus may be used for heating ball-grid-array connections, of which those described herein are representative.  
         [0062]     The invention has been illustrated in application to the testing of a flip-chip device. However, the temporary BGA connection of any device, including other chip scale packages (CSP), is enhanced by this process and apparatus.  
         [0063]     It is apparent to those skilled in the art that various changes and modifications, including variations in heating procedures and structures, may be made to the BGA connection method and apparatus of the invention as described herein without departing from the spirit and scope of the invention as defined in the following claims.