Patent Publication Number: US-11043468-B2

Title: Lead-free solder joining of electronic structures

Description:
BACKGROUND 
     The present exemplary embodiments pertain to the joining of electronic structures such as semiconductor devices to a laminate substrate and, in turn, the laminate substrate to other laminate substrates. 
     First, second and third level packages require the joining of multiple components onto the substrate. The first level package may be a chip scale package (CSP) to receive a semiconductor device, also referred to as a chip. The second level package may be a daughter card onto which one or more CSPs may be joined. The third level package may be a mother card (also known as a mother board) onto which one or more daughter cards may be joined. 
     There is a need to reduce the thermal exposure with each attachment of a component to a substrate and to avoid having the joints of the earlier joined components remelted to the point they move. If the earlier joined components move, the movement can lead to potential issues such as chip package interactions defects, solder extrusion, solder spitting, etc. 
     During the leaded period of microelectronics, it was possible to have joints made with high melt temperature leaded solder and low melt temperature leaded solder. 
     The introduction of lead free solders has brought a number of challenges with respect to the joining of multiple components on multiple levels of packages. The concept of having a high melt temperature solder joined to a low melt temperature solder was not pursued mainly due to a reduced delta in the melting point of the different lead free solders and their availability to have controlled plating on the chip side and availability of options on the substrate side. 
     BRIEF SUMMARY 
     The various advantages and purposes of the exemplary embodiments as described above and hereafter are achieved by providing, according to an aspect of the exemplary embodiments, a method of joining a semiconductor structure comprising: forming an underbump metallurgy comprising a metal on a semiconductor device; forming lead free solder on the underbump metallurgy, the lead free solder comprising an alloy of tin and bismuth of the composition at least 85 weight percent bismuth, remainder tin; reflowing the lead free solder at 240-260° C. so that the lead free solder reflows to form solder bumps and react with the underbump metallurgy to raise the liquidus temperature of the solder bumps by incorporating the metal from the underbump metallurgy; forming pads of a low melting temperature lead free solder on a substrate, the solder comprising an alloy of tin, bismuth and silver with the composition 56-58 weight percent bismuth, 0.5-1.5 weight percent silver, remainder tin; and joining the reflowed bumps to the pads of solder at 200 to 230° C. 
     According to another aspect of the exemplary embodiments, there is provided a semiconductor structure comprising: a semiconductor device having an underbump metallurgy comprising a metal; a plurality of lead free solder bumps on, and alloyed with, the underbump metallurgy, the lead free solder bumps comprising an alloy of tin, bismuth and the metal from the underbump metallurgy, the plurality of solder bumps having a liquidus temperature of at least 260° C.; a substrate for receiving the semiconductor device; and lead free solder pads for joining the lead free solder bumps to the substrate, the lead free solder pads comprising tin, bismuth and silver and having a maximum liquidus temperature that is at least 10° C. less than the liquidus temperature of the lead free solder bumps. 
     According to a further aspect of the exemplary embodiments, there is provided a method of joining a semiconductor structure comprising: forming a lead free solder ball having a liquidus temperature less than 230° C. on a first substrate; forming a low melting temperature lead free solder having a liquidus temperature of 230° C. or less; alloying the low melting temperature lead free solder with an underball metallurgy on a second substrate, the alloying causing the liquidus temperature of the low melting temperature lead free solder to rise to 250 to 295° C. to form a high melting temperature lead free solder pad; coining the high melting temperature lead free solder pad to form a pattern of dimples on a top of the high melting temperature lead free solder pad; placing the lead free solder ball in direct contact with the pattern of dimples in the high melting temperature lead free solder pad; heating at 240 to 260° C. the lead free solder ball and the high melting temperature lead free solder pad to cause the lead free solder ball and the high melting temperature lead free solder pad to join. 
     According to yet another aspect of the exemplary embodiments, there is provided a semiconductor structure comprising: a first substrate; a second substrate having underball metallurgy; a lead free solder ball having a liquidus temperature less than 230° C. on the first substrate; and a high melting temperature lead free solder pad on the underball metallurgy on the second substrate joining the lead free solder ball to the second substrate, the high melting temperature lead free solder pad having a liquidus temperature of 250 to 295° C., the high melting temperature lead free solder pad having a pattern of dimples on a top surface of the high melting temperature lead free solder pad that make direct contact with the lead free solder ball. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
       The features of the exemplary embodiments believed to be novel and the elements characteristic of the exemplary embodiments are set forth with particularity in the appended claims. The Figures are for illustration purposes only and are not drawn to scale. The exemplary embodiments, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a flow chart for an exemplary method of joining a first substrate to a second substrate in which a high melting temperature lead free ball is joined to opposing substrates by lead free solders. 
         FIGS. 2A to 2C  are cross sectional views to illustrate the exemplary method of  FIG. 1  wherein: 
         FIG. 2A  depicts a high melting temperature lead free ball joined to a first substrate by a low melting temperature lead free solder; 
         FIG. 2B  depicts the low melting temperature lead free solder that has been reflowed to be a high melting temperature solder and a second substrate having a low melting temperature lead free solder; and 
         FIG. 2C  depicts the joining of the high melting temperature lead free ball to the low melting temperature lead free solder on the second substrate. 
         FIG. 3  is a flow chart for an exemplary method of joining a semiconductor device to a first level package. 
         FIGS. 4A to 4D  are cross sectional views to illustrate the exemplary method of  FIG. 3  wherein: 
         FIG. 4A  depicts lead free solder on underbump metallurgy on a semiconductor device; 
         FIG. 4B  depicts the reflow of the lead free solder to form a lead free solder bump which has alloyed with the underbump metallurgy to increase its melting temperature; 
         FIG. 4C  depicts the formation of low melting temperature solder on a first level package; and 
         FIG. 4D  depicts the joining of the lead free solder bump to the low melting temperature solder on the first level package. 
         FIG. 5  is a flow chart for an exemplary method of joining a first substrate to a second substrate having a dimpled interface. 
         FIGS. 6A to 6D  are cross sectional views to illustrate the exemplary method of  FIG. 5  wherein: 
         FIG. 6A  depicts the formation of a low melting temperature lead free solder bump on a first substrate; 
         FIG. 6B  depicts the formation of a high melting temperature lead free solder pad alloyed with underbump metallurgy on a second substrate; 
         FIG. 6C  depicts the coining of the high melting temperature lead free solder pad to form dimples on a surface of the high melting temperature lead free solder pad; and 
         FIG. 6D  depicts the joining of the low melting temperature lead free solder bump on the first substrate with the high melting temperature lead free solder pad on the second substrate and the dimpled interface between the low melting temperature lead free solder bump and the high melting temperature lead free solder pad. 
         FIG. 7  is a plan view of the dimples in the high melting temperature lead free solder pad as a result of coining. 
         FIG. 8A  is a side cross sectional view of another exemplary embodiment of the dimples in the high melting temperature lead free solder pad as a result of coining and  FIG. 8B  is a plan view of the dimples in  FIG. 8A . 
         FIG. 9A  is a side cross sectional view of a further exemplary embodiment of the dimples in the high melting temperature lead free solder pad as a result of coining and  FIG. 9B  is a plan view of the dimples in  FIG. 9A . 
     
    
    
     DETAILED DESCRIPTION 
     The following definitions for melting point, liquidus, solidus and organic laminate substrate are used herein. Elements have a melting point above which the element is liquid and below which is solid. 
     Alloys may be illustrated by a phase diagram. In a simple two element alloy system, such as a eutectic system, the alloy may have a eutectic point (similar to a melting point) above which the element is liquid and below which is solid. More often, the alloy may have two phase regions which may be defined by an upper boundary, called the liquidus, and a lower boundary, called the solidus. Above the liquidus, the alloy is completely liquid. The liquidus thus denotes for each possible alloy composition in the two phase region the temperature at which freezing begins during cooling or at which melting is completed upon heating. The solidus indicates for each possible alloy composition in the two phase region the temperature at which melting begins upon heating or at which freezing is completed upon cooling. Below the solidus, the alloy is completely solid. Between the solidus and liquidus, liquid and solid phases are both present. 
     An organic laminate substrate may have a core of, for example, glass particle filled epoxy and may further have a top metallic plane and a bottom metallic plane. 
     As noted previously, multiple components need to be joined to various substrates at different times. A corollary is that components and the substrates may need to be reworked because of components that may have failed during testing. Reworking requires control of solder on the components and substrates so reworking may be accomplished without affecting every other component. 
     Further, it is desired to have a known mass of solder on every ball grid array of a substrate when it is removed. In this way, it is possible to avoid the need to remove solder from the laminate to which the ball grid array is attached and to add solder to the ball grid array. 
     Moreover, copper sphere (hereafter copper ball) stand offs may be used in the ball grid array. However, the copper balls can lead to a focusing of the current and have the potential to reduce electromigration performance. 
     Accordingly, it is proposed to change from a copper ball to either A) a Cu6Sn5 or Cu3Sn ball (liquidus &gt;700° C.), or B) an SnAgCuNiAu alloy ball (liquidus about 250 to 280° C.) comprising (in weight percent) 0.1 to 6% copper, 0.1 to 4% silver, 0.1 to 1% gold, 0.1 to 2% nickel, balance tin. 
     The lower conductivity of these balls will make the electrical resistivity of the joint higher. Since the current comes in through the via, this will force the current from the center toward the edge of the joint. Due to the higher melting point and the higher activation energy to move an atom out of the matrix of the intermetallic vs. the solder, the electromigration performance is now enhanced. 
     The limiting of the pre-solder on whichever side the ball is placed leads to that side becoming alloyed with the ball. Upon melting during rework, the side with the sphere maintains all of the solder and ball. The side without the ball melts leading to about half of the solder going with the ball and half remaining on the other side. 
     Referring to the Figures in more detail, and particularly referring to  FIGS. 1 and 2 , there is illustrated an exemplary embodiment of a method of joining a semiconductor structure and the semiconductor structure. 
     The method includes forming a first low melting temperature lead free solder having a liquidus temperature less than 230° C. on a first substrate, box  10  and then placing a lead free ball having a liquidus temperature equal to or greater than 250° C. on the first low melting temperature lead free solder, box  12  ( FIG. 1 ). The lead free ball may have a composition comprising copper and tin (Cu6Sn5 or Cu3Sn (liquidus &gt;700° C.)), or a tin/silver/copper/nickel/gold (SnAgCuNiAu) alloy (liquidus about 250 to 280° C.). 
     Referring to  FIG. 2A , first substrate  30  has an underbump metallurgy (UBM) pad  32  of copper or nickel. First low melting temperature lead free solder  34  has been formed on the UBM pad  32 . The first low melting temperature lead free solder  34  may be a tin/silver/copper (SAC) alloy having a liquidus temperature less than about 230° C. One example of a suitable SAC alloy may be SAC 105 having a composition of 1 weight percent silver, 0.5 weight percent copper, remainder tin and having a liquidus temperature of 227° C. The lead free ball  36  is placed on the first low melting temperature lead free solder  34 . 
     The substrate  30  with the first low melting temperature lead free solder  34  and lead free ball  36  undergoes a heating step, typically 1-3 minutes to reflow the first low melting temperature lead free solder  34  at a temperature less than the liquidus of the lead free ball  36 , box  14  ( FIG. 1 ). For example, this temperature may be at a typical chip join temperature of 240 to 260° C. 
     The reflowing is followed by annealing at a temperature of around 150° C., typically for 60 to 300 minutes depending on an annealing temperature of 140 to 165° C., with the shorter annealing times corresponding to the higher annealing temperatures, to convert the first low melting temperature lead free solder  34  into a higher melting temperature lead free solder having a liquidus temperature of 240 to 250° C., box  16  ( FIG. 1 ). The first low melting temperature lead free solder  34  is chosen so that after annealing, there is at least a 10° C. liquidus temperature difference between the higher melting temperature lead free solder  38  and the lead free ball  36  with the lead free ball  36  having the higher liquidus temperature. 
     Referring now to  FIG. 2B , the reflowed first low melting temperature lead free solder is now shown as a higher melting temperature lead free solder  38  joining the lead free ball  36 . As a result of the reflowing and annealing steps, the first low melting temperature lead free solder  34  alloys with the lead free ball  36  to form a higher melting temperature lead free solder  38  to lock the lead free ball  36  in place. At this point, the higher melting temperature lead free solder  38  may have a liquidus temperature of 240 to 250° C. Again, the liquidus temperature of higher melting temperature lead free solder  38  should be at least 10° C. less than the liquidus temperature of the lead free ball  36 . 
     Also shown in  FIG. 2B  and described in box  18  ( FIG. 1 ) is second substrate  40  having wiring  42 , for example copper wiring, UBM pad  44  and a second low melting temperature lead free solder  46 . The second low melting temperature lead free solder  46  may also be a SAC alloy having a liquidus of 220° C. or less. One example of a suitable SAC alloy may be SAC 405 having a composition of 4 weight percent silver, 0.5 weight percent copper, remainder tin and having a liquidus temperature of 220° C. 
     The lead free ball  36  is placed in direct contact with the second low melting temperature lead free solder  46 , box  20  ( FIG. 1 ) and then heated at 240 to 260° C. for typically 1 to 3 minutes to cause the second low melting temperature lead free solder  46  to reflow and join with the lead free ball  36 , box  22  ( FIG. 1 ). The joining temperature is adjusted so as to be less than the liquidus temperatures of the higher melting temperature lead free solder  38  and the lead free ball  36 . 
     Referring now to  FIG. 2C , the lead free ball  36  has been placed into contact with the second low melting temperature lead free solder  46  and heated at 240 to 250° C. to join the lead free ball  36  to the second low melting temperature lead free solder  46  which in turn joins first substrate  30  to second substrate  40 . 
     In one exemplary embodiment, the first substrate  30  is a first level package such as a chip scale package and the second substrate  40  is an organic laminate daughter card. 
     In another exemplary embodiment, the first substrate  30  is an organic laminate daughter card and the second substrate  40  is a first level package such as a chip scale package. 
     In a further exemplary embodiment, the first substrate  30  is an organic laminate daughter card and the second substrate  40  is an organic laminate mother card. 
     In yet another exemplary embodiment, the first substrate  30  is an organic laminate mother card and the second substrate  40  is an organic laminate daughter card. 
       FIG. 2C  illustrates a semiconductor structure which may include a first substrate  30  and a second substrate  40 . Interposed between the first and second substrates  30 ,  40  may be a lead free ball  36  having a liquidus temperature equal to or greater than 250° C. The lead free ball  36  comprises at least copper and tin and may include the alloys previously mentioned. There is a high melt lead free solder  38  joining the lead free ball  36  to the first substrate  30 . The high melt lead free solder  38  has a liquidus temperature at least 10° C. less than the liquidus temperature of the lead free ball  36 . The semiconductor structure further includes a low melt lead free solder  46  joining the lead free ball  36  to the second substrate  40 . The low melt lead free solder has a liquidus temperature of 220° C. or less. 
     During reworking of the exemplary embodiments, the first substrate  30  having the higher melting temperature lead free solder  38  and the lead free ball  36  may be separated from the second substrate  40  with all of the higher melting temperature lead free solder  38  and the lead free ball  36  remaining with the first substrate  30 . Some portion, approximately half, of the second low melting temperature lead free solder  46  will remain attached to the lead free ball  36 , making reworking simpler. 
     Tin/bismuth (Sn/Bi) are proposed for joining a semiconductor device to a first level package, such as a CSP. The problem with tin/bismuth alloys is that they are stiff which has led to raised stress at the joint between the semiconductor device and the first level package. 
     In an exemplary embodiment, it is proposed to use two different tin/bismuth alloys to create a joint with low stress during chip join. One tin/bismuth alloy is 85 weight percent or more of bismuth, remainder tin. The tin/bismuth alloy is used to form solder balls on a semiconductor device. A tin/bismuth/silver alloy is placed on the first level package to enable joining of the semiconductor device to the first level package. Only enough mass of the tin/bismuth/silver alloy is used to enable joining but will be consumed during low temperature anneal. The tin/bismuth/silver alloy may comprise about 5 to 10% weight percent of the final solder joint mass. After joining, the joined components may be underfilled. If needed, a low temperature anneal at about 150° C., for example in the range of 140 to 165° C., may be used to fully consume the tin/bismuth/silver alloy into the solder joint. 
     Referring to the Figures in more detail, and particularly referring to  FIGS. 3 and 4 , there is illustrated an exemplary embodiment of a method of joining a semiconductor structure and the first level package. 
     The method includes forming an UBM metallurgy comprising a metal on a semiconductor device, box  50  ( FIG. 3 ) followed by forming lead free solder on the underbump metallurgy, box  52  ( FIG. 3 ). The lead free solder may comprise an alloy of tin and bismuth of the composition at least 85 weight percent bismuth, remainder tin. 
     Referring now to  FIG. 4A , there is depicted a semiconductor device  62  having UBM  64  and lead free solder  66 . The UBM  64  may be plated and may comprise nickel or copper. The lead free solder  66  may, for example, be screened onto the UBM  64 . 
     The lead free solder may be reflowed at 240-260° C. for typically 1 to 3 minutes so that the lead free solder reflows to form solder bumps and react with the UBM to raise the liquidus temperature of the solder bumps by incorporating the metal from the underbump metallurgy, box  54  ( FIG. 3 ). 
     Referring now to  FIG. 4B , the lead free solder  66  has been reflowed to form solder bumps  68 . The lead free solder  66  reacts with the UBM  64  to incorporate nickel or copper from the UBM  64  which substantially raises the liquidus temperature of the solder bumps from below 250° C. to over 260° C. 
     Pads of a low melting temperature lead free solder are formed on a first level package substrate, such as a chip scale package. The low melting solder may comprise an alloy of tin, bismuth and silver. The tin/bismuth/silver alloy may have the composition, in weight percent, 56-58% bismuth, 0.5-1.5% silver, remainder tin and having a liquidus temperature of about 204° C., box  56  ( FIG. 3 ). 
     Referring now to  FIG. 4C , semiconductor device  62  having solder bumps  68  is shown. Additionally shown is first level package substrate  70  having wiring  72 , copper for example, terminal metallurgy  74  and low melting tin/bismuth/silver alloy solder  76 . 
     The solder bumps are joined to the pads of lead free low melting solder at about 200-230° C., for example 215° C., box  58 . 
     Referring now to  FIG. 4D , the solder bump  68  is placed into direct contact with the pad  76  of lead free low melting solder and then heated to about 215° C. to reflow the lead free low melting solder pad  76  and join the lead free low melting solder pad  76  with the solder bump  68 . 
     If needed, the semiconductor structure comprising the semiconductor device  62 , solder bumps  68 , first level package  70  and the lead free low melting solder pad  76  may be annealed at about 150° C., for example, 140 to 165° C., for about 60 to 300 minutes depending on temperature to fully consume the lead free low melting solder. That is, the lead free low melting solder pad  76  becomes alloyed with the solder bump  68  so that it is no longer just a tin/bismuth/silver alloy and may come closer in composition to the composition of the solder bump  68 . 
       FIG. 4D  illustrates a semiconductor structure which may include a semiconductor device  62  having an UBM  64  comprising a metal. Also included may be a plurality of lead free solder bumps  68  on, and alloyed with, the UBM  64 . The lead free solder bumps  68  may comprise an alloy of tin, bismuth and the metal from the UBM  64 . In one exemplary embodiment, the UBM  64  may be copper or nickel so that the metal included in the solder bumps  68  may be the copper or nickel. The plurality of solder bumps  68  have a liquidus temperature of at least 260° C. 
     The semiconductor structure further includes a substrate  70  for receiving the semiconductor device  62 . The substrate  70  is a first level package and may be a CSP. 
     The semiconductor structure further includes lead free solder pads  76  for joining the lead free solder bumps  68  to the substrate  70 . The lead free solder pads comprising tin, bismuth and silver and having a maximum liquidus temperature that is at least 10° C. less than the liquidus temperature of the lead free solder bumps. 
     Laminates typically do not have a copper pillar, pedestal or electrolytic nickel terminal metallurgy and as such they may have a lower electromigration performance for power and ground connections as compared to the semiconductor side which may have a combination of all of these. The semiconductor side may have solder bumps, socall C4 connections, for connecting to the first level package. 
     Electromigration performance is impacted both by the local current density at the solder interface and by the melting point of the solder itself. 
     It is proposed to utilize a high melting point solder on the laminate that forms initially at a low temperature. In this way, the laminate/solder interface will have a high melting point solder which improves the electromigration performance by about 50 mA/C4 connection. 
     Electromigration considerations are important for power and ground connections due to their higher current density. Signal connections, having a lower current density, are less affected by electromigration. Accordingly, in one exemplary embodiments, it is proposed to modify the laminate connections for power and ground connections. 
     Referring to the Figures in more detail, and particularly referring to  FIGS. 5 and 6 , there is illustrated an exemplary embodiment of a method of joining a semiconductor structure and the semiconductor structure. 
     The method includes forming a lead free solder ball having a liquidus temperature less than 230° C. on a first substrate, box  80  ( FIG. 5 ). 
     Referring now to  FIG. 6A , there is shown a first substrate  92  having UBM  94  and a lead free solder ball  96 . The lead free solder ball  96  has a liquidus temperature less than 230° C. The UBM  94  may be copper or nickel. For purposes of illustration and not limitation, the lead free solder ball  96  may comprise any tin/silver/copper alloy, such as SAC 105 or SAC 405 mentioned previously, so long as the liquidus temperature is less than 230° C. 
     The method further includes forming a low melting temperature lead free solder having a liquidus temperature of 230° C. or less, box  82  ( FIG. 5 ) and alloying the low melting temperature lead free solder with an underball metallurgy on a second substrate, the alloying causing the liquidus temperature of the low melting temperature lead free solder to rise to about 250 to 295° C. to form a high melting temperature lead free solder pad, box  84  ( FIG. 5 ). 
     Referring now to  FIG. 6B , there is shown a second substrate  98  having UBM  100  and the high melting temperature lead free solder pad  102 . 
     The high melting temperature lead free solder pad  102  may be formed according to two methods. In one method, the low melting temperature lead free solder may be screened as solder paste onto UBM  100  and then reflowed above its liquidus temperature of about 230° C. to remove any organic materials in the solder paste. The reflow also pulls in the terminal metal to raise the liquidus temperature to about 250 to 295° C. In this method, low melting temperature lead free solder may be transformed to high melting temperature lead free solder pads  102 . 
     In a second method, droplets of molten low melting temperature lead free solder may be dropped onto UBM  100  which then react with the UBM  100  to form the high melting temperature lead free solder pads  102 . 
     The starting low melting temperature lead free solder may comprise an alloy of tin and one or more of bismuth, gold, silver, copper and nickel. The table below illustrates some exemplary embodiments of low melting temperature lead free solders and the preferred UBM for each solder. 
     
       
         
           
               
               
             
               
                   
               
               
                 UBM 
                 Solder Composition 
               
               
                   
               
             
            
               
                 Nickel 
                 SnBi: &gt;85 weight % Bi, remainder Sn 
               
               
                 Nickel 
                 SnAu: 1-2 weight % Au, remainder Sn 
               
               
                 Nickel 
                 SnAuCuAg: 0.5-1 weight % Au, 0.5-1 weight % 
               
               
                   
                 Cu, 1-6 weight % Ag, remainder Sn 
               
               
                 Nickel + Au coating 
                 SnAgCuNi: 1-6 weight % Ag, 0.5-1 weight % 
               
               
                   
                 Cu, 0.2 weight % Ni, remainder Sn 
               
               
                 Nickel + Au coating 
                 SnBi: &gt;85 weight % Bi, remainder Sn 
               
               
                 Copper 
                 SnAgCuNiAu: 1-6 weight % Ag, 0.5-1 weight % 
               
               
                   
                 Cu, 0.2 weight % Ni, 0.5-1 weight % Au, 
               
               
                   
                 remainder Sn 
               
               
                 Copper 
                 SnBi: &gt;90 weight % Bi, remainder Sn 
               
               
                 Copper 
                 SnAuNi: 1-2 weight % Au, 0.2 weight % Ni, 
               
               
                   
                 remainder Sn 
               
               
                   
               
            
           
         
       
     
     The method further includes coining the high melting temperature lead free solder pad to form a pattern of dimples on a top of the high melting temperature lead free solder pad, box  86  ( FIG. 5 ). 
     Referring now to  FIG. 6C , coining apparatus  104  having projections  106  presses down on the high melting temperature lead free solder pad  102  to form a pattern of dimples (or depressions)  108  on a top  110  of the high melting temperature lead free solder pad  102 .  FIG. 7  is a plan view of just the high melting temperature lead free solder pad  102  having the pattern of dimples  108  on the top  110  of the high melting temperature lead free solder pad  102 . 
     The lead free solder ball is placed in direct contact with the pattern of dimples in the high melting temperature lead free solder pad, box  88  ( FIG. 5 ) and then the lead free solder ball and the high melting temperature lead free solder pad are heated to about 240 to 260° C. to cause the lead free solder ball and the high melting temperature lead free solder pad to join, box  90  ( FIG. 5 ). 
     Referring now to  FIG. 6D , the lead free solder ball  96  is placed in direct contact with the pattern of dimples  108  in the high melting temperature lead free solder pad  102 . Then the lead free solder ball  96  and the high melting temperature lead free solder pad  102  are heated to about 250° C. to cause the lead free solder ball  96  and the high melting temperature lead free solder pad  102  to join. 
     In one exemplary embodiment, the first substrate is a first level package such as a CSP and the second substrate is an organic laminate daughter card. 
     In another exemplary embodiment, the first substrate is an organic laminate daughter card and the second substrate is an organic laminate mother card. 
     The exemplary embodiments produce a structure within the joint of high melt solder that keeps an ideal shape to spread current on the ground side of the laminate. 
     It is the copper/solder interface that leads to electromigration fails. The exemplary embodiments move and spread the current from the laminate to the interior of the joint away from the copper/solder interface. The movement of the interface reduces the localized current density. 
       FIG. 6D  illustrates a semiconductor structure which may include a first substrate  92 , a second substrate  98  having UBM  100 , a lead free solder ball  96  on the first substrate and a high melting temperature lead free solder pad  102  on the UBM  100  on the second substrate  98  that joins the lead free solder ball  96  to the second substrate  98 . The lead free solder ball  96  has a liquidus temperature less than 230° C. The high melting temperature lead free solder pad  102  has a liquidus temperature of about 250 to 295° C. 
     The high melting temperature lead free solder pad  102  has a pattern of dimples  108  on a top surface  110  of the high melting temperature lead free solder pad  102  that make direct contact with the lead free solder ball  96 . 
     Referring now to  FIGS. 8A and 8B , there is illustrated another exemplary embodiment of a pattern of dimples  114  on a top surface  116  of a high melting temperature lead free solder pad  112 . In this exemplary embodiment, there is one dimple  114  in the approximate center of the high melting temperature lead free solder pad  112 . 
     Referring now to  FIGS. 9A and 9B , there is illustrated another exemplary embodiment of a pattern of dimples  120  on a top surface  122  of a high melting temperature lead free solder pad  118 . In this exemplary embodiment, there are concentric rings  120  of dimples arranged around the approximate center of the high melting temperature lead free solder pad  118 . 
     It will be apparent to those skilled in the art having regard to this disclosure that other modifications of the exemplary embodiments beyond those embodiments specifically described here may be made without departing from the spirit of the invention. Accordingly, such modifications are considered within the scope of the invention as limited solely by the appended claims.