Abstract:
The present invention is drawn to a method of making solder bump interconnections or BGA ranging from chip-level connections to either single chip or multichip modules, flip-chip packages and printed circuit boards connections. According to the method of the present invention, a die wafer or a substrate with a conductive contact location is positioned in close proximity and aligned with a mold wafer having a pocket corresponding to the contact location of the die wafer. A source of a molten solder is also provided which interconnects with the mold wafer. The molten solder from the source is introduced into the pocket of the mold wafer such that the molten solder wets the contact location aligned with the pocket. Before the molten solder inside the pocket is allowed to solidify, the die wafer and the mold wafer are separated from each other. Some of the molten solder is attracted to the contact location of the die wafer and it separates from the remaining molten solder in the pocket so as to assume the shape of a partially sphere. The method of the present invention is especially useful for batch processing in the formation of a large number of solder balls.

Description:
RELATED APPLICATIONS  
       [0001]    This is a continuation of U.S. patent application Ser. No. 09/143,996, filed on Aug. 31, 1998, entitled “Method of Forming a Solder Ball”, which is incorporated herein by reference.  
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. The Field of the Invention  
           [0003]    The present invention is directed to semiconductor device assemblies. More specifically, the present invention is directed to a method for forming connections, such as ball grid arrays, on a semiconductor chip, substrate or semiconductor package.  
           [0004]    2. Background of the Invention  
           [0005]    As integrated circuits become larger and more complex, the demand for the increased number of external connections on chips, semiconductor substrates and packages has rapidly grown. The most common methods for connection of the packaged devices to external systems are wire bonding, tape automated bonding and solder bumping. Among these technologies, solder bumping provides the highest packaging density with less packaging delay. Therefore, solder bumping or creation of ball or bumped grid arrays (BGAs) is rapidly developing as the technology of choice for high input/output count integrated circuits. A BGA design is part of an integrated circuit package in which the input and output points are solder bumps (or balls) arranged in a grid pattern. BGA designs are used in various package configurations, including a single chip or multichip applications.  
           [0006]    Solders have been commonly used as interconnecting material for electronic packaging and assembly. A typical ball grid array package has a large number of solder balls or bumps disposed on a surface of the package. The package surface is usually formed from an electrically insulating material with a pattern of metalized pads disposed on it. These pads connect to circuity of a semiconductor device within the package. BGA packaging technology offers superior thermal, electrical, and size performance compared to other existing package technologies. One challenge affecting BGA technology, however, is to achieve the interconnect density required to route area array devices or packages, at a cost compatible with the commercial marketplace. Another concern is uniformity of the balls across a semiconductor device during fabrication of solder ball arrays. Finally, a badly bonded ball or voids in the solder balls can lead to faulty operation of the device.  
           [0007]    Several different methods have been developed for forming ball grid arrays or solder bumps. Many of these methods use preformed solder balls and place these preformed balls on the package contact pads. For example, some methods use reflow soldering which is a process for joining parts to a substrate by depositing solder paste, placing parts (for example, preformed solder balls), heating until the solder fuses, and allowing them to cool in a joined position. Since it is very difficult to ensure that the correct amount of paste is applied to the electrical contacts, these methods are complicated, time consuming and not cost-effective.  
           [0008]    As an alternative, a solder paste is disposed on the contact pads in specifically measured quantities. Thereafter the paste is heated and while it melts, it assumes the spherical shapes to form the ball grid contacts. This method also has a number of disadvantages because of the difficulties with measuring the exact quantities of solder paste and dealing with the elasticity, viscosity and other important characteristics of the paste. The smaller contact pads and narrower spacing between the contact pads require a higher level of precision in solder paste deposition and in component placement, and require higher quality and consistency in solder paste. Moreover, the shape and the size of the resulting balls are often not uniform because it is difficult to control the wettable exposed area of the contact pad covered by the solder paste while it is melting. Therefore, this and similar methods inevitably result in a complex manufacturing process with rigorous process control and the resulting higher costs.  
           [0009]    Another conventional method of creation of the solder bumps is by evaporation. Evaporation does not create uniform solder ball heights and can be time-consuming. Similarly, wire bonding with solder has not been able to yield consistent ball size and adhesion strength.  
           [0010]    One example of the method for forming raised bump contacts on a substrate is disclosed in the U.S. Pat. No. 5,381,848 to Trabucco which is fully incorporated herein by reference. In that patent, a substrate upon a surface of which the solder bumps are formed is inserted into a two-halves mold with a number of the specially formed recesses. The molten solder thereafter is introduced into the recesses. Upon cooling inside the recesses, molten solder solidifies and forms bumps on a substrate in the shape of the recesses of the mold. Only after solidification of the solder, the substrate is removed from the mold. Another invention that requires the molten solder to solidify before the device is removed from the mold is disclosed in the U.S. Pat. No. 5,244,143 to Ference at al. Both of the above-mentioned inventions still require strict quality and process control and inherent maintenance of the mold, including the verification of the precise formation of each recess in the mold to assure the identical shape and size of the resulting solder bumps. Any imperfections in the shape or dimensions of the recesses of the mold result in the nonuniform balls, and thus, potentially defective contacts. Moreover, some damage to the bumps may occur while opening the mold and removing the device from the mold.  
           [0011]    Therefore, with the requirements of the increased complexity and density of the BGAs, there is an increased need for a method for forming solder bumps that is simple, cost-effective and time-effective. There is also a need for a method that allows for batch processing of the highly uniform solder balls that requires relatively low investment in manpower and complicated equipment. It would be also advantageous to create a method of forming solder bumps that lends itself to high-volume production, and that has a low defects rate and a higher yield.  
           [0012]    In light of the foregoing, it would be an advantage in the art to devise a time- and cost-effective solder ball formation process that forms solder balls having substantially uniform dimension, preferably in batch quantities, on a substrate.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention is drawn to a method of making solder bump interconnections ranging from chip-level connections to either single chip or multichip modules, flip-chip packages and printed circuit board connections. The method of the present invention is especially useful for batch processing.  
           [0014]    In the microelectronics industry, a “substrate” refers to one or more semiconductor layers or structures which include active or operable portions of semiconductor devices. The term “semiconductor substrate” is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in assemblies comprising other materials. A semiconductor device refers to a semiconductor substrate upon which at least one microelectronic device has been or is being batch fabricated. In the context of this document the term a “die wafer” is intended to encompass “a substrate,” “a semiconductor substrate,” “a semiconductor device,” a printed circuit board, and various packages for various levels of interconnection.  
           [0015]    In accordance with the invention as embodied and broadly described herein, there is provided a die wafer or a substrate having a conductive contact location on its surface. This die wafer or substrate is placed in close proximity and aligned with a mold wafer having a pocket corresponding to the conductive contact location of the die wafer. A source of a molten solder is also provided to the mold wafer. The molten solder from the source is introduced into the pocket of the mold wafer such that the molten solder wets the conductive contact location aligned with the pocket. The molten solder that wets the conductive contact location is attracted and adheres to the conductive contact location. Before the molten solder inside the pocket is allowed to solidify, the die wafer and the mold wafer are separated from each other. The molten solder that adheres to the conductive contact location of the die wafer separates from the remaining molten solder in the pocket of the mold wafer. Upon separation from the remaining molten solder in the pocket of the mold wafer, natural forces act upon the molten solder that adheres to the conductive contact location so that it assumes a partially spherical shape. The molten solder in the partially spherical shape then solidifies to form the solder bump at the conductive contact location on the die wafer.  
           [0016]    According to one aspect of the present invention, a large number of the uniform solder bumps can be simultaneously created on respective conductive contact locations on a substrate or die wafer. The present invention achieves the goal of simplicity. It is cost and time effective, and does not require a complex manufacturing process. Moreover, the present inventive method furthers uniformity of the resulting ball grid array due to the uniformity of natural forces acting upon the molten solder at each conductive contact location so as to form substantially uniform partially spherical solder balls thereat.  
           [0017]    Other features and advantages of the invention will become apparent in light of the following description thereof  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    In order that the manner in which the above-recited and other advantages of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:  
         [0019]    [0019]FIG. 1 is a cross sectional schematic representation of the apparatus for forming solder bumps according to the present invention, the apparatus including a die wafer having a conductive contact location upon a surface thereof, and a mold wafer having a pocket corresponding to the conductive contact location of the die wafer.  
         [0020]    [0020]FIG. 2 is a cross sectional schematic representation of the apparatus seen in FIG. 1, wherein the pocket of the mold wafer corresponding in alignment to the conductive contact location of the die wafer are brought in close proximity, and molten solder is introduced into the pocket of the mold wafer so as to wet to the conductive contact location.  
         [0021]    [0021]FIG. 3 is a cross sectional schematic view of an alternative embodiment of the inventive method wherein the die wafer has a conductive contact location that is recessed such that its lowermost surface and the lowermost surface of the die wafer are substantially flush.  
         [0022]    [0022]FIG. 4 a is cross sectional schematic view of the apparatus of FIG. 2, further showing separation of the mold wafer and die wafer, where a portion of the molten solder in the pocket adheres to the conductive contact location and takes on a partially spherical shape.  
         [0023]    [0023]FIG. 5 is a cross sectional schematic view of an alternative embodiment of the apparatus seen in FIG. 1, wherein a distance or space is left between the die wafer and the mold wafer during solder ball formation.  
         [0024]    [0024]FIG. 6 is a cross sectional schematic view of another alternative embodiment of the present invention, and featuring a gasket between the mold wafer and the die wafer.  
         [0025]    [0025]FIGS. 7 and 8 are cross sectional schematic views corresponding to an optional step that can be conducted in the inventive method, including deformation of a partially spherical solder ball.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    Advantages of the present invention will become readily apparent to those skilled in the art to which the invention pertains from the following detailed description, wherein preferred embodiments of the invention are shown and described in the disclosure by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.  
         [0027]    With reference to FIG. 1, an exemplary system for implementing the method of the present invention includes a die wafer  10  having a surface  12  and a mold wafer  20  having a backside  26 . If die wafer  10  and mold wafer  20  are positioned one above another, backside of mold wafer  20  is the side of mold wafer  20  that is opposite to the side which is adjacent die wafer  10 . As was previously explained, the term “die wafer,” as used herein, broadly refers to a number of devices, such as a single-chip wafer, multiple-chip wafer, a substrate, a printed circuit board, packages for various levels, or other assemblies requiring solder connections.  
         [0028]    Surface  12  of die wafer  10  has a conductive contact location  14 , which can be, for example, a BGA pad or a bond pad for a flip chip device. Conductive contact location  14  is preferably metallic and has the ability to be wetted by molten solder  30  and to form a strong bond with the portion of molten solder  30  that will wet conductive contact location  14 . One example of the conventionally used material is copper, or copper having a layer of gold thereon. Surface  12  of die wafer  10  adjacent to conductive contact location  14 , on the other hand, is preferably made of a solder non-wettable material, so that molten solder  30  does not adhere to it. Surface  12  may have just one contact location  14  or as many conductive contact locations  14  as desired. Conductive contact locations  14  may be arranged in any desired pattern. Conductive contact locations  14  are formed on die wafer  10  prior to the process of the present invention by any suitable conventional method and may have to be activated with flux so as to allow molten solder  30  to bond to the metal thereof  
         [0029]    As seen in FIG. 1, mold wafer  20  has a pocket  22 . It is preferred to have at least as many pockets  22  on mold wafer  20  as there are conductive contact locations  14  on die wafer  10  and arranged in the same pattern, such that each contact location  14  has at least one corresponding pocket  22 . These pockets  22  may be formed in mold wafer  20  by any of the existing methods, for example, by etching. Any other suitable method is also appropriate. Many different shapes and sizes of pockets  22  may be chosen. In a preferred embodiment of the present invention shown in FIG. 1, conductive contact location  14 , when aligned with pocket  22 , is completely surrounded by pocket  22  so that a perimeter of pocket  22  is at least as large as the perimeter of the corresponding contact location  14 .  
         [0030]    Mold wafer  20  communicates with a source of molten solder  30 . In the context of the present invention, the term “solder” refers to an alloy that is used to join or seal metals by wetting the surfaces thereof and forming a joint by causing molecular attraction between the solder and the metal. It is preferred to use soft solders that have melting points up to approximately 600° F. Preferably, soft solders suitable for use with the present invention are those substantially composed of tin-lead alloys. The use of any other suitable solder is also contemplated within the scope of the present invention.  
         [0031]    The source of molten solder  30  may be positioned adjacent mold wafer  20  or can be positioned in a distant location (not shown) that communicates with mold wafer  20  by any available means. By way of example and not limitation, such communication may be through a conduit. In one preferred embodiment shown in FIG. 1, the source of molten solder  30  is positioned adjacent mold wafer  20 . In that embodiment, communication between the source of molten solder  30  and mold wafer  20  is accomplished through a passage  24  extending from a center of pocket  22  in mold wafer  20 .  
         [0032]    Passage  24  in communication with pocket  22  extends through backside  26  of mold wafer  20 . There may be as many passages  24  as there are pockets  22  in mold wafer  20 , so that each pocket is connected to the source of molten solder  30 . The passage or passages  24  may be formed, for example, by laser drilling. However, any other conventional method for formation of passages  24  can be used with the present invention. Passage  24  in FIG. 1 is shown only for illustrative purposes and any other shape and form of passage  24  may be utilized. In addition, any suitable form of connection between the source of molten solder  30  and pocket  22  of mold wafer  20 , is contemplated within the scope of the term “passage,” as used in the present invention.  
         [0033]    As shown by way of example in FIG. 2, the method of the present invention contemplates positioning of die wafer  10  and mold wafer  20  in close proximity with each other and in alignment of conductive contact location  14  of die wafer  10  and pocket  22  of mold wafer  20 . The term “close proximity” includes within its scope the touching of the adjacent surfaces of die wafer  10  and mold wafer  20 , as shown in the preferred embodiment of FIG. 2, where die wafer  10  and mold wafer  20  are positioned so that there is substantially no space between their adjacent surfaces. The movement and positioning of the die and mold wafers  10 ,  20  against each other may be accomplished by many different ways. For example, in one preferred embodiment, mold wafer  20  may be stationary at a predetermined location and die wafer  10  is moved relative to mold wafer  20 . For purposes of separating the wafers  10 ,  20 , die wafer  10  may be moved in a vertical direction up and down relative to mold wafer  20 . In another preferred embodiment, die wafer  10  may be stationary and it is mold wafer  20  that is moved. Yet in another preferred embodiment, both die wafer  10  and mold wafer  20  may be moved relative to each other.  
         [0034]    After die wafer  10  and mold wafer  20  are positioned and aligned against each other as previously discussed, a molten solder  30  from a source is caused to flow into and fill pocket  22  of mold wafer  20  by any suitable method of introduction, for example by heating and/or under pressure. Some examples of the suitable methods of introduction of the molten solder are listed in the U.S. Pat. No. 5,381,848 to Trabucco, which is incorporated herein by reference. Molten solder  30  wets conductive contact location  14  and adheres to it. As can be appreciated by one skilled in the art, the amount of molten solder  30  within pocket  22  can be controlled employing any suitable means such as by controlling the volume of molten solder  30  forced into passage  24  by an overfeeding of the source of molten solder  30 .  
         [0035]    In another preferred embodiment of the present invention shown in FIG. 2, conductive contact location  14  is recessed within die wafer  10  such that it lies flush with surface  12  of die wafer  10 . As in the previously preferred embodiment, conductive contact location  14 , when aligned with pocket  22 , is substantially completely surrounded thereby such that a perimeter of pocket  22  is at least as large as the perimeter of the corresponding contact location  14 . Die wafer  10  and mold wafer  20  are articulated together such that surface  12  of die wafer  10  makes contact with mold wafer  20 . Solder within pocket  22  fills pocket  22  so as to be substantially flush with mold wafer  20 . After die wafer  10  is articulated against mold wafer  20 , a pressure “spike” is sent into molten solder  30 , thereby causing molten solder  30  to force more solder through channel  24  and to make contact with conductive contact location  14 . Surface tension between conductive contact location  14  and molten solder  30  within pocket  22 , and the size of pocket  22  forming a perimeter around conductive contact location  14  causes a measured amount of solder to adhere to conductive contact location  14 . The pressure spike is of a measured duration and severity so as to cause a preferred amount of solder to adhere to conductive contact location  14 .  
         [0036]    In an embodiment of the inventive method, it is desirable to heat die wafer  10  to a temperature in a range from about 175° C. to about 350° C. prior to wetting conductive contact location  14  with molten solder  30 . The heating of die wafer  10  enables a controlled rate at which molten solder  30  solidifies at conductive contact location  14 .  
         [0037]    As seen in FIGS. 3 and 4, after molten solder  30  wets conductive contact location  14  on die wafer  10 , but before molten solder  30  in pocket  22  solidifies, introduction of molten solder  30  into pocket  22  of mold wafer  20  stops and separation of mold wafer  20  and die wafer  10  begins. For example, as was previously explained, mold wafer  20  preferably is stationary and the separation is achieved by moving die wafer  10  away from mold wafer  20 . As an alternative, only mold wafer  20  may be movable or both wafers may be moved away from each other. While the distance between the respective wafers is increasing, surface tension causes molten solder  30  attracted to conductive contact location  14  to form a solder ball  40  in a partially or generally spherical shape at such conductive contact location  14 , as seen in FIG. 3. Solder ball  40  is formed at conductive contact location  14  of die wafer  10  and thereafter quickly begins to air cool and solidify. Both complete formation and solidification of solder ball  40  happens outside pocket  22  of mold wafer  20 . Therefore, the shape of solder ball  40  does not necessarily conform to the shape of pocket  22  and is therefore not dependent on precise measurements of all dimensions of pocket  22 .  
         [0038]    In one preferred embodiment of the present invention, die wafer  10  is positioned vertically above mold wafer  20  so that, in addition to the surface tension, the natural force of gravity allows the suspended molten solder  30  at conductive contact location  14  to assume a partially spherical shape.  
         [0039]    Of course, any desired number of solder balls  40  may be formed simultaneously by providing a number of pockets  22  corresponding to conductive contact locations  14 . All of conductive contact locations  14  have the identical shape and size, the whole surface of each contact location is wetted by the equal amount of molten solder  30 , and all other physical conditions are identical. As a result, the identical forces act at each contact location which leads to the formation of the substantially uniform array of solder balls.  
         [0040]    [0040]FIG. 5 illustrates one variation of the embodiments of FIG. 2 and is illustrated in particularity for the embodiment of FIG. 2. Therein, it can be seen that a semiconductor substrate or die wafer  10  is positioned in close proximity with mold wafer  20  such that there is a clearance distance “d” between the adjacent surfaces of the die and the mold wafers  10 ,  20 . This embodiment allows for a clearance distance “d” and is useful in the method of the present invention when die wafer  10  is located above mold wafer  20 . The distance “d” between the adjacent surfaces of die wafer  10  and mold wafer  20  may be in the range of about 5 to about 100 microns, depending upon factors such as the ball size and pitch between the balls. It is also preferred to position die wafer  10  above mold wafer  20  such that conductive contact location  14  slightly projects into the corresponding pocket  22 . The clearance distance “d” helps to assure that except for conductive contact location  14 , there is no contact between molten solder  30  and surface  12  of die wafer  10 .  
         [0041]    Generally, the surface of die wafer  10  is made of any suitable non-conductive material that does not attract molten solder  30 . As an alternative, surface  12  of die wafer  10  may be covered with a layer of solder non-wettable material. Mold wafer  20  is also made from a material that does not adhere to molten solder  30 . For example, mold wafer  20  may be made of silicon. Therefore, only conductive contact locations  14  are conductive and attract molten solder  30 . It is undesirable that surface  12  of die wafer  10  be contaminated by contact with spilled molten solder  30  that might wet some areas of surface  12  of die wafer  10  other than conductive contact locations  14 . Therefore, maintaining of the distance “d” between die wafer  10  and mold wafer  20  assists in avoiding undesirable wetting of surface  12  of die wafer  10 .  
         [0042]    [0042]FIG. 6 depicts an alternative embodiment, which is especially useful for forming ball grid arrays of the present invention. If surface  12  of die wafer  10  and an adjacent surface of mold wafer  20  are not sealed against each other, molten solder  30  may spill on to portions thereof other that at conductive contact locations  14 . To assure no such spillage, a sealing member  50  is placed between die wafer  10  and mold wafer  20  to surround each pocket. By way of example and not limitation, sealing member  50  may be in the form of a gasket. Die wafer  10  and mold wafer  20  are pressed against each other and sealing member  50  serves as a seal between the adjacent surfaces of die wafer  10  and mold wafer  20  in the area surrounding conductive contact locations  14  that are aligned with pockets  22 . Only after the seal between the adjacent surfaces of die wafer  10  and mold wafer  20  is created, molten solder  30  is introduced into pockets  22 . As a result, molten solder  30  in pocket  22 , while allowed to wet conductive contact location  14 , cannot spill over and wet the adjacent surface  12  of die wafer  10  surrounding conductive contact location  14 . Therefore, sealing member  50  may be utilized to avoid the undesirable wetting of the surfaces surrounding conductive contact locations  14  by the spilled over molten solder  30 .  
         [0043]    The preferred alternative embodiment depicted in FIG. 3 may also be carried out using sealing member  50  as a gasket wherein a pressure spike is passed through molten solder  30  such that it bulges and makes contact with conductive contact location  14  but is contained by sealing member  50 . Because sealing member  50  substantially resists flow of solder onto surface  12  of die wafer  10  except where conductive contact location  14  occurs, a measured amount of molten solder  30  is contacted with conductive contact location  14 .  
         [0044]    Sealing member  50  is preferably made of any suitable material that can form an effective seal with die wafer  10  and mold wafer  20  under pressure and that will not interact or damage die wafer  10 . Examples of such material are silicone and elastomers. Any other suitable material, however, is contemplated within the scope of the present invention.  
         [0045]    [0045]FIGS. 7 and 8 show the optional step of the present invention which allows, if desired, to modify the shape of solder balls  40  made by the above described method of the present invention by which a solder ball  40   a  with one flat side may be created. At the end of the process of formation of the solder ball  40  through the steps described in relation to FIGS.  1 - 4 , die wafer  10  with the attached solder balls  40  is positioned away from mold wafer  20 . Die wafer  10  and mold wafer  20  may be brought back towards each other so that each solder ball  40  is aligned with and inserted into the corresponding pocket  22  of mold wafer  20 .  
         [0046]    Prior to the insertion of solder ball  40  into pocket  22 , pocket  22  preferably does not contain any solid or molten solder  30  therein, which is accomplished by any suitable conventional method. By way of example and not limitation, the remaining molten solder  30  may be forced back from pocket  22  through passage  24  into the source thereof by creating a negative pressure in passage  24 . As long as no molten solder  30  is left in pocket  22  itself, the fact that some molten solder  30  can still be present in passage  24  is not critical. After molten solder  30  has partially solidified, molten solder  30  is pressed into mold wafer  20 . Preferably, pocket  22  will be at a temperature above the reflow temperature. A portion of molten solder  30  that contacts pocket  22  should turn to a liquid state. All of the ball of molten solder  30 , however, should not turn to a liquid state.  
         [0047]    As seen in FIG. 7, after molten solder  30  is removed from pockets  22 , die wafer  10  and mold wafer  20  are moved towards each other until solder ball  40  is pressed against a bottom  28  of pocket  22 . The result is a flattened solder ball  40   a  as shown in FIG. 7. Since all of solder balls  40  are moved simultaneously an equal distance with equal forces into the similarly and corresponding shaped pockets  22 , this optional step allows the creation of an array of the uniformly modified solder balls  40   a . Care must be taken, however, not to leave solder ball  40   a  within pocket  22  for a period sufficient to cause solder ball  40   a  to entirely reliquify. An alternative method for reshaping solder ball  40   a  is to apply solder to die wafer  10  and to allow both surface  12  of die wafer  10  and mold wafer  20  to cool below the liquidus temperature of molten solder  30  where solder ball  40   a  makes contact with surfaces of pocket  22  and with conductive contact location  14 . By this embodiment, solder ball  40   a  will take the shape of pocket  22 . Molten solder  30  within passage  24 , if present at all, will separate from solder ball  40  once die wafer  10  and mold wafer  20  are separated.  
         [0048]    As can be appreciated by one skilled in the art, the shape of solder balls  40  may be that of any desirable shape by choosing the corresponding shape of pocket  22  of mold wafer  20 . More than one side of solder ball  40  can also be reshaped. The above embodiments can also be used to substantially planarize a BGA. That is, the height of all of the solder balls can be made substantially uniform for a contact or seating plane where contact of solder ball  40   a  is assured to be made with bottom  28  of pocket  22 .  
         [0049]    It will be understood by one skilled in the art that BGAs may be formed  8  simultaneously on more than one die wafer  10  without departing from the spirit of the present invention.  
         [0050]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. It will be understood by on of ordinary skill in the art that details which are not essential to the inventive features for simplicity have been omitted from the description and the drawings, and that such details may be readily incorporated without undue experimentation. The described embodiments are to be considered in all respects only as illustrated and not restrictive. 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.