Abstract:
A system comprising a ball grid array (“BGA”) substrate adapted to electrically couple to an application board using a plurality of solder balls, and a film adapted to abut the application board and the BGA substrate, the film comprising a plurality of perforations, the solder balls adapted to couple to the application board through the perforations.

Description:
BACKGROUND  
       [0001]     A ball grid array (“BGA”) package is a type of chip package wherein solder balls are used to electrically connect the BGA package to a structure external to the package, such as a printed circuit board (“PCB”). The solder balls conduct electrical signals between a chip inside the package and the external structure. A BGA package is electrically coupled to a PCB using the solder balls during a solder reflow process. During a solder reflow process, the solder balls are heated such that the solder balls melt (i.e., “reflow”) and form electrical connections (i.e., metallic bonding) with the PCB.  
         [0002]     Many BGA packages have heatsinks coupled to a surface of the BGA package opposite the solder balls.  FIG. 1  shows one such BGA package  100  abutting a heatsink  102 . The BGA package  100  comprises a chip  10  abutting a substrate  20 . The BGA package  100  is electrically coupled to a PCB  104  by way of multiple solder balls  106  that are coupled to the substrate  20  at solder joints  108 . The heatsink  102  is assembled abutting the BGA package  100  after the BGA package  100  is reflowed to the PCB  104 . The heatsink  102  is assembled abutting the BGA package  100  either through adhesive attach, spring clipping, or screw and backing plate assembly. The weight of the heatsink  102  may add stress to the solder balls  106  and the solder joints  108 , thus damaging the solder joints  108 . In cases where the heatsink  102  is screwed to a backing plate  50  using screws  52  as shown in  FIG. 1 , a compressive force caused by the heatsink  102  and the screws  52  also may cause the solder joints  108  to be damaged. Damaged solder joints  108  may render the BGA package  100  useless.  
         [0003]     The stress resulting from the weight and compressive force from the heatsink  102  also may cause the solder balls  106  to be compressed in between the BGA package  100  and the PCB  104  to a degree greater than in a typical solder reflow process. This compression causes each solder ball  106  to creep and progressively expand toward adjacent solder balls  106 , as shown in  FIGS. 2   a - 2   c . Specifically,  FIG. 2   a  shows the solder balls  106  prior to creeping.  FIG. 2   b  shows the solder balls  106  expanding toward each other due to compression between the substrate  20  and the PCB  104  (i.e., caused by the weight and/or compression of the heatsink  102 /screws  52 /backing plate  50  assembly). As shown in  FIG. 2   c , a sufficient amount of creeping under compression may cause at least some of the solder balls  106  to come into electrical contact with each other, resulting in multiple short circuits. These short circuits may render the BGA package  100  and/or the PCB  104  useless.  
         [0004]     One possible solution to such a problem is to apply a polymer underfill between the substrate  20  and the PCB  104 . However, applying an underfill prevents the package  100  from being removed from the PCB  104 . For example, if the package  100  does not function properly, the package  100  cannot be removed from the PCB  104  and replaced with a properly functioning package. Leaving an improperly-functioning package  100  on the PCB  104  substantially increases cost, particularly in applications such as servers and telecommunications.  
       BRIEF SUMMARY  
       [0005]     The problems noted above are solved in large part by a solder joint support film for BGA packages under heatsink compression. One exemplary embodiment may be a system comprising a ball grid array (“BGA”) substrate adapted to electrically couple to an application board using a plurality of solder balls, and a film adapted to abut the application board and the BGA substrate, said film comprising a plurality of perforations, the solder balls adapted to couple to the application board through said perforations. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     For a detailed description of exemplary embodiments of the invention, reference will now be made to the accompanying drawings in which:  
         [0007]      FIG. 1  shows a BGA package electrically coupled to a PCB and a heatsink assembled abutting the package;  
         [0008]      FIGS. 2   a - 2   c  show the progressive compression creeping of solder balls as the substrate is pushed closer to the PCB due to the compressive load from the heatsink;  
         [0009]      FIG. 3  shows a thin film having multiple perforations, in accordance with a preferred embodiment of the invention;  
         [0010]      FIG. 4   a  shows the thin film abutting the substrate, in accordance with embodiments of the invention;  
         [0011]      FIG. 4   b  shows a PCB abutting the substrate and thin film configuration of  FIG. 4   a , in accordance with embodiments of the invention;  
         [0012]      FIG. 4   c  shows the thin film between the substrate and the PCB, in accordance with embodiments of the invention; and  
         [0013]      FIG. 5  shows a flow chart in accordance with embodiments of the invention. 
     
    
     NOTATION AND NOMENCLATURE  
       [0014]     Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections.  
       DETAILED DESCRIPTION  
       [0015]     The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment.  
         [0016]     Presented herein is a device that supports BGA package solder joints and prevents solder ball short circuiting. Specifically, a perforated thin film is deposited between a BGA package and a PCB to provide mechanical support to the solder joints and the BGA package during a solder reflow process. The perforated thin film also prevents the solder balls from coming into electrical contact with each other due to stress applied by a heatsink abutting the BGA package.  
         [0017]      FIG. 3  shows a top view of a thin film  300  comprising a plurality of perforations  302 . The perforations  302  preferably are produced to align with a BGA package solder ball pattern with which the thin film  300  is to be used, although any arrangement of perforations  302  may be used. Likewise, the thin film  300  may have dimensions of any suitable size. In particular, the thin film  300  preferably has a thickness substantially similar to that of the diameter (e.g., height) of the solder balls  106 . The thin film  300  may have a thickness greater than approximately 25.0 micrometers, although the scope of disclosure is not limited to these dimensions.  FIG. 4   a  shows a cross sectional side view of the chip  10  abutting the BGA substrate  20 . The BGA substrate  20  is electrically coupled to the multiple solder balls  106 . The thin film  300  is coupled to the BGA substrate  20  using an adhesive (e.g., epoxy) such that at least some of the solder balls  106  are at least partially within perforations  302  of the thin film  300 .  
         [0018]      FIG. 4   b  shows the configuration of  FIG. 4   a  during a solder reflow process, wherein the BGA substrate  20  is electrically coupled to the PCB  104  using the solder balls  106 . Because the heatsink  102  abuts the chip  10 , the solder balls  106  and corresponding solder joints  108  are subjected to mechanical stress, as described above. However, because the thin film  300  abuts the BGA substrate  20  and the PCB  104 , the thin film  300  supports the BGA substrate  20  and the solder joints  108 . In this way, the BGA substrate  20  and the solder joints  108  are not subjected to so much stress that solder ball short circuits form or the solder joints  108  become damaged as described above.  
         [0019]      FIG. 4   c  shows a detailed view of the BGA substrate  20  coupled to the PCB  104  by way of the solder balls  106 , and the thin film  300  situated therebetween. The stress applied to the BGA substrate  20  and the solder balls  106  by the heatsink  102  causes the solder balls  106  to be compressed, as described above. This compression causes the solder balls  106  to horizontally expand toward adjacent solder balls  106 . However, because the thin film  300  is situated between some or all pairs of solder balls  106 , the solder balls  106  do not expand to the degree that the solder balls  106  would expand in the absence of the thin film  300 . Furthermore, for the same reason, the likelihood of two solder balls  106  causing a short circuit by coming into electrical contact with each other is considerably low or virtually nonexistent. Also, unlike underfill material, because the thin film  300  is not permanently fixed between the substrate  20  and the PCB  104 , the thin film  300  may allow for replacement of an improperly-functioning package  100 . Enabling such package replacements may substantially reduce costs compared to those incurred by using an underfill material between the substrate  20  and the PCB  104 .  
         [0020]     The thin film  300  may be fabricated using any suitable process such as that shown in  FIG. 5 . The liquid photo imaging process of  FIG. 5  may begin with exposing a film material to light in accordance with the design of the thin film  300  (block  502 ). In this way, at least some portions of the film are chemically altered. The process may be further continued by processing or developing the film using etchants, such that at least some of the portions of the film are etched away, leaving a film having a pattern substantially similar to the pattern of the thin film  300  or some other desired thin film pattern (block  504 ). Finally, the film is cured, such as by heating the film in an oven until the film is dry (block  506 ). The order of the acts depicted in  FIG. 5  may be altered as desired. The scope of disclosure is not limited to the specific process shown in  FIG. 5 . Any process that produces the thin film  300  and the perforations  302  in the thin film  300  (e.g., mechanical drill process, mechanical punching process, laser drill process) may be used. Furthermore, although the thin film  300  preferably is produced using polyimide, any suitable (e.g., nonconductive) material may be used.  
         [0021]     The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.