Patent Publication Number: US-2007114266-A1

Title: Method and device to elongate a solder joint

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is a divisional of U.S. application Ser. No. 10/667,008, filed on Sep. 17, 2003. The disclosure of this prior application from which priority is claimed is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to solder joints. More particularly, the present invention relates to a method and device to elongate a solder joint.  
      2. Description of the Related Art  
      A solder joint is formed when a solder deposit between two substrates is subjected to reflow soldering. During reflow soldering, the solder deposit is heated such that it melts and wets a solderable surface on a substrate. The solder deposit solidifies upon subsequent cooling, forming the solder joint.  
      Solder joints serve two functions in an electronic assembly; they provide mechanical support to hold various components in the electronic assembly together and electrical support to form electrical connections within a circuit. Examples of solder joints include chip-to-interposer connections such as controlled collapse chip connections (C4 joints), package-to-board connections in, for example, Surface Mount Technology (SMT), and chip-to-board connections such as in chip-on-board (COB) technology.  
      During field application, the electronic assembly undergoes a temperature cycle each time it powers on or off. Due to differences in coefficients of thermal expansion (CTEs), substrates in the electronic assembly expand and contract to different degrees during the temperature cycle. This differential expansion and contraction of the substrates results in a movement of one end of a solder joint relative to another, which puts a strain on the solder joint. Because the electronic assembly undergoes numerous temperature cycles over its service life, the solder joints are subjected to repeated applications of strain, resulting in fatigue failure of the solder joints, which shortens the service life of the electronic assembly.  
      Studies have shown that the fatigue life of a solder joint, that is, the number of applications of strain that a solder joint can sustain before fatigue failure, can be improved by elongating the solder joint. An elongated solder joint is more compliant and is therefore better able to absorb the strain caused by the temperature cycling. Consequently, various methods to elongate a solder joint have been proposed.  
      One such method is disclosed in U.S. Pat. No. 4,545,610 issued to Lakritz, et al, wherein a solder extender on a substrate is positioned over a solder mound on a semiconductor chip and reflowed to form an elongated solder joint. A spacer is used to maintain a predetermined spacing between the semiconductor chip and the substrate.  
      A drawback of this method is that it involves numerous processing steps. Each processing step increases the complexity of the manufacturing process and adds to the cost of production. In particular, the processing steps involving the use of vapour deposition techniques to form the solder extender on the substrate and then to deposit a layer of low melting metal onto a top surface of the solder extender make this method of elongating a solder joint expensive.  
      In addition, because the probability of defects occurring in the solder joint increases with each processing step, the reliability of an elongated solder joint formed with this method is compromised.  
      Another method for elongating a solder joint is disclosed in U.S. Pat. No. 5,968,670 issued to Brofman, et al. In this method, the solder joint is elongated when a spring, restrained in a compressed state by solder, is released during solder reflow.  
      Unfortunately, this method also involves numerous processing steps which, as discussed previously, increase the complexity of the manufacturing process and the cost of production, and reduce the reliability of the solder joint.  
      Additionally, because some of the components involved in this method are of an infinitesimal dimension, the handling of these components poses a problem. For example, it is difficult to manipulate the tiny spring into a suitable orientation for insertion into a minute cavity in a graphite boat mold to form an expandable solder bump. This difficulty in the handling of the components contributes to the complexity of the process.  
      In view of the foregoing, it is desirable to have a method for elongating a solder joint that involves a minimal number of processing steps. It is also desirable to have a method that does not involve the handling of minute components.  
     SUMMARY OF THE INVENTION  
      The present invention fills these needs by providing a method and device to elongate a solder joint. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device or a method. Several inventive embodiments of the present invention are described below.  
      One embodiment of the present invention provides a method to elongate a solder joint. The method begins by forming an elongator on a first substrate. The elongator comprises an expander and an encapsulant to encapsulate the expander. A solder joint is formed to connect the first substrate to a second substrate. Thereafter, the encapsulant is softened to release the expander from a compressed state to elongate the solder joint.  
      The elongator may be formed on the first substrate by providing a mold having a first mold cavity and a second mold cavity. The first substrate is disposed in the first mold cavity, while an expander is disposed in the second mold cavity. The expander is compressed and an encapsulant is introduced into the mold to encapsulate the expander to form the elongator on the first substrate.  
      The first substrate may be one of group consisting of a chip, an interposer, a package, a board, a series of interposers, a series of packages and a wafer. The first substrate may be subjected to singulation.  
      The elongator may be formed on the first substrate by one of a group consisting of an injection molding process, a compression molding process, a transfer molding process and a casting process.  
      The solder joint to connect the first substrate to the second substrate may be formed by melting a plurality of solder deposits to wet a solderable surface to form the solder joint.  
      The encapsulant is preferably an electrical insulator. More preferably, the encapsulant is a thermoplastic such as polyamide or polyacetal. Most preferably, the thermoplastic has a softening temperature of approximately 40° C. higher than a melting point of the plurality of solder deposits.  
      The expander may be a corrugated strip. Preferably, a first end of the corrugated strip overlaps a second end of the corrugated strip.  
      In another embodiment of the invention, a device to elongate a solder joint is provided. The device to elongate a solder joint comprises a substrate having an elongator formed on it. The elongator includes an expander in a compressed state and an encapsulant to encapsulate the expander.  
      The substrate may be one of a group consisting of a chip, an interposer, a package, a board, a series of interposers, a series of packages and a wafer.  
      In yet another embodiment of the invention, an electronic assembly is provided. The electronic assembly comprises a first substrate coupled to a second substrate by a solder joint and an elongator coupled between the first substrate and the second substrate. The elongator, which comprises an expander and an encapsulant to encapsulate the expander, is formed on the first substrate.  
      Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings. To facilitate this description, like reference numerals designate like structural elements.  
       FIGS. 1   a  to  1   c  illustrate a method to form an elongator on a substrate in accordance with one embodiment of the present invention.  
       FIG. 2   a  illustrates a perspective view of an expander in accordance with one embodiment of the present invention.  
       FIG. 2   b  illustrates a side view of an expander in accordance with one embodiment of the present invention.  
       FIG. 3  illustrates a plan view of a substrate with an elongator affixed thereto in accordance with one embodiment of the present invention.  
       FIGS. 4   a  to  4   c  illustrate a method to elongate a solder joint in accordance with one embodiment of the present invention.  
       FIGS. 5   a  to  5   c  illustrate a method to form an elongator on a substrate in accordance with another embodiment of the present invention.  
       FIG. 6  illustrates a plan view of a wafer with an elongator affixed thereto in accordance with another embodiment of the present invention.  
       FIG. 7  illustrates a plan view of a series of interposers with an elongator affixed thereto in accordance with another embodiment of the present invention.  
       FIGS. 8   a  to  8   c  illustrate a method to elongate a solder joint in accordance with another embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A method and device to elongate a solder joint are provided. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be understood, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention.  
       FIGS. 1   a  to  1   c  illustrate a method to form an elongator  10  on a substrate  12  in accordance with one embodiment of the present invention. A mold  14  to form elongator  10  on substrate  12  is illustrated in  FIG. 1   a . Mold  14  comprises a first mold half  16  having a first mold cavity  18 , a second mold half  20  having a second mold cavity  22 , and a nozzle  24 . Substrate  12  is disposed in first mold cavity  18 , while an expander  26  is disposed in second mold cavity  22 .  
      Substrate  12  may be a chip, an interposer, a package or a board. In this embodiment, substrate  12  includes a plurality of solder deposits  28  with a height H S . In an alternative embodiment, substrate  12  may include a solderable surface.  
       FIGS. 2   a  and  2   b  illustrate a perspective view and a side view, respectively, of expander  26  in accordance with one embodiment of the present invention. Expander  26  comprises a corrugated strip  30 .  
      An alternating series of grooves and ridges  32  in corrugated strip  30  confers a quality of resilience to corrugated strip  30 . Accordingly, expander  26  is of an uncompressed height H u  as illustrated in  FIG. 2   b . Uncompressed height H u  of expander  26  may correspond to a desired height H d  of a solder joint.  
      A first end  34  of corrugated strip  30  may overlap a second end  36  of corrugated strip  30 , as shown in  FIG. 2   b , to accommodate a compression of expander  26 . Alternatively, a spacing (not illustrated) may be provided between first end  34  and second end  36  of corrugated strip  30  to accommodate the compression of expander  26 .  
      Corrugated strip  30  may be economically manufactured by stamping out of a sheet of metal such as stainless steel. A desired resilience of expander  26  may be achieved by selecting an appropriate material and suitable dimensions, in terms of length, width and thickness, for corrugated strip  30 . A geometry of corrugated strip  30  may correspond to an area on substrate  12  which is not populated by the plurality of solder deposits  28  or a solderable surface.  
      Other embodiments of expander  26  include a linear compression spring and other resilient devices.  
      With reference to  FIG. 1   b , expander  26  is compressed to a height H c  and brought into contact with substrate  12  when first mold half  16  is brought into contact with second mold half  20 . Compressed height H c  of expander  26  is preferably less than height H s  of the plurality of solder deposits  28 .  
      An encapsulant  38  in a molten state is injected into mold  14  through nozzle  24 . Encapsulant  38  fills up a space  40  in second mold cavity  22  around expander  26 .  
      Encapsulant  38  has a softening temperature that is approximately 40° C. higher than a melting point of the plurality of solder deposits  28 . Such a softening temperature allows encapsulant  38  to remain rigid at a solder reflow temperature, that is, a temperature at which the plurality of solder deposits  28  is melted to form a solder joint, while softening at a temperature of approximately 10° C. above the solder reflow temperature.  
      Encapsulant  38  is preferably an electrical insulator such as a thermoplastic to prevent elongator  10  from short-circuiting the solder joint. Additionally, by eliminating the risk of short-circuiting, a larger volume of encapsulant  38  may be used to encapsulate expander  26 . The larger volume of encapsulant  38  provides a degree of structural reinforcement against stress and strain to the solder joint during temperature cycling and also against mechanical shock, thereby enhancing the reliability of the solder joint. Examples of suitable thermoplastic encapsulants include polyamide and polyacetal.  
      Mold  14  is allowed to cool once space  40  is filled. Upon cooling, encapsulant  38  solidifies, forming elongator  10  with expander  26 . Expander  26  is restrained in a compressed state by encapsulant  38 . Consequently, elongator  10  is of a height H c  corresponding to height H c  of compressed expander  26 .  
      First mold half  16  is then separated from second mold half  20  as illustrated in  FIG. 1   c . Thereafter, substrate  12  with elongator  10  affixed thereto is removed from first mold cavity  18 .  
      The use of an injection molding process is advantageous in that the injection molding process provides a quick and economical way of forming elongator  10  on substrate  12 . Although the injection molding process is used to form elongator  10  on substrate  12  in this embodiment, other processes such as compression molding, transfer molding and casting may also be employed.  
       FIG. 3  illustrates a plan view of substrate  12  with elongator  10  affixed thereto in accordance with one embodiment of the present invention. Elongator  10  is coupled to an area on substrate  12  that is not populated by the plurality of solder deposits  28 . Accordingly, the geometry of elongator  10  may correspond to an area on substrate  12  that is not populated by the plurality of solder deposits  28 .  
      Similarly, in the alternative embodiment where substrate  12  includes a solderable surface, elongator  10  will be coupled to an area on substrate  12  that is not populated by the solderable surface. Accordingly, the geometry of elongator  10  may correspond to such an area.  
       FIGS. 4   a  to  4   c  illustrate a method to elongate a solder joint  50  in accordance with one embodiment of the present invention. Referring first to  FIG. 4   a , a plurality of solder deposits  52  on a first substrate  54  is positioned to oppose a solderable surface  56  on a second substrate  58 . An elongator  60  is formed on first substrate  54 . In an alternative embodiment, elongator  60  may be formed on second substrate  58 .  
      Elongator  60  comprises an expander  62  encapsulated by an encapsulant  64 . Expander  62  is restrained in a compressed state by encapsulant  64 . Expander  62  may be a corrugated strip, a linear compression spring or other resilient devices. Encapsulant  64  has a softening temperature that is approximately 40° C. higher than a melting point of the plurality of solder deposits  52  and is preferably an electrical insulator such as polyamide or polyacetal.  
      First substrate  54  may be a chip or a package. Correspondingly, second substrate  58  may be an interposer or a board depending on a desired assembly. For example, in an instance where first substrate  54  is a chip, second substrate  58  may be an interposer or a board. Accordingly, solder joint  50  forms a chip-to-interposer or a chip-to-board connection, respectively, between first substrate  54  and second substrate  58 .  
      Heat may be applied to melt the plurality of solder deposits  52  via a reflow process. At a solder reflow temperature, that is, a temperature above the melting point of the plurality of solder deposits  52 , each of the plurality of solder deposits  52  melts and wets a corresponding solderable surface  56  to form solder joint  50  as illustrated in  FIG. 4   b . Solder joint  50  couples first substrate  54  to second substrate  58  to form an electronic assembly  66 .  
      Because the softening temperature of encapsulant  64  is higher than the solder reflow temperature, encapsulant  64  remains sufficiently rigid to prevent a release of expander  62  from its compressed state.  
      Electronic assembly  66  is then subjected to further heating to raise the reflow temperature to the softening temperature of encapsulant  64 . At the softening temperature, encapsulant  64  begins to soften.  
      As encapsulant  64  softens, expander  62  is gradually released from its compressed state, resulting in a gradual elongation of solder joint  50 . The rate at which solder joint  50  elongates depends on the compliance of expander  62  and the viscosity of encapsulant  64 . The use of a more compliant expander  62  and a more viscous encapsulant  64  will result in a slower release of expander  62  from its compressed state, and a correspondingly slower rate of elongation.  
      With reference to  FIG. 4   c , electronic assembly  66  is cooled when solder joint  50  attains a desired height H d . Upon cooling, solder joint  50  and encapsulant  64  solidify. Elongator  60  serves as a reinforcement for solder joint  50 , increasing its resistance to stress and strain caused by temperature cycling and also to mechanical shock, thereby enhancing the reliability of solder joint  50 .  
       FIGS. 5   a  to  5   c  illustrate a method to form an elongator  100  on a substrate  102  in accordance with another embodiment of the present invention. A mold  104  to form elongator  100  on substrate  102  is illustrated in  FIG. 5   a . Mold  104  comprises a first mold half  106  having a first mold cavity  108  and a second mold half  110  having a second mold cavity  112 . Substrate  102  is disposed in first mold cavity  108 , while an expander  114  is disposed in second mold cavity  112 .  
      Substrate  102  may be a series of interposers, a series of packages or a wafer with a plurality of chips. Process efficiency is improved by mass-producing elongator  100  on substrate  102 . Second mold cavity  112  is designed to accommodate a corresponding expander  114  for each chip, interposer or package. A nozzle  116  is provided for each chip, interposer or package. In this embodiment, each chip, interposer or package includes a plurality of solder deposits  118  with a height H s ′. In an alternative embodiment, each chip, interposer or package may include a solderable surface.  
      Expander  114  is of an uncompressed height H u ′. Uncompressed height H u ′of expander  114  may correspond to a desired height H d ′ of a solder joint. As discussed previously, expander  114  may be a corrugated strip, a linear compression spring or other resilient devices.  
      Referring next to  FIG. 5   b , expander  114  is compressed to a height H c ′ and brought into contact with substrate  102  when first mold half  106  is brought into contact with second mold half  110 . Compressed height H c ′ of expander  114  is preferably less than height H s ′ of the plurality of solder deposits  118 .  
      An encapsulant  120  in a molten state is injected into mold  104  through nozzle  116 . Encapsulant  120  fills up a space  122  in second mold cavity  112  around expander  114 .  
      Encapsulant  120  has a softening temperature that is approximately 40° C. higher than a melting point of the plurality of solder deposits  118 . Such a softening temperature allows encapsulant  120  to remain rigid at a solder reflow temperature, that is, a temperature at which the plurality of solder deposits  118  are melted to form a solder joint, while softening at a temperature of approximately 10° C. above the solder reflow temperature. As discussed previously, encapsulant  120  may be a thermoplastic such as polyamide or polyacetal.  
      Mold  104  is allowed to cool once space  122  is filled. Upon cooling, encapsulant  120  solidifies to form elongator  100  with expander  114 . Expander  114  is restrained in a compressed state by encapsulant  120 . Consequently, elongator  100  is of a height H c ′ corresponding to height H c ′ of compressed expander  114 . Because second mold cavity  112  is designed to accommodate a corresponding expander  114  for each chip, interposer or package, elongator  100  may be formed on each chip, interposer or package.  
      First mold half  106  is then separated from second mold half  110  as illustrated in  FIG. 5   c . Thereafter, substrate  102  with elongator  100  affixed thereto is removed from first mold cavity  108 .  
      The use of an injection molding process is advantageous in that the injection molding process provides a quick and economical way of mass-producing elongator  100  on a plurality of interposers, a series of packages or a plurality of chips.  
      Although the injection molding process is used to form elongator  100  on substrate  102  in this embodiment, other processes such as compression molding, transfer molding and casting may also be employed.  
       FIG. 6  illustrates a plan view of a wafer  150  with an elongator  152  affixed thereto in accordance with another embodiment of the present invention. Wafer  150  comprises a plurality of chips  154 , each of which includes a plurality of solder deposits  156 . Elongator  152  is coupled to an area on each of the plurality of chips  154  that is unpopulated by the plurality of solder deposits  156 . Accordingly, the geometry of elongator  152  may correspond to an area on each of the plurality of chips  154  that is unpopulated by the plurality of solder deposits  156 .  
      Wafer  150  may be separated into individual chips by scribing or sawing. Each individual chip may be used to elongate a solder joint with an interposer or a board using the method illustrated in  FIGS. 4   a  to  4   c.    
       FIG. 7  illustrates a plan view of a series of interposers  200  with an elongator  202  affixed thereto in accordance with another embodiment of the present invention. The series of interposers  200  comprises a plurality of interconnected interposers  204 , each of which includes a solderable surface  206 . Elongator  202  is coupled to an area on each of the plurality of interconnected interposers  204  that is unpopulated by solderable surface  206 . Accordingly, the geometry of elongator  202  may correspond to an area on each the plurality of interconnected interposers  204  that is unpopulated by solderable surface  206 .  
      The series of interposers  200  may be subjected to singulation along a singulation line  208  to separate the plurality of interconnected interposers  204  into individual interposers. Each individual interposer may be used to elongate a solder joint with a chip using the method illustrated in  FIGS. 4   a  to  4   c.    
      Alternatively, the series of interposers  200  may be used to elongate a solder joint  210  with a chip  212  prior to singulation as illustrated in  FIGS. 8   a  to  8   c . Referring first to  FIG. 8   a , solderable surface  206  is positioned to oppose a plurality of solder deposits  214  on chip  212 .  
      Elongator  202  comprises an expander  216  encapsulated by an encapsulant  218 . Expander  202  is restrained in a compressed state by encapsulant  218 . Expander  216  may be a corrugated strip, a linear compression spring or other resilient devices. Encapsulant  218  has a softening temperature that is approximately 40° C. higher than a melting point of the plurality of solder deposits  214  and is preferably an electrical insulator such as polyamide and polyacetal.  
      Heat is applied to melt the plurality of solder deposits  214  via a reflow process. At a solder reflow temperature, that is, a temperature above the melting point of the plurality of solder deposits  214 , each of the plurality of solder deposits  214  melts and wets a corresponding solderable surface  206 , forming solder joint  210  as illustrated in  FIG. 8   b . Solder joint  210  couples each of the plurality of interconnected interposers  204  to chip  212  to form an electronic assembly  220 .  
      Because the softening temperature of encapsulant  218  is higher than the solder reflow temperature, encapsulant  218  remains sufficiently rigid to prevent a release of expander  216  from its compressed state.  
      Electronic assembly  220  is then subjected to further heating to raise a reflow temperature to the softening temperature of encapsulant  218 . At the softening temperature, encapsulant  218  begins to soften.  
      As encapsulant  218  softens, expander  216  is gradually released from its compressed state, resulting in a gradual elongation of solder joint  210 . The rate at which solder joint  210  elongates depends on the compliance of expander  216  and the viscosity of encapsulant  218 . The use of a more compliant expander  216  and a more viscous encapsulant  218  will result in a slower release of expander  216  from its compressed state, and a correspondingly slower rate of elongation.  
      With reference to  FIG. 8   c , electronic assembly  220  is cooled when solder joint  210  attains a desired height H d ′. Upon cooling, solder joint  210  and encapsulant  218  solidify. Elongator  202  serves as a reinforcement for solder joint  210 , increasing its resistance to stress and strain caused by temperature cycling and to mechanical shock, thereby enhancing the reliability of solder joint  210 .  
      The series of interposers  200  may then be singulated along a singulation line  224  to separate the series of interposers  200  into a plurality of individual electronic assemblies.  
      An advantage of the present invention is that it requires a minimal number of processing steps. Because an elongator is formed directly on one of the substrates forming an electronic assembly, no further processing steps are required to position and to affix the elongator to the substrate individually. By minimizing the number of processing steps required, manufacturing costs and the probability of defects in the solder joint are minimized.  
      Additionally, by forming the elongator directly on one of the substrates forming the electronic assembly, the manufacturing process is simplified as the elongator does not then require individual handling.  
      Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention. Furthermore, certain terminology has been used for the purposes of descriptive clarity, and not to limit the present invention. The embodiments and preferred features described above should be considered exemplary, with the invention being defined by the appended claims.