Abstract:
Methods for mounting electrical components on a substrate and securely retaining the components are described. The methods include altering solder paste compositions, interposed between component retentive pins and retentive through holes, during a reflow process. Electronic assemblies including circuit boards and electrical components mounted thereto are also described. In one of the electronic assembly embodiments, materials originally associated with a mounted electrical component migrate into solder paste coupling the electrical component to the circuit board.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a division of U.S. patent application Ser. No. 10/255,312, filed Sep. 26, 2002, the contents of which are hereby incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to surface mounted electrical components having improved retentive properties, and methods for mounting and retaining electrical components on substrates.  
         BACKGROUND OF THE INVENTION  
         [0003]    Various methods of electrically connecting components such as connectors and integrated circuit devices to printed boards are well known in the art. Surface mount technology is increasingly being employed as cost-effective method. Examples of surface mount technology include pin grid arrays (PGA&#39;s), ball grid arrays (BGA&#39;s), column grid arrays (CGA&#39;s), and land grid array (LGA&#39;s). Generally with such methods, a solder paste is introduced between surfaces to be joined, the solder paste reflowed, and then cooled to form a mechanical and electrically conductive joint.  
           [0004]    As the name implies, ball grid arrays (BGA&#39;s) utilize a grid or array of solder bumps or balls arranged on one side of an electrical component to effectuate electrical and mechanical connection with a printed board. The ball attachment sites are typically referred to as lands or pads. Solder past is disposed on the lands (and/or on the balls), such as through screening or masking techniques, and the electrical component is then positioned on the board so that the balls and lands are registered. The board is then processed at an elevated temperature sufficient to cause the solder and at least a portion or all of the solder ball to flow and fuse with the underlying land/pad to form the electrical interconnection. See, for example, U.S. Pat. No. 6,325,644 assigned to the assignee of this patent, and patents within that patent, all of which is incorporated by reference herein.  
           [0005]    Locator pins extending from the electrical component may be employed to facilitate the initial alignment of the electrical component. The locator pins are inserted into through holes located in the board at locations along the periphery of the array of lands/pads. Pin diameters may be slightly larger than the corresponding through hole diameters so that an interference fit is achieved. Alternatively, the pin diameters may be smaller than the through hole diameters. In this scenario, adhesive or solder paste may be applied in and around the through holes to help retain the electrical component on the substrate.  
           [0006]    By selecting locator pins with a relatively smaller diameter and applying solder within the through holes, the BGA balls are allowed to float (that is, not rigidly restricted) during a reflow process resulting in self-alignment and ultimately a low stress solder joint. Component retention however may be compromised with this configuration when the board is manipulated before the reflow heat is substantially dissipated, or when the board is rotated so that additional electrical components can be mounted on other available surface areas with subsequent reflow steps.  
           [0007]    Accordingly, there is a need for a method of mounting electrical components on a substrate that provides for a low stress solder joint while effectively retaining the mounted components during ensuing double-sided handling and/or processing during a second reflow step.  
         SUMMARY OF THE INVENTION  
         [0008]    Methods for mounting electrical components on substrates are provided by the present invention. In accordance with a first preferred embodiment of the present invention, there has now been provided a method for mounting and retaining an electrical component on a substrate that is subjected to at least two thermal cycles. The method comprises the steps of: disposing solder paste of a first solder composition into a retentive through hole of the substrate; inserting a retentive pin extending from the electrical component and having a relatively smaller diameter into the retentive through hole; subjecting the substrate to a first thermal cycle such that material associated with at least one of the retentive through hole and retentive pin migrates into the solder paste to create a mixed solder composition that has a higher melting point than the first solder composition; and then subjecting the substrate to a second thermal cycle having a peak temperature less than the melting point of said mixed solder composition.  
           [0009]    In accordance with another preferred embodiment of the present invention, there has now been provided a method for making an electronic assembly. The method comprises the steps of: providing a circuit board including a retentive through hole having a wall; providing an electrical connector having a retentive pin extending therefrom, wherein the retentive pin has a diameter smaller than the retentive through hole and is made from a material comprising a precious metal; disposing solder paste of a first solder composition into the retentive through hole; inserting the retentive pin into the retentive through hole; and reflowing the solder paste such that at least some of the precious metal originally associated with the retentive pin migrates into the solder paste to create a mixed solder composition.  
           [0010]    In accordance with yet another preferred embodiment of the present invention, there has now been provided a method for mounting electrical components on opposing sides of a substrate. The method comprises the steps of: providing a substrate including first and second sides and first and second retentive through holes; providing a first electrical component including a first retentive pin extending therefrom, wherein the first retentive pin has a diameter smaller than that of the first retentive through hole, and has plating material thereon; disposing solder paste of a first solder composition into said first retentive through hole; juxtaposing the first electrical component on the first side of the substrate, and inserting the first retentive pin into the first retentive through hole; heating the substrate such that at least a portion of the plating material migrates from the first retentive pin into the solder paste, wherein a mixed solder composition is created which has a higher melting point than that of said first solder composition; disposing solder paste of the first solder composition into the second retentive through hole; providing a second electrical component including a second retentive pin extending therefrom, wherein the second retentive pin has a diameter smaller than that of the second retentive through hole; juxtaposing the second electrical component on the second side of the substrate, and inserting the second retentive pin into the second retentive through hole; and heating the substrate to a temperature sufficient to reflow the first solder composition and below the melting point of the mixed solder composition.  
           [0011]    Electronic assemblies are also provided by the present invention. In accordance with a preferred embodiment of the present invention, there has now been provided an electronic assembly comprising a circuit board including a retentive through hole having a wall; an electrical component including a housing and retentive pin extending therefrom mounted on the circuit board, wherein the retentive pin is disposed within said retentive through hole and has a diameter less than that of the retentive through hole; and reflowed solder disposed in the retentive through hole and surrounding at least a portion of the retentive pin, wherein the reflowed solder includes material originally associated with at least one of the retentive pin and the retentive through hole wall, and wherein the reflowed solder has a melting point greater than a melting point of pure solder.  
           [0012]    In accordance with another preferred embodiment of the present invention, there has now been provided an electronic assembly comprising a circuit board including a retentive through hole; an electrical component including a housing and retentive pin extending therefrom mounted on the circuit board, wherein the retentive pin is disposed within the retentive through hole and has a diameter less than that of the retentive through hole; and reflowed solder disposed in the retentive through hole and surrounding the retentive pin, wherein at least about 0.5% by weight of said reflowed solder is an additive comprising a precious metal.  
           [0013]    In accordance with yet another preferred embodiment of the present invention, there has now been provided an electronic assembly comprising a circuit board including a retentive through hole; an electrical component including a housing and retentive pin extending therefrom mounted on the circuit board, wherein the retentive pin is disposed within the retentive through hole, has a diameter less than that of the retentive through hole, and has plating material thereon; and reflowed solder disposed in the retentive through hole and surrounding the retentive pin, wherein the reflowed solder comprises at least about 0.5% by weight of the plating material that has migrated from the retentive pin.  
           [0014]    These and various other features of novelty, and their respective advantages, are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of aspects of the invention, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated preferred embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a perspective view of an electronic component designed and configured for surface mounting to a substrate.  
         [0016]    [0016]FIG. 2 is a plan view of a circuit board including surface mounting lands.  
         [0017]    [0017]FIG. 3 is a partial side view of an electronic assembly embodiment provided by the present invention showing an electronic connector mounted to a circuit board including ball grid array interconnections  
         [0018]    [0018]FIG. 4 is a partial perspective view of an electronic assembly embodiment provided by the present invention showing an electronic connector mounted to a circuit board including ball grid array interconnections and retentive pins disposed in solder-filled through holes.  
         [0019]    [0019]FIG. 5 is a partial cross-sectional view of an electronic component retentive pin disposed in a substrate retentive through hole with solder surrounding the retentive pin.  
         [0020]    [0020]FIG. 6 is a partial perspective view of an electronic assembly embodiment provided by the present invention including electronic components surface mounted to both sides of a circuit board. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    Preferred embodiments of the present invention are directed to surface mounted electrical components having improved retentive properties and methods for making the same. Referring now to the drawings, wherein like reference numerals designate corresponding structure throughout the views, and referring in particular to FIG. 1, an electrical component  10  in the form of a connector is shown including housing  20 , an array of solder balls  21  extending from a housing lower surface  22 , and retentive pins  25  extending from housing lower surface  22  and spaced apart from solder balls  21 . Each of solder balls  21  is coupled to an end of one of a plurality of conventional terminals (not shown) that reside within housing  20 . Opposing ends of the terminals can be engaged by an electrical component, such as, for example, an integrated chip or mating electrical connector.  
         [0022]    Electrical connector  10  is suitable for surface mounting to a substrate, such as the printed circuit board  40  shown in FIG. 2. Printed circuit board  40  includes a first side  42 , an array of lands or pads  41  disposed on side  42  that correspond to the array of solder balls  21 , and retentive through holes  45  that are designed and configured to accept retentive pins  25 .  
         [0023]    Referring to FIGS. 3 and 4, to mount electrical connector  10  to circuit board  40 , solder paste  50  is first applied to lands or pads  41 , and in and/or around retentive through holes  45 . Numerous techniques readily understood by one skilled in the art, such as, for example screen-printing, screening or masking, may be used to apply the solder paste. Solder paste  50  is preferably a eutectic alloy of tin and lead having a melting point of approximately 183 degrees Celsius. Electrical connector  10  is placed in juxtaposition with board  40 , with individual solder balls  21  roughly aligned with corresponding individual lands or pads  41  by inserting retentive pins  25  into the solder containing retentive through holes  45 . The board/component assembly is then reflowed (subjected to a thermal cycle) preferably in a stationary or belt type furnace that employs convection or radiation heat sources. The solder reflow may be performed in an environment selected from a group comprising dry nitrogen, forming gas or hydrogen. The reflow process typically includes a thermal cycle that subjects the board-component assembly to a peak temperature (or temperature at or above the melting point of the solder paste) for a time period between 15 and 150 seconds. The reflow process is conventional and well documented in the literature.  
         [0024]    Referring now to FIG. 5, retentive pins  25  preferably have a diameter  26  that is smaller than the diameter  46  of the retentive through holes, such that the component being surface mounted to the board is allowed to float. For example, the pin diameters  26  may be approximately 0.9 mm and the corresponding through hole diameters  46  approximately 1.1 mm; although the specific diameters and difference between the two diameters may be greater or smaller than this. This configuration promotes self-aligning of balls  21  with lands  41 , and results in a low stress solder joint.  
         [0025]    After mounting the electrical connector  10  to board  40  (as can be seen in FIG. 2), it may be desirable to further process the board/component assembly at temperatures above the melting point of solder paste  50  without affecting solder bonds previously formed. The present invention provides method embodiments to improve the retention of mounted electrical components employing retentive pins having diameters less than the through hole diameters, by altering the solder composition within the retentive through holes  45  during or after the initial component mounting. To alter the solder composition, retentive pins  25  are plated with materials including precious metals such as gold, palladium, platinum, silver, rhodium, iridium, osmium, ruthenium, and rhenium. During the first reflow (or shortly thereafter), at least a portion of the precious plating material migrates from retentive pins  25  and into solder paste  50  residing within retentive through holes  45 , thereby creating a mixed solder composition that includes an additive made up of the plating material. Due to the presence of the additive, the melting point of the mixed solder composition is higher than the melting point of the original solder composition (i.e., melting point of pure solder). The melting point of the mixed solder composition should now preferably be at least 10 degrees Celsius higher than the melting point of the original (or first) composition. Accordingly, the board assembly may be further processed to a temperature at or above the melting point of the first solder composition and below the melting point of the mixed solder composition without concern that the mounted electrical component will become separated from the board.  
         [0026]    In alternative embodiments, a wall  47  (see FIG. 5) of retentive through holes  45  may be plated with material that will migrate into the solder paste during or after an initial reflow process. Or both the through hole walls  47  and the retentive pins  25  may be plated with similar or dissimilar materials for migration into the solder paste. The migrated material may also originate from a base composition of the retentive pins and through hole walls, rather than from plating material overlying a similar base composition. Preferably, retentive pins  25  are the sole source of the migrating material. In preferred embodiments, retentive pins  25  are made from a lead-brass alloy base material, with a first plating layer of nickel at a thickness of from about 0.00127 to about 0.00635 mm, and a second plating layer of gold or palladium at a similar thickness.  
         [0027]    One instance where two or more reflow steps (two thermal cycles) may occur is when electrical components are mounted on both sides  42  and  43  of circuit board  40 . A preferred method embodiment for mounting electrical components on opposing sides of a circuit board begins with a step of applying solder paste  50  to lands  41  and in and around retentive through holes  45  on side  42  of board  40 . The solder composition at this point preferably consists essentially of tin and lead. Referring again to FIG. 4, first electrical component  10  is movably coupled to side  42  by inserting retentive pins  25  comprising a gold or palladium plating into the solder containing through holes  45 . Board  40  is then subjected to a first thermal cycle including a peak temperature of about 183 degrees Celsius to reflow solder paste  50 . During the first thermal cycle, at least a portion of the gold or palladium plating migrates from retentive pins  25  and into solder paste  50  residing within through holes  45 . The solder composition within through holes  45  now comprises tin, lead, and gold or palladium, and has a melting point higher than 183 degrees Celsius. The amount of gold or palladium present in the reflowed solder will typically vary, with higher concentrations proximate retentive pins  25 . In preferred embodiments, the gold or palladium (or other plating material) is present in the reflowed solder in an amount of at least about 0.5% by weight.  
         [0028]    Referring now to FIG. 6, board  40  is flipped and side  43  is prepared for mounting a second electrical component thereon. The same solder paste  50  composition (consisting essential of tin and lead) used for mounting the electrical component  10  on board side  42  is now applied to lands  141  and in and around retentive holes  145  located on board side  43 . A second electrical component  110  (similar or dissimilar to component  10 ) comprising an array of solder balls  121  and retentive pins  125  is movably coupled to board  40  in a similar fashion to that of component  10 . Board  40  is subjected to a second thermal cycle including a peak temperature of about 183 degrees Celsius to reflow solder paste  50 . During the second thermal cycle, reflowed solder in retentive through holes  45  does not melt, and therefore, first electrical component  10  is securely held on board  40 . Retentive pins on the second electrical component  110  may or may not include materials that will migrate into the solder paste residing in retentive though holes  145 . A cost savings, via materials and processing, may be realized by not including a plating material on the retentive pins (or on the walls of through holes  145 ) extending from electrical component  110 . Obviously, if retentive pins on electrical component  110  are made with a material comprising a precious metal, then migration will likely occur during the second thermal cycle.  
       EXAMPLES  
     Example 1  
       [0029]    Table 1 includes parameters of two preferred embodiments along with the calculated solder compositional change after a first thermal cycle.  
                                         TABLE 1                                   Gold Plating   Palladium Plating                                    Unplated pin diameter, mm   0.90   0.90       Plating thickness, mm   0.0051   0.0051       Through hole diameter, mm   1.10   1.10       Solder volume, mm 3     0.57   0.57       Migration, % volume   60   40       Migrated material in reflowed solder, %   6.10   4.20       volume       Original solder melting point, ° Celsius   183   183       Altered solder melting point, ° Celsius   226   235                  
 
       Example 2  
       [0030]    Un-plated retentive pins having a diameter of 0.90 mm were plated with gold or palladium at a thickness of approximately 0.0051 mm. A eutectic tin-lead solder paste was applied in and around retentive holes extending through a board. The retentive through holes had a diameter of 1.10 mm. The plated retentive pins were inserted into the retentive through holes and the board subjected to a thermal cycle sufficient to reflow the solder paste. Samples of the altered solder were taken at three positions, as shown in FIG. 5: 1) proximate the retentive pin—P 1 ; 2) proximate the wall of the retentive through hole—P 3 ; and 3) an intermediate position—P 2 . The solder samples were analyzed via mass spectrometry. Table 2 shows the amount of plating material (% by weight) present in the solder after completing the thermal cycle.  
                                     TABLE 2                           Solder Sample   Plating Material in Solder,       Pin Plating Material   Position   % by weight                                Gold   P1   14.88       Gold   P2   5.84       Gold   P3   4.58       Palladium   P1   19.62       Palladium   P2   6.05       Palladium   P3   1.78                  
 
         [0031]    As can be seen in Table 2, the plating material diffuses all the way through the solder disposed within the retentive through holes. That is, the plating material presence in the solder is not isolated to the solder region nearest to the material source (retentive pin). In addition, Table 2 illustrates that gold diffuses more effectively than palladium through the tin-lead solder once the material is leached off of the retentive pins.  
         [0032]    It is to be understood that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure, manufacture of, and function of the invention, the disclosure is illustrative only. Accordingly, changes may be made in detail, especially in matters of shape, size and arrangement of structural features, as well as, sequences of manufacturing steps, within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.