Abstract:
There is provided a method for applying solder to an element on a surface of a substrate. The method comprises the steps of (a) placing a mold over the surface, where the mold includes a conduit that contains the solder, and (b) heating the solder to a molten state so that the solder flows from the conduit onto the element. The conduit enjoys two degrees of horizontal freedom with respect to the surface such that the conduit becomes substantially aligned with the element when the solder is in the molten state. There is also provided a system for applying solder to an element on a surface of a substrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to soldering, and more particularly, to applying solder to an element on a substrate. The technique is particularly suitable for applying solder columns to bottom surface metallurgy (BSM) pads on a chip carrier. 
     2. Description of the Prior Art 
     Electronic packaging generally contains many levels of packages and interconnections. A first level package may connect one or more silicon chips on a ceramic substrate carrier. A second level package may interconnect one or more such ceramic substrate carriers on an organic board. 
     The ceramic substrate is connected to the organic board by pins that are typically rigid and made of metal. The rigid pins are brazed on the ceramic substrate with a suitable braze material such as a gold-tin alloy. Ceramic substrates with an array of such pins, i.e., pin grid arrays (PGA), are subsequently plugged into a pin connector or wave soldered to an array of plated through-holes on the organic board. This connection system has disadvantages such as the through-holes limiting the number of wiring channels available in the board. Another disadvantage is the high cost associated with the braze material, the rigid metal pins, and the pin connectors or plated through-holes. 
     U.S. Pat. No. 4,914,814 to Behun et al. describes how these disadvantages can be avoided by using solder column connection (SCC) technology, which is also known as ceramic column grid array (CCGA) technology. Generally, CCGA technology is less expensive than PGA technology. CCGA technology also provides an improved electrical interconnection that can better withstand stresses associated with thermal expansion mismatch between a ceramic chip carrier and a supporting circuit board. 
     To connect a ceramic chip carrier to a supporting circuit board using CCGA technology, the chip carrier is soldered to the board using solder columns, which are typically 90% lead and 10% tin. The solder columns are formed and one end is attached to metallized pads on a surface of the ceramic chip carrier. Such pads are provided by a technique known as bottom surface metallurgy (BSM). Then the other end of the solder columns, opposite to the ceramic chip carrier, is attached to the circuit board. 
     One problem associated with the CCGA assembly process at the module level occurs when the solder columns do not properly join to the metallized pads of the chip carrier BSM surface. The problem occurs when there is a misalignment between the solder columns and the BSM pads. Pitches of 1.27 mm and 1.00 mm between BSM pads, center to center, are conventionally available. For the 1.00 mm pitch, the BSM pads have a diameter of about 0.8 mm with a spacing of about 0.2 mm between adjacent columns. For the 1.27 mm pitch, the BSM pads have a diameter of about 0.86 mm with a spacing of about 0.41 mm between adjacent BSM pads. As such, the aforementioned problem is more pronounced for the 1.00 mm pitch, but it is also apparent with the 1.27 mm pitch. Defects due to misalignment result in a lower product yield, a loss of material and an increased cost due to rework of the CCGA assembly. Another problem is the formation of excess solder, i.e., solder “blobs”, on the chip carrier&#39;s BSM surface due to upward force from molten solder during a solder reflow operation. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved method for applying solder to an element on a substrate, such as a BSM pad on a chip carrier. 
     It is another object of the present invention to provide such a method that reduces possibility of an excess solder “blob” forming between the elements. 
     It is a further object of the present invention to provide such a method that utilizes a solder surface tension effect to self-center the solder on the element. 
     These and other objects of the present invention are achieved by a first method for applying solder to an element on a surface of a substrate, comprising (a) placing a mold over the surface, where the mold includes a conduit that contains the solder, and (b) heating the solder to a molten state so that the solder flows from the conduit onto the element. The conduit enjoys two degrees of horizontal freedom with respect to the surface such that the conduit becomes substantially aligned with the element when the solder is in the molten state. 
     A second method for applying a solder column to an element on a surface of a chip carrier, comprises (a) securing the chip carrier in a fixture, (b) applying a solution having a first flux concentration onto the element, (c) positioning a mold in the fixture over the chip carrier such that a conduit in the mold is in a preliminary alignment with the element, where the conduit contains the solder column, (d) applying a solution having a second flux concentration onto a end of the solder column remote from the element, where the first flux concentration is greater than the second flux concentration, and (e) heating the solder column to a molten state so that the solder column flows from the conduit onto the element. The conduit enjoys two degrees of horizontal freedom with respect to the chip carrier such that the conduit becomes substantially aligned with the element when the solder column is in the molten state. 
     A first embodiment of the present invention is an apparatus for applying solder to an element on a surface of a substrate. The apparatus comprises (a) a base for holding the substrate, and (b) a mold that includes a conduit for containing the solder, where the mold is placed on the base over the surface. The conduit enjoys two degrees of horizontal freedom with respect to the surface such that the conduit becomes substantially aligned with the element when the solder is in a molten state. 
     A second embodiment of the present invention is an apparatus for applying a solder column to an element on a surface of a chip carrier, comprising (a) a base for securing the chip carrier, (b) a mold for positioning on the base over the chip carrier, where the mold includes a conduit that contains the solder, and (c) a weight for placement on the mold to limit vertical freedom of the mold with respect to the surface. The conduit enjoys two degrees of horizontal freedom with respect to the chip carrier such that the conduit becomes substantially aligned with the element when the solder column is in a molten state. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a fixture, and a pictorial representation of a method, for aligning solder columns with elements on a substrate, in accordance with the present invention. 
     FIG. 2 is a flowchart of a method for applying solder to an element on a surface of a substrate, in accordance with the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     The present invention provides for a fixture and method for aligning solder columns with elements on a substrate. The fixture and method are suited for aligning solder columns with bottom surface metallurgy (BSM) input/output (I/O) pads on a chip carrier. 
     The present invention increases production yield when attaching the solder columns onto the chip carrier. Chip carriers are used as an interconnection between a module and a card or printed circuit board. The fixture allows a mold that contains the solder columns to move freely horizontally along X and Y axes with respect to the chip carrier during solder reflow, and provides a weighted element to restrain undesired vertical movement along the Z axis due to solder surface tension upward forces. More particularly, the invention utilizes the solder surface tension effect to self-center the solder columns with respect to the chip carrier BSM pads along the X and Y axis, and applies a weight for mass balance in the Z axis. This technique significantly reduces cast column rework that would otherwise occur due to misalignment. 
     FIG. 1 is an illustration of a fixture, generally indicated by reference numeral  100 , for aligning solder columns with elements on a substrate, in accordance with the present invention. Fixture  100  includes a base  105 , an alignment plate  110 , a mold  115  and a weight  120 . 
     FIG. 1 also shows a substrate, e.g., chip carrier  125 , with a plurality of elements, e.g. BSM pads  130 , configured in an array on its top surface. BSM pads  130  are metallic contacts, typically plated with gold and nickel by a metallurgy process. The present invention is described herein in the context of attaching solder columns to a BSM pads  130 , however, the solder is not restricted in form to that of a column, and any suitable conductor, such as a wire, or component, such as an integrated circuit, can be soldered to BSM pads  130 . 
     Base  105  has a cavity  135  into which chip carrier  125  is placed. Cavity  135  allows chip carrier  125  to enjoy two degrees of horizontal freedom, that is, along an X axis and along a Y axis, over a limited spatial range. Base  105  also includes alignment pins  140 . Preferably base  105  is made of graphite, and alignment pins  140  are made of stainless steel. 
     Mold  115  includes a plurality of channels or conduits  145  that contain the solder that will eventually be applied to the BSM pads  130 . Conduits  145  are configured in an array such that each of conduits  145  correspond with, and ultimately will align with, one of the plurality of BSM pads  130 . The solder is preferably a mix of 90% lead and 10% tin. It can be installed into conduits  145  by any conventional process, however the preferred installation techniques are either of an injection molding process or a mechanical vibration and vacuum process. In the injection molding process, injection molded solder (IMS) is injected, while in a molten state, into conduits  145 . In the mechanical vibration and vacuum process, also known as a pin load, pre-cut solder segments or solder wires are loaded into conduits  145 . 
     Mold  115  is placed over or on top of chip carrier  125 . It has alignment holes  150  that mate with alignment pins  140  of base  105  and help to ensure the alignment of conduits  145  and BSM pads  130 . Alignment holes  150  have a diameter that is approximately 0.014 to 0.018 inch larger than the diameter of alignment pins  140 . Thus, mold  115 , and accordingly, channels  145 , is allowed a limited degree of horizontal freedom along the X and Y axes. This freedom also allows molten solder from conduits  145  to move freely in the X and Y axes with respect to BSM pads  130  during furnace reflow of the solder. Mold  115  is preferably made of graphite 
     Weight  120  has alignment holes  155  that mate with alignment pins  140  of base  105 . It is placed on top of mold  115  and applies a downward force on mold  115  and restrains vertical movement of mold  115 , i.e., along the Z axis. 
     Alignment plate  110  is a template that is temporarily placed on top of chip carrier  125  when chip carrier  125  is in cavity  135 , to facilitate a preliminary alignment of BSM pads  130  of chip carrier  125  with conduits  145  of mold  115 . Alignment plate  110  has several apertures  165  that allow a user of fixture  100  to visually inspect the orientation of chip carrier  125  within cavity  135 . The spacing between the centerlines of apertures  165  relative to alignment holes  160  is the same as the spacing between the centerlines of corresponding conduits  145  relative to alignment holes  150 . 
     During the preliminary alignment, the user places one end of apertures  165  adjacent to BSM pads  130 , and views BSM pads  130  through the other end of apertures  165 . The user typically performs the visual inspection with the aid of a microscope or other suitable magnifying device. Apertures  165  are slightly larger than BSM pads  130 , and preferably slope out to have a larger dimension on the side through which the user is looking than on the side adjacent to BSM pads  130 . For example, as shown in FIG. 1, apertures  165  may have side walls that slope out at an angle of about 45 degrees from a vertical axis. The sloped side walls permit for easier viewing of the relationship between BSM pads  130  and the end of apertures  165  adjacent thereto. The user performs the preliminary alignment by adjusting the position of chip carrier  130  within cavity  135  for a best fit between apertures  165  and BSM pads  130 . After completion of the preliminary alignment, alignment plate  110  is removed from base  105 , and chip carrier  125  is secured in its pre-aligned position. 
     FIG. 2 is a flowchart of a method  200  for applying solder to an element on a surface of a substrate, in accordance with the present invention. Method  200  is described below with reference to the elements of FIG. 1, in the context of aligning solder columns with BSM pads. Method begins with step  205 . 
     In step  205 , chip carrier  125  is placed within cavity  135  of base  105 . Method  200  then progresses to step  210 . 
     In step  210 , alignment plate  110  is placed on top of chip carrier  125  and chip carrier  125  is preliminarily aligned within cavity  135 . The preliminary alignment more specifically refers to a desired alignment relationship between BSM pads  130  and conduits  145  when mold  115  is placed over chip carrier  125  (see step  220 , below). Alignment plate  110  has several apertures  165  to permit a visual inspection of the relationship between apertures  165  of alignment plate  110  and BSM pads  130 . Upon completion of this preliminary alignment, alignment plate  110  is removed and chip carrier  125  is secured in its pre-aligned position within base  105 , by way of a set screw or other conventional locking arrangement. If the solder surface tension self-centering effect is sufficient to yield a satisfactory alignment between conduits  145  and BSM pads  130 , then step  210  is not required. Method  200  then progresses to step  215 . 
     In step  215 , a flux solution is applied to the surface of BSM pads  130  to remove oxides therefrom. The flux solution preferably has a flux concentration of about 8% to 12% rosin-based in an organic solvent, such as iso-propyl-alcohol (IPA). Method  200  then progresses to step  220 . 
     In step  220 , mold  115 , which contains the solder columns in conduits  145 , is placed over chip carrier  125 . Alignment holes  150  mate with alignment pins  140 . Because of the preliminary alignment performed during step  210 , the solder columns are substantially aligned with BSM pads  130 . Method  200  then progresses to step  225 . 
     In step  225 , a flux solution is applied to the top surface of mold  115 , and more specifically, to the end of the solder columns opposite of, or remote from, BSM pads  130 , to create a localized reducing atmosphere. The flux solution preferably has a flux concentration of about 2% to 6% rosin-based in an organic solvent, such as IPA. Note that the flux concentration on the surface of BSM pads  130  (see step  215  ) is greater than that applied to the surface of mold  115  (in the current step) because solder flows to an area of higher flux concentration, and in this case, the solder is intended to flow toward BSM pads  130 . After completion of step  225 , method  200  progresses to step  230 . 
     In step  230 , weight  120  is placed on top of mold  115 . Weight  120  counteracts an upward force from molten solder that ordinarily occurs during a solder reflow operation. It reduces the possibility that solder column will join together as “blobs” during reflow. Weight  120  is selected as an optimized mass balance along the Z axis. If weight  120  is too heavy, then it will unduly restrict the movement of mold  115  along the X and Y axes, and consequently, when the solder is in its molten state, the self-centering effect of the solder surface tension will be ineffective. If weight  120  is too light, then it will not adequately restrain the aforementioned vertical force to prevent solder “blobs”. Table 1 lists several different chip carrier configurations, and weights that are considered practical for those configurations. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 BSM Pad 
                 BSM Pad 
                 Weight (grams) 
                 Weight (grams) 
               
               
                 Pitch (mm) 
                 Count 
                 Pin Load Process 
                 IMS Process 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1.27 
                 624 
                 15 to 20 
                 16 to 22 
               
               
                 1.27 
                 831 
                 50 to 58 
                 55 to 64 
               
               
                 1.27 
                 1088 
                 62 to 70 
                 68 to 77 
               
               
                 1.00 
                 1247 
                 66 to 74 
                 72 to 81 
               
               
                 1.00 
                 1657 
                 88 to 94 
                  96 to 103 
               
               
                   
               
             
          
         
       
     
     Note that the optimized weights for weight  120  depend, in part, on whether the solder columns were installed into conduits  145  of mold  115  by the pin load process or by the IMS process. For example, given a BSM pad pitch of 1.00 mm and a BSM pad count of 1657, a weight of 88 to 94 grams is appropriate for the pin load process, whereas 96 to 103 grams is appropriate for the IMS process. This is due, in part, to the fact that in the IMS process the solder is installed in a molten state into conduits  145 , and thus conduits  145  are substantially, completely filled with solder. In contrast, in the pin load process, the solder is installed in a solid state in the form of solder segments or solder wires, and thus conduits  145  typically are not completely filled with solder. Because of this difference in the quantity of solder in conduits  145 , a greater weight, approximately 10% on average, is suggested for use if conduits  145  were loaded using the IMS process as compared to being loaded using the pin load process. 
     Also in step  230 , assembly  100  is heated, for example in a furnace, to transform the solder columns in conduits  145  to a molten state. The molten solder flows from conduits  145  to BSM pads  130 , thus resulting in the formation of a cast solder columns at each of BSM pads  130 . Because of the surface tension of reflowed solder and because of the loose fit between alignment holes  105  of mold  115  and alignment pins  140  of base  105 , the molten solder from conduits  145  moves with relative freedom in the X and Y directions, and self-centers the solder column with BSM pads  130 . Note that if the solder surface tension self-centering effect is adequate to achieve a desired level of column alignment on its own, then the preliminary adjustment in step  210  is not required. 
     It should be understood that various alternatives and modifications can be devised by those skilled in the art. For example, although the present invention is described in the context of applying solder columns to BSM pads on a chip carrier, it is also suitable for applying solder to other types of surface elements, such as pads on a printed circuit board or contacts on a surface-mount component. The present invention is intended to embrace all such alternatives, modifications and variances that fall within the scope of the appended claims.