Abstract:
A method of forming and applying a solder mass comprised of depositing solder paste containing a carrier and a solder onto a first substrate, not wettable by said solder; reflowing the solder paste on the first substrate to cause the solder to coalesce into a solder mass; and transferring the solder mass from the first substrate to a second substrate.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a method of solder sphere formation and specifically to methods of forming an array of satellite-free solder spheres for use in connection with chip package assemblies and the like.  
         [0002]     It is well known in the surface mount technology art to mount components on one or both surfaces of a substrate using masses of solder in the shape of spheres or balls. For example, solder balls are commonly used as a means for attaching and interconnecting chip packages to printed circuit boards. It is therefore necessary to form an array of accurately dimensioned solder balls on a substrate, so as to accommodate the attachment of components to the substrate.  
         [0003]     Various methods exist to create solder balls of different dimensions. Smaller solder balls, approximately 50 micrometers in diameter, are commonly formed by reflow of controlled thicknesses of metal that have been deposited at desired sites by an electroplating or vapor phase technique. Larger solder balls, approximately 300 micrometers in diameter, are usually produced as discrete spheres. The spheres are deposited onto a receiving surface by first placing excess quantities of solder spheres or the like onto a metal plate having openings which correspond to landings on the receiving substrate. After depositing an excess quantity of solder spheres onto the metal plate, the solder spheres will fall through each opening in the shape of spheres and onto the receiving substrate. Solder flux typically is applied to the receiving substrate to hold the balls in place after the metal plate and surplus spheres have been removed. Solder spheres are then reflowed to attach them securely to the wettable metal on the receiving substrate.  
         [0004]     Processes for forming and depositing solder spheres of intermediate size ranging from approximately 50 micrometers to 300 micrometers, however, are not as accurate and present many problems. Forming spheres with intermediate dimensions require depositing a thickness of metal that is thicker than can be achieved at an economic rate by wet plating and vapor phase techniques. Furthermore, coalescing smaller solder spheres into an intermediate size is difficult to accomplish. Smaller solder spheres are more difficult to handle because they are light and move under the influence of static electricity. They therefore fail to remain in position when placed on a substrate. It is for this reason that single solder spheres smaller than 125 micrometers but larger than 50 micrometers are uncommon.  
         [0005]     Several attempts have been made to create solder spheres of intermediate size. For example, in the process of screen printing, very fine solder spheres (smaller than 50 micrometers in diameter) are mixed with flux and the resulting paste is forced through a stencil, or apertures on a metal plate, to the substrate below. The solder paste is reflowed causing the finer solder spheres to coalesce to form a single larger sphere. However, screen printing has several drawbacks. Screen printing is limited by the size of the opening that can be reliably made in the stencil. Additionally, the process is further limited by the need for a minimum amount of metal between adjacent holes of the stencil to ensure that the stencil will survive the printing process. Indeed, screen printing solder paste at below 150 micrometers pitch requires advanced technology, but achieves only moderate yields.  
         [0006]     Another method of achieving solder formation of an intermediate size involves the use of a syringe connected to a fine needle. The syringe dispenses solder paste to a desired location and in desired amounts using computer-controlled stages. Like screen printing, dispensing solder paste through a syringe presents several drawbacks. First, it is not possible to obtain consistent volumes of solder when the solder spheres are reflowed. This is due to the inability of the fine solder balls to successfully coalesce into a larger sphere. The remaining residues, commonly referred to as satellites, remain scattered across the solder paste. The satellites are mobile and prone to causing electrical shorts, mechanical interference to micro electro-mechanical system (“MEMS”) devices, and blocking all or part of the light path in photonics devices such as image sensors.  
         [0007]     It is therefore desirable to find a cost-effective method of creating one or more accurately dimensioned solder spheres that are free of satellites.  
       SUMMARY OF THE INVENTION  
       [0008]     Preferred methods according to the present invention are designed to overcome the shortcomings associated with prior art methods of solder sphere formation by providing a method of easily producing solder spheres that are free from satellites of solder.  
         [0009]     A method of forming and applying a solder mass in accordance with one aspect of this invention comprises depositing solder paste containing a carrier and a solder onto a first substrate not wettable by the solder, and reflowing the solder paste on the first substrate to thereby cause the solder to coalesce into the solder mass. Typically, the carrier wets the first substrate prior to conclusion of the reflowing. The method preferably includes the function steps of transferring the solder mass from the first substrate to a second substrate, which desirably includes a solder-wettable surface area.  
         [0010]     The first substrate is preferably aligned with the second substrate prior to the transfer of the solder mass from the first substrate to the second substrate. In order to allow for accurate alignment of the solder mass on the first substrate with the desired location on the second substrate, the coefficient of thermal expansion of the first substrate is matched with the coefficient of thermal expansion of the second substrate. Alternatively, first and second substrates are aligned with each other at least in part by viewing the second substrate through the first substrate, which is optically transparent. Glass is preferably used as the optically transparent first substrate, and various types of glass may be used in accordance with this invention, such as frosted glass. In yet another alternative step of alignment, recesses on the surface of the first substrate may be used to define the locations of one or more solder spheres that will correspond to locations on the second substrate. Once the first and second substrates are aligned, the first substrate can then be removed away from the second substrate, so as to remove any residual solder paste and residual solder satellites.  
         [0011]     The initial deposition of the solder paste may occur at more than one location on the first substrate so that more than one solder mass may be transferred to the second substrate simultaneously.  
         [0012]     The amount of solder paste deposited onto the first substrate can be varied in order to obtain solder masses of different sizes preferably ranging from 50 micrometers to 300 micrometers in diameter.  
         [0013]     Another method of forming and applying a solder mass in accordance with another aspect of this invention concerns reflowing the solder mass on the second substrate, preferably a pin, onto a third substrate.  
         [0014]     A method of making a unit comprised of a solder mass on a first substrate not wettable by solder in accordance with another aspect of the invention is also disclosed. The method comprises depositing solder paste containing a carrier and a solder onto the first substrate and reflowing the solder paste on the first substrate to thereby cause the solder to coalesce into the solder mass.  
         [0015]     In this method of making a unit, solder paste may also be deposited onto more than one location on the first substrate so as to allow formation of an array of solder masses. The deposition of the solder paste may be at locations which correspond to locations on a second substrate. Units formed in this manner can be handled, shipped and stored for later use in transferring the solder masses to a second substrate.  
         [0016]     A further aspect of the invention provides unit comprising a first substrate and a plurality of masses of a solder disposed on a surface of said first substrate but not metallurgically bonded to said first substrate. Such a unit can be fabricated by the methods discussed above, or by other methods. Such a unit can be handled, shipped and stored as an article of commerce to provide a ready-made set of solder masses. A related aspect of the method includes applying such a unit to transfer solder balls to a second substrate.  
         [0017]     These and other features and characteristics of the present invention will be apparent from the following detailed description of preferred embodiments, which should be read in light of the accompanying drawings in which corresponding reference numbers refer to corresponding parts throughout the several views. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIGS. 1A-1D  are sequential diagrammatic front cross-sectional views showing the sequence in fabricating and transferring a solder mass in accordance with an embodiment of the present invention;  
         [0019]      FIG. 2A  is a top view of an alternate intermediate glass substrate that may be used in accordance with other embodiments of the present invention;  
         [0020]      FIG. 2B  is a cross-sectional view of another alternate intermediate glass substrate that may be used in accordance with other embodiments of the present invention; and  
         [0021]      FIGS. 3A-3E  are sequential diagrammatic cross-sectional views showing the sequence in fabricating and transferring a solder mass in accordance with an alternative embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]      FIGS. 1A-1D  illustrate a method of solder mass or ball formation in accordance with one embodiment of the present invention. Referring to  FIG. 1A , there is shown a reusable or intermediate substrate  112  that is preferably not wettable by solder. The intermediate substrate has opposed first and second surfaces  111 ,  113  and is preferably used to facilitate the initial formation of a solder sphere of desired size. As will be explained in greater detail herein, the intermediate substrate  112  is preferably comprised of glass, such as borosilicate glass.  
         [0023]     In a first step of the process, a predetermined amount of a source of solder is deposited onto the intermediate substrate  112 . In a preferred embodiment, the source of solder is a commercially available solder paste that contains at least solder and a carrier, such as flux. As shown in  FIG. 1A , solder paste  110  is deposited onto the intermediate substrate  112  using a known method of deposition such as screen printing, use of a syringe, or the like. The solder paste  110  contains small masses  109  of solder that are suspended in flux  115 .  
         [0024]     Because its ability to flow permits distribution of the solder paste, use of solder paste  110  to form solder spheres is desirable. Furthermore, solder paste  110  is unaffected by static charge, which can disrupt formation of a unified solder sphere. These factors aid in keeping the solder paste  110  in its original position on the intermediate substrate  112 . Accordingly, it is possible to achieve accurate placement and transfer of a newly formed solder sphere when transferring the solder sphere from intermediate substrate  112  to a receiving substrate as discussed below.  
         [0025]     Referring to  FIG. 1B , once the solder paste  110  is deposited onto the intermediate substrate  112 , the solder paste is heated or reflowed to formulate a solder sphere  116 . Although the present invention is not limited by any theory of operation, it is believed that during reflow, the contact angle between the flux wetted to the intermediate substrate  112  and the small masses  109  of solder in surface tension force help to sweep the small masses  113  of solder toward the center of the dispensed solder paste  110 . The majority of the solder found in the solder paste  110  coalesces to form a single large solder sphere  116 . In some cases, however, not all of the solder is able to coalesce, thereby leaving behind residual satellites  114  of solder surrounding the solder sphere  116 . In either event, the reflowed solder is allowed to cool. The flux  115  remains bonded to the intermediate substrate  112  so that although the solder sphere  116  and any remaining satellites  114  are not metallurgically bonded to the intermediate substrate  112  (since the intermediate substrate  112  is not wettable by solder), they will remain affixed to it by solidified flux residues. This helps to position the solder sphere  116  precisely at the center of the site on the intermediate substrate where the solder paste  110  was originally dispensed.  
         [0026]     As shown in  FIG. 1C , the receiving substrate  118  has opposed first and second sides  117 ,  119 . Although the receiving substrate  118  is generally not wettable by solder, it preferably contains a contact pad or other feature  121  that is wettable by solder. The receiving substrate may be any element to which a solder sphere is to be applied as, for example, an unpackaged microelectronic element such as a semiconductor chip; a semiconductor wafer; a packaged microelectronic element; or a circuit board. The intermediate substrate  112  is preferably inverted and aligned with the receiving substrate  118  so that the first side  111  of the intermediate substrate  112  and the first side  117  of the receiving substrate  118  are facing one another. The intermediate substrate  112  is aligned with the receiving substrate  118  so that solder sphere  116  is then aligned directly on top of the contact pad  121 . It should be appreciated that the position of the receiving substrate is not limited to the position shown in  FIGS. 1C-1D  (below the intermediate substrate). In this regard, the position of the receiving substrate will preferably determine where the intermediate substrate  112  needs to be positioned (i.e., above, below, to the left, to the right, or at an angle to the receiving substrate  118 ) in order to align the solder sphere  116  on the intermediate substrate  112  with the contact pad  121  on the receiving substrate  118 . The substrates may be provided with fiducial marks which can be observed by a human observer or a machine vision system.  
         [0027]     The preferable use of transparent glass as the intermediate substrate  112  can simplify the process of aligning and verifying that the solder sphere  116  has been accurately positioned on the receiving substrate  118 . By looking through a transparent glass intermediate substrate, an individual is able to visually align the solder sphere  116  with the contact pad  121 . In an automated process, a robotic vision system may be used to align the solder sphere  116  on the intermediate substrate  112  with the receiving substrate  118 .  
         [0028]     The use of glass as the intermediate substrate  112  is also preferred because its wide range of expansivities assists in accurate alignment of a solder sphere on a receiving substrate. Typically, solder spheres formed on an intermediate substrate are formed at locations that will correspond to locations, such as contact pads, on the receiving substrate. Because the solder sphere  116  is transferred to the receiving substrate  118  at the transfer temperature, and not at room temperature, the alignment of the solder sphere  116  with the contact pad  121  on the receiving substrate  118  must occur at the temperature of transfer. However, the coefficient of thermal expansion of certain types of glass closely parallels the coefficient of thermal expansion of silicon from room temperature to the reflow temperature of the solder (i.e., 20° C. to 300° C.), allowing silicon and glass to expand at the same rate. It is therefore possible for the position of the solder sphere  116  on a glass intermediate substrate to always correspond to a desired location on a receiving substrate comprised of silicon. Thus, when it is desired to deposit a solder sphere  116  onto a chip, wafer, or device comprised of silicon, the use of glass as the intermediate substrate permits accurate placement of the solder sphere onto a silicon receiving substrate.  
         [0029]     Returning back to  FIG. 1C , once the solder paste  110  containing the solder sphere  116  is aligned with contact pad  121  on the receiving substrate  118 , the solder paste  110  is reflowed again to remelt the solder, as well as the flux  115  residues solidified in the solder paste  110  that bind the solder sphere  116  to the intermediate substrate  112 . The second reflow process allows the solder sphere  116  to wet onto the contact pad  121  of the receiving substrate  118 .  
         [0030]     Referring to  FIG. 1D , while the flux  115  is molten, the intermediate substrate  112  is then lifted away from the receiving substrate  118 . Solder satellites  114  which may remain on the intermediate substrate  112  will be removed from the receiving substrate  118  by lifting the intermediate substrate  112  away from the receiving substrate  118 . Such solder satellites  114  will remain trapped by the surface tension of the molten flux on the intermediate substrate  112  when it is pulled away. As a result, the solder sphere remains on the receiving substrate while remaining satellites  114  trapped in the flux  115  are removed. This process greatly reduces the amount of satellites  114  on the receiving substrate  118  by comparison to the number of satellites which would be formed on the second substrate at direct deposition of the solder paste on the second substrate.  
         [0031]     The aforementioned steps describe the process of removing the intermediate substrate  112  away from the receiving substrate  118  when the flux is molten. In an alternative embodiment, the intermediate substrate  112  may also be lifted away from the receiving substrate  118  after the flux has solidified. One method contemplates the use of a mechanical means, such as a robotic arm, to lift the intermediate substrate  112 . Another method contemplates the use of chemical means, such as a solvent for the flux to dissolve solidified flux residues that retain the solder sphere to the intermediate plate. Dissolution of the solidified flux residues eliminates the bond between the intermediate substrate  112  and the solidified flux, thereby allowing removal of the intermediate substrate  112  away from the receiving substrate  118 .  
         [0032]     After transfer of the solder balls to the second substrate, the second substrate can be mounted to a further substrate (not shown) as, for example, a printed circuit board using the solder ball to bond contact pads  121  of the second substrate to a contact pad (not shown) of the further substrate.  
         [0033]     Because solder spheres, when molten, will not wet to a glass substrate, the process may be performed several times on the same receiving substrate  112 , such that spheres of different or similar dimensions and/or compositions may populate the same receiving substrate. At the conclusion of the process, the flux residues and solder satellites  114  may be removed from the intermediate substrate  112  using appropriate solvents. The intermediate substrate  112  can then be cleaned and ready for reuse.  
         [0034]     The size of the final solder sphere  116  at each desired location on the receiving substrate  118  directly corresponds to the amount of solder paste  110  originally dispensed onto the intermediate substrate  112 . One may, therefore, obtain solder spheres of different sizes by varying the amount of solder paste applied. Determining the amount of solder paste necessary to achieve solder spheres of different sizes will depend, however, on the percentage of solder originally found in the solder paste.  
         [0035]      FIGS. 1A-1D  depict the formation and application of only one solder ball. However, the process most typically is performed so as to form and transfer numerous solder spheres simultaneously. Thus, multiple deposits of solder paste  110  may be deposited onto the intermediate substrate, so that more than one solder sphere is formed at one time. The multiple deposits of solder paste may be applied in an array of locations on the intermediate substrate corresponding to the array of contact pads or other solder-wettable features on the second substrate. Thus, after reflowing to form solder spheres, all of the spheres can be aligned with respective pads simultaneously by aligning the intermediate substrate with the second substrate.  
         [0036]     In the alternative embodiment of an intermediate substrate shown in  FIG. 2A , several etched pits  152  are provided on a glass intermediate substrate  154  to allow for the formation of more than one solder sphere on the intermediate substrate  154 , and to increase accurate placement of a solder paste onto the intermediate substrate  112 . As shown in  FIG. 2B , the etched pits  152 ′ may be created in a variety of shapes and sizes.  FIG. 2B  illustrates several recesses of different shapes and sizes in the same intermediate substrate. In practice, any of these shapes (and other shapes) can be used, but typically all of the recesses in a given substrate would be substantially identical. The etched pits  152  and  152 ′ are recesses in the surface of the glass intermediate substrate  112  which can be formed using wet etching. The etched pits  152  are disposed in the array corresponding to the desired placement of the solder spheres, i.e., in an array corresponding to the array of contact pads or other features on a chip or other substrate. This is beneficial because the flux  115  in the solder paste  110  is wetted onto the surface of the intermediate surface  112  allowing surface forces to pull the puddle of dispensed paste toward the center of the etched pits  152 , thereby centering the masses of paste in the locations of the array. Additionally, the recesses may be filled with solder paste by wiping excess material across the face of the plate. The solder in the recesses will be in the form of a molten solder puddle. Thus, the incorporation of such etched pits  152  aids in increasing the positional accuracy of a final solder sphere.  
         [0037]     It should be further appreciated that once the solder paste  110  has been reflowed and solidified into one or more solder spheres on the intermediate substrate, the combination of the intermediate substrate with the newly solidified solder sphere or spheres (See  FIG. 1B ) may be stored away as a unit, and subsequent steps performed at a later time when it is desired to finally transfer the solder sphere  116  to a receiving substrate  118 . Typically, such a unit includes an array of solder spheres. Such units may be especially useful when it is desired to keep arrays of solder spheres in storage that will correspond to one or more contact pads or other conductive features on a receiving substrate. Units with the intermediate substrate and solder spheres attached thereto by solidified flux can be fabricated in mass production and used in the same plant, or in a separate plant to transfer the solder spheres to the second substrate. The plant that performs the transfer to the second substrate need not have the equipment required to dispense solder paste.  
         [0038]     Referring to  FIGS. 3A-3E , an alternative method of solder formation and transfer, in accordance with the present invention, is shown. As shown in  FIG. 3A , a solder sphere  116 ′ is formed on an intermediate surface  112 ′, preferably glass. The intermediate surface  112 ′ has opposed first and second sides,  111 ′ and  113 ′. As previously disclosed herein, solder paste  110 ′ is deposited on the intermediate substrate  112 ′ and reflowed to form a large solder sphere  116 ′.  
         [0039]     Referring to  FIG. 3B , a first receiving substrate  120  preferably comprising a pin  122 , such as a Socketstrate® pin, is aligned with the solder sphere  116 ′ found on the intermediate substrate  112 ′. (Socketstrate® is a trademark owned by Tessera, Inc. and disclosed in pending U.S. Application No. 60/583,108 filed on Jun. 25, 2004; U.S. Application No. 60/583,109 filed Jun. 25, 2004; and an application entitled, “Microelectronic Packages and Method Therefor,” filed on Jun. 25, 2004, which are incorporated herein by reference.)  
         [0040]     The pin  122  has a pin head  126 , a first angled sidewall  128  and a second angled sidewall  130 . It is possible to achieve close pitches of solder components by transferring the solder sphere  116 ′ directly onto the pin  122  and then aligning the pin with a desired object located on a second receiving substrate  123 .  
         [0041]     Referring to  FIG. 3C , the pin  122  is placed into the solder sphere  116 ′, such that the head  126 , and first and second angled walls  128 ,  130  are enveloped by the solder sphere  116 ′. Thereafter, the solder paste  110  is reflowed and the solder sphere  116 ′ is able to wet to the pin  122 , as well as dewet from the intermediate surface  112 ′.  
         [0042]     Referring to  FIG. 3D , the intermediate substrate  112 ′ is pulled away from the first receiving substrate  120 , or pin  122 . When the intermediate substrate  112 ′ is removed, residual satellites  114 ′ of solder that may remain are removed, but the solder sphere  116 ′ remains on the pin.  
         [0043]     Referring to  FIG. 3E , there is a second receiving substrate  123 , such as a silicon chip or a printed circuit board, that has opposed first and second sides  117 ′ and  119 ′. The pin  122 , which carries the solder sphere  116 ′, is placed onto the first side  117 ′ of the second receiving substrate  123 . The solder sphere  116 ′ is then again reflowed to attach the pin  122  to a land  150  on the receiving substrate  118 ′.  
         [0044]     Although the invention herein has been described with reference to particular embodiments and preferred dimensions or ranges of measurements, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. Additionally, it is to be appreciated that the present invention may take on various alternative orientations. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.