Abstract:
The present invention relates to improvements in forming and transferring solder bumps for use in mounting integrated circuit substrates on chip carrier packages. A mold having cavities for the solder bumps is held in contact with a substrate and a compressible device. As the temperature is increased to melt the solder in the cavities, at an appropriate time and temperature, the compressible device is caused to decompress resulting in the mold separating from the substrate and leaving formed solder bumps on the contacts on the substrate. Various mechanisms are described to cause the force holding the mold and substrate together to decrease.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     This invention relates to electronic packages for integrated circuits, including methods and apparatus related to manufacturing, and in particular, electronic chip carrier packages with solder bump electrical connections between chip substrate pads and the chip carrier package. 
     2. Background of the Invention 
     Current manufacturing techniques utilize two primary methods for coupling integrated circuit chip substrates to chip carrier packages. The first is wire bonding, whereby each of the I/O pad terminals on a chip substrate is sequentially wired to corresponding pads on a chip carrier package. The second method is flip chip attachment (FCA) in which all the I/O pads on a chip are first terminated with a solder material. The chip is then flipped over and the solder bumps are aligned and reflowed in a reflow furnace to facilitate all of the I/O connections with bonding pads on the chip carrier package. An advantage achieved by the flip chip process is its suitability to high I/O density and greater reliability of the interconnections—formed as compared to wire bonding processes. 
     There are a variety of methods presently used to form solder bumps on a chip substrate. Often, the formation of solder bumps is carried out by a method of evaporating lead and tin mixtures through a mask for producing the desired configuration of solder bumps on the chip substrate. Techniques of electrodeposition of such mixtures have also been used to produce solder balls in flip chip packaging. 
     Another popular technique is the process of solder paste screening. However, the continued evolution of integrated circuit manufacturing processes toward progressively higher density circuits necessitates a correspondingly higher I/O pad density and tighter pitch constraints for pad terminals. For current processes, it is not unusual for a design to contain more than 1000 I/O pads. As a result, the solder paste screening technique becomes less practical to implement. Moreover, since the solder paste is normally applied directly to the substrates through a screen mask which contains holes aligned to the paste receiving pads on the substrate, any problems occurring during the process may also result in substantial rework of the substrate, thereby increasing the probability of damage to the substrate and significantly impacting manufacturing throughput. 
     A more recently developed injection molded solder technique dispenses molten solder instead of solder paste. An advantage of this process results from very little volume change occurring between the molten solder and the resulting solder bump. This process is typically practiced by first filling with solder a mold containing holes or cavities aligned to the pads on the substrate. Next, the filled mold is placed in close proximity to a substrate which contains solder receiving pads and onto which flux material has typically been dispensed in a thin layer over the substrate. When the solder in the mold is heated to a melting temperature in a furnace, surface tension reduction causes the solder to ball up and intimately contact the solder receiving pad, which is normally covered with gold or other solder wetting alloy. A wiper may be used after the molten solder fills the holes to remove excess solder. However, when this technique is used on large substrates, the balling up of the solder may be insufficient to ensure intimate contact between the solder in the mold cavities and solder receiving pads on the substrate and thus the solder balls may not adequately adhere to the substrate contact pads. 
     One prior art technique for overcoming the difficulties of known processes in forming solder bumps for integrated circuit to package interconnections is described in U.S. Pat. No. 6,003,757 entitled “Apparatus for Transferring Solder Bumps and Method of Using,” issued Dec. 21, 1999 and commonly assigned to International Business Machines Corporation. This patent describes a method and apparatus to maintain a solder mold in intimate contact with the solder receiving substrate, for example a semiconductor wafer, during a solder reflow operation such that the solder in the mold is transferred to solder wettable pads on the receiving substrate. A uniform pressure on the mold substrate assembly is necessary to ensure that all solder segments from the mold cavities are able to contact all solder wettable pads on the substrate at the time that the solder becomes molten. As described in U.S. Pat. No. 6,003,757, the apparatus applies such a uniform pressure until physical disassembly by human intervention as when opening the lid of the clamshell assembly of the apparatus releases the pressure. Such human intervention must occur after the mold-substrate assembly has exited the reflow furnace and cooled. Due to the pressure, the molten solder has maintained the shape of the mold cavity in which it was located and in effect, is somewhat adhering to most or all of the mold cavity surfaces. Although this is not a metallurgical bond in the sense of the solder-substrate pad interface, which is a strong metallurgical bond, separating the mold from the cavity nonetheless requires a certain tensile force and care must be taken to avoid any shearing motion. Both of these latter conditions risk unintended separation of the metallurgical bond between the solder and substrate pad. To reduce such risk, the mold-substrate assembly is subjected to a second solder reflow operation after the uniform pressure has been physically released. At this stage without any compressive forces present and with gravitational forces minimized by orienting the assembly such that the lighter substrate is on top, the remelted solder, now metallurgically bonded to the substrate pad, will tend to partially ball up, thus forcing the mold and the substrate to partially separate and facilitate physical separation of the two. Once successfully separated, it is often desired to have perfectly rounded solder bumps on the substrate in order to optimize subsequent assembly operations, suggesting yet another solder reflow operation of the substrate alone. 
     SUMMARY OF INVENTION 
     The present invention relates to reducing the difficulties in forming and transferring uniform solder bumps from a grid array solder mold containing individual solder elements to an integrated circuit substrate. The present invention provides techniques to alleviate the above described deficiencies and drawbacks in the prior art. 
     The apparatus and method of the present invention provides for the removal of intimate contact between the mold and the substrate at a prescribed point in the process during the reflow operation. By doing so, the transfer of solder from a mold to solder wettable substrate pads of a substrate, the separation of mold from the substrate and the uniform balling shape of the transferred solder bumps on the substrate, can be accomplished by a single solder reflow process instead of two or three as may be required in the prior art. 
     According to one aspect of the present invention, a method is provided of transferring solder bumps from a mold to a substrate having a plurality of pads by providing a base member and a substrate located thereon and positioning a mold having a plurality of solder elements on said substrate such that each solder element contacts a corresponding pad on the substrate and the mold contacts at least one compressible device located on the base member. The mold is caused to compress the compressible device and the solder elements are heated such that each solder element melts and metallurgically bonds to a corresponding substrate pad. The compressible device is caused to decompress and thereby separate the substrate and the mold while the solder elements are still molten. As the solder cools and hardens, each solder element remains on its corresponding substrate pad and forms a semi-spherical solder bump. 
     According to another aspect of the present invention, an apparatus is provided for transferring solder bumps from a mold to a substrate having a plurality of pads where the apparatus has a base member and a substrate located thereon. A mold having a plurality of solder elements is positioned on the substrate such that each solder element contacts a corresponding pad on the substrate and the mold contacts a compressible device located on the base member. A compressive force is applied to the mold causing the compressible device to contract. A reflow heating element melts the solder elements and causes each of the solder elements to transfer to a corresponding pad. A compressive force is applied to the mold thereby decompressing the compressible device and causing the substrate and mold to separate while the solder elements are molten resulting in each solder element remaining on a corresponding substrate pad in the form of a semi-spherical solder bump. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other advantages of the present invention will be better understood with reference to the following drawings wherein like reference numbers represent like elements of the invention embodiments: 
         FIG. 1  illustrates an isometric drawing of a transfer fixture apparatus for mounting and holding a substrate wafer or similar object to which solder bumps are transferred in accordance with a preferred embodiment of the present invention; 
         FIG. 2  shows a cross-sectional view of the apparatus shown in  FIG. 1  as indicated by the arrows A—A, showing more details of the compressible devices of the transfer fixture apparatus and the mechanisms to compress and decompress the compressible devices, according to a preferred embodiment of the present invention; 
         FIG. 3  illustrates the positioning of the elements of a preferred embodiment of the invention prior to heat being applied to melt the solder slug of the transfer fixture apparatus; 
         FIG. 4  illustrates the positioning of elements as in  FIG. 3  wherein the temperature has been raised to cause the solder slug to melt; and 
         FIG. 5  shows the positioning of elements as in  FIGS. 3 and 4  wherein the mold has been separated from the substrate. 
     
    
    
     DETAILED DESCRIPTION 
     The preferred embodiments of the present invention disclose methods and apparatus that provide for opposing forces to occur within a transfer assembly to thereby enhance the transfer of solder bumps from a mold to a substrate. A backing plate and lid member of the transfer fixture continue to provide a compressive force to a mold and substrate assembly while a base member is provided with compressible devices forced against the mold outside of the interface area between the mold and substrate. While the compressive forces imparted by the backing plate and lid member exceed the decompressing forces of the compressible devices, the mold and substrate remain in contact with each other. As soon as the compressive forces resulting from the backing plate and lid member are eliminated or sufficiently reduced, the decompression of the compressible devices results in the mold moving upward and away from the substrate. In order to achieve the advantages of the subject invention, in a preferred embodiment, a thermally ductile buffer is provided to eliminate or sufficiently reduce the forces applied to the lid member and backing plate while the mold, substrate assembly and transfer fixture mechanism are still in a reflow furnace and the solder bumps to be transferred are still in a molten state. 
       FIG. 1  is a perspective view of transfer fixture  10  for implementing a preferred embodiment of the present invention. Fixture  10  comprises a moveable lid  11  attached to base member  12 , with hinge  20 . Lid  11  may be of any appropriate design and shape and as shown in  FIG. 1 , lid  11  in the shape of the letter “H” is suitable. A compressing pin  15  is attached to lid  11 , which will be described with reference to  FIG. 2 . Mold  13  overlays a substrate or wafer on which solder bumps are to be formed. The solder bumps are transferred and attached to wettable receiving pads on the substrate. The wafer or substrate is not readily shown in  FIG. 1  but will be apparent and further described with reference to subsequent drawing figures. Backing plate  14  is configured to uniformly exert pressure on mold  13 . Any appropriate design of backing plate  14  may be employed as long as substantially uniform pressure is applied to mold  13 . The pressure is initiated by forces resulting from compressing pin  15  which applies pressure to backing plate  14  through a thermally ductile buffer  16  which may be physically altered by the application of heat. In the preferred embodiment of the invention, the thermally ductile buffer  16  may be a slug of solid solder with a higher melting temperature than the solder used to form the solder bumps on the substrate. 
       FIG. 2  illustrates a cross-sectional view of the preferred transfer fixture of  FIG. 1 . Substrate  17  is placed on base member  12  over which mold  13  is positioned. Mold  13  is shown abutting against compressible devices  18 . Compressible device  18  includes one or more devices suitably positioned between mold  13  and base  12  of fixture  10 . Compressible device  18  could be, for example, a spring. When force is applied to compressing pin  15 , mold  13  contacts substrate  17  and compressible device  18  is compressed. Mold  13  has a plurality of cavities in which solder elements have been placed and these solder elements in turn contact pads on the surface of substrate  17 . As shown in  FIG. 2 , when lid  11  is closed against base  12 , compressing pin  15  is forced against solder slug  16 , which in turn causes pressure to be applied to backing plate  14  and mold  13 , thereby compressing compressible devices  18 . 
     Compressing pin  15  is preferably implemented with a spring-loaded pin. An interference fit is designed between pin  15  and lid  11 . When lid  11  is closed against base member  12 , a force is transmitted from pin  15  to subsequent elements below pin  15 , namely solder slug  16 , backing plate  14 , mold  13 , substrate  17  and base member  12 . The interference fit will cause spring-loaded pin  15  to compress thereby exerting a force against slug  16  and in turn against backing plate  14  and against mold  13  which causes compressible devices  18  between mold  13  and base  12  to compress. 
     Compressing pin  15  and compressible devices  18  are designed to provide sufficient compressive forces to ensure adhesion of the solder bumps to the wetted surface of the substrate pads. Material selection of backing plate  14  is such that it is non-wettable to the solder of solder slug  16 . Fixture  10  also comprises an appropriate mechanical device (not shown), vacuum or otherwise, to hold substrate  17  against base  12 , such that during separation of mold  13  from substrate  17 , the surface tension forces of the molten solder between said mold  13  and said substrate  17  are overcome and substrate  17  remains on base  12 . 
     In summary, with reference to  FIGS. 1 and 2 , lid  11  contains compressing pin  15  which abuts against backing plate  14  by means of a slug of solid solder  16  between pin  15  and backing plate  14 . Backing plate  14  in turn abuts against the top side of the assembly of mold  13  and substrate  17 . Thermally ductile solid solder element or slug  16  is made of a material with a higher melting point than the solder elements used in cavities of mold  13  yet a lower melting point than the peak temperature that is achieved in the reflow temperature profile as appropriately selected for the solder elements in mold  13 . Thus, solder slug  16  will only melt after the solder elements in mold  13  to be transferred have melted and wetted the solder wettable pads of substrate  17 . When solder slug  16  melts, it can no longer act as a solid interface between compressing pin  15  and backing plate  14  and hence backing plate  14  no longer exerts a significant compressive force against the assembly of mold  13  and substrate  17 . At this point, compressive device or devices  18  located between base member  12  and mold  13  will force mold  13  upward, thus separating mold  13  from substrate  17 . However, substrate  17  is held in place on base member  12  for a sufficient length of time whereby the transferred solder balls have wetted the pads on substrate  17  and are still molten. When the cavities of mold  13  no longer restrict the shape of the transferred solder bumps, the solder bumps are free to revert to their lowest energy shape which tend towards spherical. The shape is determined by the existence of metallurgical bonds between each one of the solder bumps and a solder wettable pad on substrate  17  upon which the solder bump is affixed. 
     Examples of selections for the solder alloy constituting solder slug  16  will now be described. If the solder bumps to be transferred to substrate  17  are made of eutectic tin/lead (63% Sn, 37% Pb) with a melting point of 183 degrees C., typical solder reflow profiles may have a maximum peak temperature of 215 to 230 degrees C. For the purpose of this example, the average peak reflow temperature is assumed to be 225 degrees C. Accordingly, one possible alloy for solder slug  16  is Sn 3.5 Ag 0.7 Cu (hereinafter referred to as SAC), which melts at 217 degrees C. When the solder reflow temperature profile reaches 183 degrees C., the solder bumps will start to melt and wet the corresponding pads on substrate  17 . As the solder reflow temperature profile reaches 217 degrees, the SAC solder slug  16  will melt. This releases or decreases the compressive forces between pin  15  and backing plate  14  and therefore mold  13 . Compressible devices  18  between base member  12  and mold  13  then force mold  13  upward and away from substrate  17  which is being held on base  12 . As mold  13  moves away from substrate  17 , backing plate  14  is similarly caused to move upward, forcing the liquid SAC solder slug  16  to flow around compressive pin  15 . This process continues as the solder reflow temperature profile of the solder bumps rises to a peak of 225 degrees C. and then descends to 217 degrees C. At 217 degrees C., the SAC solder slug  16  solidifies in its position around compressive pin  15  with backing plate  14  and mold  13  still being pushed upward position by the compressible devices  18  on base member  12 . The eutectic solder bumps remain molten at 217 degrees C. and are no longer restricted in shape by the cavities of mold  13 . Instead, the solder bumps attach to the pads of substrate  17  and tend to ball up into a semi-spherical shape because of their adherence to respective pads on the substrate. The solder bumps will subsequently solidify in this shape as the temperature profile descends below 183 degrees C. 
       FIGS. 3 ,  4  and  5  illustrate various stages of the formation of the solder bumps. Note that substrate  17  is shown mounted on foam  34  on base  12  in all three figures. Foam  34  is optional in realizing the beneficial results of the invention, but is found to be useful in achieving a more uniform pressure being applied between mold  13  and wafer or substrate  17 . In any case, as previously described, substrate  17  is appropriately held against base  12 . Mold  13  has a plurality of cavities  30  containing solder elements  31 . Substrate  17  has a plurality of solder wettable pads  32  corresponding to each of the cavities  30  of mold  13 .  FIG. 3  shows the initial position of the various elements of apparatus  10  once lid  11  has been appropriately attached to fixture frame  33  and compressing pin  15  applies pressure to backing plate  14  through solder slug  16 . At a temperature range up to 183 degrees C., at which point solder elements  31  begin to melt, solder elements  31  in mold cavities  30  have not yet wet pads  32  on substrate  17 . However, as the temperature profile increases, solder elements  31  begin to melt and the temperature rises from 183 degrees C. to 217 degrees C., the arrangement as shown in  FIG. 3  continues to exist, but solder elements or bumps  31  in mold cavities  30  have melted and wet the respective pads  32  on substrate  17 . 
     With reference to  FIG. 4 , the diagrammatic representation of transfer fixture  10  is shown as the temperature rises above 217 degrees C. At this point solder slug  16  melts and begins to flow around compressing pin  15 , thereby reducing the compressive forces applied by lid  11  through pin  15  on backing plate  14  and mold  13 . This results in backing plate  14  and mold  13  being pushed upward by compression devices  18 . As can be seen, compression devices  18  are elongated and decompressed in  FIG. 4  when compared with the illustration in  FIG. 3 . 
     With reference to  FIG. 5 , the diagrammatic representation of fixture  10  is shown at the temperature between 217 degrees C. and 225 degrees C. Solder bumps  31  are no longer restrained by cavities  30  in mold  13  but are wetted to pads  32  on substrate  17  and begin to ball up as shown. Once fixture  10  is removed from the reflow furnace and the temperature profile is allowed to descend below 183 degrees C., solder bumps  31  will solidify while remaining attached to pads  32  on substrate  17 . Solder bumps  31  will cure and harden in the spherical configuration shown in  FIG. 5 . 
     Other methods and structures could be used to achieve similarly intended results of eliminating and reducing the compressive forces applied by lid  11  in fixture  10  at the appropriate time where solder elements  31  from mold  13  have transferred to pads  32  on substrate  17  while they are still in a molten state. These alternative embodiments would be apparent to one of ordinary skill in this art. For example, a spring mechanism for imparting compressive forces on backing plate  14  resulting from compressing pin  15  could be designed to diminish the applied forces at a prescribed time, where the time is correlated to the time dependent temperature profile of the reflow furnace. Alternatively, the release or decrease of the applied force could be affected at a prescribed temperature by the use of a temperature sensor. In addition, the spring mechanism could be made of a material that has a temperature-dependent spring constant such that at the desired temperature the spring constant is sufficiently low that the compressing pin  15  exerts little or no compressive force on backing plate  14  and mold  13 , permitting the compressible devices  18  to decompress and move mold  13  away from substrate  17  as described above. 
     While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention. The scope of the invention should be limited only by the language of the claims which follow.