Patent Publication Number: US-7211888-B2

Title: Encapsulation of pin solder for maintaining accuracy in pin position

Description:
This application is a divisional of U.S. patent application Ser. No. 09/965,555, filed Sep. 27, 2001 now U.S. Pat. No. 6,974,765, which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The invention relates generally to microelectronic circuits and, more particularly, to packaging of microelectronic circuits. 
   BACKGROUND OF THE INVENTION 
   In many pin grid array (PGA) packaging processes, the pins are attached to the package substrate before the corresponding die is mounted. During the subsequent die attach process, the solder connecting the pins to the substrate can melt if the associated processing temperatures are too high. If the pin solder melts, the pins can wiggle and move out of the positional ranges required for socket insertion. To prevent the pin solder from melting, lower temperature solders have traditionally been used during die attach processing so that the melting temperature of the pin solder is not exceeded. However, it is becoming more popular to use higher melting temperature solders (e.g., lead free solders) during the die packaging process. The use of such solders makes it increasingly difficult to avoid melting pin solder during packaging of microelectronic circuits. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 ,  2 ,  3 ,  4 , and  5  are a series of simplified cross-sectional side views illustrating a method for attaching pins to a microelectronic package substrate in accordance with an embodiment of the present invention; 
       FIGS. 6 ,  7 ,  8 ,  9 , and  10  are a series of simplified cross-sectional side views illustrating a method for attaching pins to a microelectronic package substrate in accordance with another embodiment of the present invention; and 
       FIGS. 11 and 12  illustrate a die attach technique that can be used in accordance with the present invention. 
   

   DETAILED DESCRIPTION 
   In the following detailed description, reference is made to the accompanying drawings that show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that the various embodiments of the invention, although different, are not necessarily mutually exclusive. For example, a particular feature, structure, or characteristic described herein in connection with one embodiment may be implemented within other embodiments without departing from the spirit and scope of the invention. In addition, it is to be understood that the location or arrangement of individual elements within each disclosed embodiment may be modified without departing from the spirit and scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, appropriately interpreted, along with the full range of equivalents to which the claims are entitled. In the drawings, like numerals refer to the same or similar functionality throughout the several views. 
   The present invention relates to methods and structures that can be used to limit movement of the pins of a microelectronic package when pin solder melts during, for example, a die attach process. A solid material is used to surround the solder joint associated with a pin so that movement of the pin is constrained even when the solder melts. Thus, the pins remain in the positional ranges required for socket insertion through out the packaging process. The inventive principles can be used in connection with a wide variety of microelectronic devices that utilize pins on a device package to provide electrical connection to an external circuit (e.g., devices using PGA&#39;s). The inventive principles are particularly beneficial for use in packaging processes that utilize high temperature (e.g., lead free) solders during die attach. 
     FIGS. 1–5  are a series of simplified cross-sectional side views illustrating a method for attaching pins to a package substrate in accordance with an embodiment of the present invention. With reference to  FIG. 1 , a substrate  10  is provided that has pin attach contact pads  12  disposed on a surface thereof. As illustrated in  FIG. 2 , a polymer material  14  (e.g., an encapsulation material) is deposited on the substrate  10  over the contact pads  12 . In at least one embodiment, the polymer material  14  is screen printed on the substrate  10 . Any of a variety of other deposition techniques can alternatively be used to deposit the polymer material  14  including, for example, spray coating, liquid dispense, and film lamination. As illustrated in  FIG. 2 , in one approach, a separate portion of polymer material  14  is applied to each individual contact pad  12  on the substrate  10 . In another approach, the polymer material  14  is applied to the contact pads  12  in predefined groups. In yet another approach, a single layer of polymer material  14  is used to cover all of the contact pads  12  on the substrate  10 . As will be appreciated, the invention is not limited by the application pattern of the polymer material  14 . Although not illustrated, openings or depressions may be provided in the polymer material  14  in the desired pin locations. 
   After the polymer material  14  has been deposited, solder balls  16  (or a similar form of solder element) are placed into the polymer material  14  in locations corresponding to the desired pin locations, as illustrated in  FIG. 3 . In one approach, the solder balls  16  are physically pressed into the polymer material  14  using mechanical means (e.g., a jig). In another approach, molten solder is dropped onto the polymer material  14  in the desired locations. If openings or depressions are provided in the polymer material  14 , the solder balls  16  may be deposited into the openings or depressions. In a preferred technique, all of the solder balls  16  are applied simultaneously. 
   After the solder balls  16  are in place, the pins  18  are attached. As shown in  FIG. 4 , in one embodiment, the pins  18  are loaded into a jig  20  that holds the pins  18  in fixed relation to one another. It should be appreciated that individual placement of the pins  18  is also possible (i.e., placement without the use of a jig) in accordance with the invention. The jig  20  is placed over and aligned with the substrate  10  so that the pins  18  align with the corresponding contact pads  12 . The assembly is heated to an appropriate temperature (typically a fixed amount higher than the melting temperature of the pin solder) and a force  22  is applied to the jig  20  in the direction of the substrate  10 . In one approach, the force of gravity is all that is used. In another approach, an additional external force is applied to the jig  20  to ensure that the solder associated with each pin  18  wets the corresponding contact pad  12 , rather than simply floating within the polymer material  14 . Although not shown, guides may be used to prevent lateral movement of the jig  20  as it moves toward the substrate  10 . After the pins  18  have contacted the corresponding contact pads  12 , the assembly is allowed to cool during which time the polymer material  14  cures. After sufficient cooling has occurred, the jig  20  is removed and the assembly of  FIG. 5  results. As illustrated in  FIG. 5 , each pin  18  is coupled to a contact pad  12  at a corresponding solder joint  24 . Significantly, the polymer material  14  has cured about the solder joints  24  in a manner that will constrain the movement of the associated pins  18  should the pin solder melt during subsequent processing. 
   The polymer material  14  that is used in the above-described process should be one that allows a pin  18  and its associated solder to penetrate through the material  14  when pressure is applied, as shown in  FIG. 4 . The polymer material  14  should also be a material that, once cured, will maintain its shape and structural integrity even if relatively high temperatures are subsequently encountered. In at least one embodiment, a polymer material  14  is used that also has fluxing capabilities to facilitate the formation of the solder joints  24 . As will be appreciated, the use of a polymer material  14  with fluxing capabilities may dispense with the need to apply a separate flux material during the pin attach process, thus reducing associated processing costs. In one approach, any of a number of commercially available “no flow” materials are used as the polymer material  14 . These materials can include, for example, Cookson 2071E, Questech EF71 or LF-8, Advanced Polymer Solutions (APS) UFR 1.0 to 1.5, Kester Solder SE-CURE® 9101, Emerson &amp; Cuming RTP-100-1, Sumotomo CRP 4700, and Loctite FF2000 and FF2200. 
     FIGS. 6–10  are a series of simplified cross-sectional side views illustrating a method for attaching pins to a package substrate in accordance with another embodiment of the present invention. With reference to  FIG. 6 , a substrate  30  is provided that has pin attach contact pads  32  disposed on a surface thereof. Solder bumps  34  (or similar solder structures) are deposited on the contact pads  32 , as illustrated in  FIG. 7  (alternatively, the solder can be applied to the contact surfaces of the pins  36  or to both the pins  36  and the contact pads  32 ). With reference to  FIG. 8 , the pins  36  are loaded into a jig  38  that is placed over and aligned with the substrate  30  (individual pin placement is also possible). The assembly is then heated to an appropriate temperature and a force  40  is applied to the jig  38  in the direction of the substrate  30 . As before, the force  40  can be gravity or an externally applied force. The assembly is then allowed to cool and the jig  38  is removed, resulting in the structure of  FIG. 9 . As shown in  FIG. 9 , each of the pins  36  is conductively coupled to a contact pad  32  at a corresponding solder joint  42 . As illustrated in  FIG. 10 , an encapsulation material  44  is next applied to the assembly in a manner that enshrouds the solder joint  42  associated with each pin  36 . The encapsulation material  44  is then allowed to cure to a hardened state before subsequent processing steps are undertaken. Similar to the previous embodiment, the hardened encapsulation material  44  will constrain the movement of the pins  36  should the associated pin solder melt during subsequent processing. 
   Any of a wide range of encapsulation materials  44  can be used in the above-described process. In one approach, for example, any of the materials normally used as underfill in microelectronic assemblies may be used as the encapsulation material  44 . This can include, for example, epoxy materials, polyimide materials (e.g., SPARK®), Dow Chemical BCB (e.g., Cyclotene®), Dexter CNB 868-10, and SEC 5230JP or 5114. In another approach, an injection molding compound is used as the encapsulation material  44 . As illustrated in  FIG. 10 , in at least one embodiment, the encapsulation material  44  is selectively applied to the solder joints of the individual pins  36 . This can be accomplished using, for example, a liquid dispense process to deposit the encapsulation material  44 . A number of other deposition techniques may alternatively be used to deposit the encapsulation material  44  including, for example, injection molding, spray coating, film coating, and others. In another approach, a single layer of encapsulation material  44  is used to enshroud the solder joints  42  of all of the pins  36  on the substrate  30 . Other techniques are also possible. 
   After the pins of a microelectronic package have been attached to the package substrate, the microelectronic die can be attached to the substrate. Often, the pins will be attached by a package vendor while the die is attached by the microelectronic device manufacturer. Other scenarios are also possible.  FIGS. 11 and 12  illustrate one possible die attach process that can be used in accordance with the present invention. It should be appreciated that many other die attach techniques can alternatively be used. As illustrated in  FIG. 11 , a substrate  50  is provided that already has pins  52  attached to a first surface thereof. The substrate  50  also has a number of die attach contact pads  54  disposed upon a second surface thereof. A microelectronic die  56  is provided that includes a number of pads  58  on a surface thereof The pads  58  on the die  56  can include, for example, solder bumps that are attached to underlying bond pads on the die  56 . In at least one embodiment of the invention, a high melting temperature, lead-free solder is used to connect the die  56  to the substrate  50 . As illustrated in  FIG. 11 , the die  56  is positioned above and aligned with the substrate  50 . The temperature of the components is increased to an appropriate level (typically a fixed amount higher than the melting temperature of the die solder) and the die  56  is brought into contact with the substrate  50  so that the pads  58  on the die  56  are coupled to corresponding contact pads  54  on the substrate  50 . The assembly is then allowed to cool. As illustrated in  FIG. 12 , an underfill material  60  may be injected into the die interconnect region to provide additional structural rigidity to the assembly. 
   Although  FIGS. 1–12  illustrate various views and embodiments of the present invention, these figures are not meant to portray microelectronic assemblies in precise detail. For example, these figures are not typically to scale. Rather, these figures illustrate microelectronic assemblies in a manner that is believed to more clearly convey the concepts of the present invention. 
   Although the present invention has been described in conjunction with certain embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.