Abstract:
A method for mechanically supporting a integrated circuit (IC) package having a column grid array (CGA) interconnection with a printed circuit board (PCB) is provided by inserting a supporting device between the IC package and the PCB after the IC package is solder attached to the PCB. The supporting device is removably and mechanically fastened to the PCB and is designed in such a shape so that the supporting device cannot come into contact with the solder columns of the CGA to cause damage or shorting. This invention eliminates the maximum retention load constraint of the IC package and enables a wide variety of thermal solution implementations without comprising reliability.

Description:
TECHNICAL FIELD 
   The present invention relates generally to printed circuit board (PCB) technology, and, in particular, to devices that mechanically support column grid array interconnects and methods of using such devices. 
   BACKGROUND OF THE INVENTION 
   Package to board interconnection has been accomplished using many different methods over the years. The industry was initially dominated by pin-through-hole (PTH) lead attachment with “integrated circuit” packages that were rectangular in shape and had rather large leads extending from the long side of the rectangle. These devices were limited in lead count and provided very rugged interconnection between the package and the printed circuit board (PCB). PTH technology was gradually replaced by surface mount technology in order to increase the number of leads, and to improve the automation of the process for attaching the devices to the boards. In the recent years, a new packaging technology, known as “ball grid array” (BGA) technology, has been developed. A BGA package consists of a silicon chip attached to the surface of a substrate. The substrate has printed circuitry that provides interconnect points for the silicon chip on the top surface, connected by fine pitch traces to an array of pads on the bottom surface. The pads on the bottom surface have attached solder spheres that serve as the interconnect points for the package to the PCB. The BGA technology allows designs with lead counts of over 1000 input/output points. In addition to the high lead count, this technology also affords many other benefits that include ease of handling, simplified device attachment and overall cost effectiveness compared to fine pitch, perimeter leaded devices. 
   The BGA technology, however, has a compromised reliability in thermal cycling. A perimeter leaded device with gull wings can be subjected to many thermal cycles without encountering stresses due to thermal coefficient of expansion (TCE) mismatch between the PCB and the device. BGA packages, on the other hand, are connected to the board with a rigid structure of solder spheres (oval shaped after reflow). When the device is operated, waste heat builds up and a temperature differential between the device and the board is created. The temperature differential, as well as the differences in TCE between the device and the board, will lead to stresses in the solder sphere attachment points, which creates a high risk of fatigue failure. 
   One solution to the thermal stability problem is the column grid array (CGA) technology, which utilize a flexible column lead in place of the solder spheres. The column leads are designed to have a lower stiffness than a solder sphere and a higher offset distance between the device and the PCB. These two features enable the leads to flex with less stress as the dimensional expansion between the device and the PCB varies. The higher offset distance reduces the stress by the square of the distance between the device and the PCB. 
   CGA has been widely used in high reliability applications. However, the thin and tall solder column interconnects in CGA are susceptible to damage due to short-term dynamic load during shock, vibration, and creep under long-term static compressive load. For example, a thermal solution that is directly attached to an integrated circuit (IC) package will subject the solder columns to shock and vibration impact, as well as long-term compressive load, and therefore should have a light mass to avoid causing excessive damages to solder columns. This limitation becomes a severe problem for large and high power IC packages that need thermal solutions with a high retention load due to heat sink mass or thermal interface requirement. The high retention load often exceeds the maximum long-term compressive load of the solder columns and causes excessive creep, bending, bucking of the solder columns, which finally results in interconnect failures such as shorting or joint failure. Accordingly, the solder columns in a CGA connection often need to be mechanically supported in these applications. The supporting device also needs to be fully fastened, so that the supporting device will not get loose and cause damages by itself. Commonly used supporting devices include posts attached between heat sink and PCB, and external frame or corner support. These devices, however, often require complicated attaching process using epoxy adhesives and consume valuable PCB real estate. 
   SUMMARY OF THE INVENTION 
   Mechanical support of an IC package having a CGA interconnection with a PCB may be provided by inserting shims between the IC package and the PCB. The shims may be mechanically and removably fastened to the PCB and may be designed in such shapes that the shims can be easily inserted into the space between the IC package and the PCB, but cannot come into contact with the solder columns of the CGA to cause damage or shorting. The maximum retention load constraint of the IC package may be substantially eliminated and a wide variety of thermal solution implementations may be enabled without compromising reliability. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments described herein are better understood in conjunction with the following drawings, in which: 
       FIG. 1  is a top view of an IC package supported by supporting devices of the embodiments described herein. 
       FIG. 2  is an isometric view of one corner shim of the embodiments described herein. 
       FIGS. 3   a ,  3   b , and  3   c  depict three embodiments of fastening a shim to a PCB with a screw. 
       FIGS. 4   a  and  4   b  depict an embodiment of fastening a shim to a PCB with a dimple. 
       FIG. 5  is a flow chart depicting a method of using the supporting device of the embodiments described herein. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following detailed description is presented to enable any person skilled in the art to make and use the embodiments described herein. For purposes of explanation, specific nomenclature is set forth to provide a thorough understanding. However, it will be apparent to one skilled in the art that the specific nomenclature is not required. Descriptions of specific applications are provided only as representative examples. Various modifications to the preferred embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be applied to other embodiments and applications. The present inventions are not intended to be limited to the embodiments shown, but are to be accorded the widest possible scope consistent with the principles and features disclosed herein. 
   The embodiments described herein are generally directed to devices and methods for supporting CGA solder columns of an IC package to prevent damage to the solder columns due to shock, vibration, and long-term compressive load. 
   With reference now to  FIGS. 1 to 5 , various embodiments of a supporting device will be described. As will be described in more detail below, the supporting device may be used in a variety of configurations to provide mechanical support for CGA solder columns on an IC package. 
     FIG. 1  shows an embodiment of the supporting device. In this embodiment, the supporting device is in the form of a corner shim  101 . In one embodiment, the supporting device may have a Y shape. The shim  101  can be made of any material that is mechanically strong enough to support an application specific integrated circuit (ASIC)  103 . Examples of the shim material include, but are not limited to, plastics, ceramics, metal, and metal alloy. Preferably, the shim  101  is made of a material with a coefficient of thermal expansion (CTE) that closely matches the CTE of the solder columns. A shim  101  with a matching CTE may be preferred in high temperature applications such as a burn-in test of semiconductor wafers and high power IC packages. In this embodiment, a shim  101  is placed at each corner of the ASIC  103 . However, it is understood that the number of shims used in a particular application may vary according to the particular requirement of the application, and that the shims  101  may be placed in other depopulated areas. For example, the shims  101  may be placed along the sides of the ASIC  103 , if there are depopulated areas along the side of the ASIC  103  that allow the insertion of the shim  101 . Preferably, the PCB  105 , ASIC  103  and shims  101  are designed to accommodate each other so that shim installation can be automated. 
   There is no particular limitation on the size and shape of the shim  101 . Generally, the size of the shim  101  is minimized to reduce consumption of PCB real estate. In the embodiments shown in  FIGS. 2 and 3 , the shims  101  are designed in such a shape that an extrusion  111  of the shim  101  can be inserted into the space between the underside of the ASIC  103  and the topside of the PCB  105 . The extrusion  111  may have a thickness that is smaller than the distance between the ASIC  103  and the PCB  105 , so that the extrusion  111  may be slipped into the space between the ASIC  103  and the PCB  105  without stretching the solder columns  107 . However, the extrusion  111  may be thick enough to provide mechanical support to the ASIC  103  and prevent shorting or joint failure due to creeping of the solder columns  107  under a long-term compressive load. In other words, there is preferably a small gap  125  between the extrusion  111  and the ASIC  103  immediately after the shim  101  is installed ( FIG. 4A ). As shown in  FIG. 4B , when the apparatus is in use, the solder columns  107  will start to creep over time due to stress and the gap  125  will be closed. When the upper surface of the extrusion  111  comes into contact with the undersurface of the ASIC  103 , the ASIC  103  is fully supported by the shims  101  and the stress on the solder columns  107  is relaxed. 
   The shim  101  may also be designed in such a shape that, after being fully inserted between the ASIC  103  and the PCB  105 , the extrusion  111  of the shim  101  will not come into contact with any of the solder columns  107  of the ASIC  103 . In the embodiments shown in  FIGS. 2 and 3 , this goal is achieved by the design of a base  113 , which would come into contact with the edges of the ASIC  103  and stop the advance of the shim  101  before the extrusion  111  comes into contact with any of the solder columns  107 . 
   In order to prevent any undesired movement of the shim  103 , the shim  103  may be mechanically and removably fastened to the PCB  105 . The shim  103  may be fastened to the PCB with a screw or with a snap-and-catch mechanism such as a dimple  119 .  FIGS. 3   a  to  4   b  illustrate several embodiments of the fastening mechanism. With reference now to  FIG. 3   a , the shim  101  is fastened by a screw  115  from the bottom of the base  113  through a hole  121  on the PCB  105 . Alternatively, the screw  115  may be installed from the top of the base  113 , through the base  113 , through the hole  121 , and into a bolster  117  underneath the PCB  105  ( FIG. 3   b ). In another embodiment, a threaded adaptor  127  is inserted into the hole  121  to convert the unthreaded hole  121  into a threaded hole for the installation of the shim  101  ( FIG. 3   c ). 
   With reference now to  FIG. 4   a , the shim  101  is pushed into a position so that the dimple  119  on the base  113  of the shim  101  clicks into the hole  121  on the PCB  105  and immobilizes the shim  101 . The dimple  119  does not have to have a tight fit with the hole  121 , so long as horizontal movement of the shim  101  is restricted by the dimple  119  and the matching hole  121  and so that the shim  101  cannot come into contact with the solder columns  107 . Since the vertical movement of the shim  101  is restricted by the extrusion  111  in between the ASIC  103  and the PCB  105 , the shim  101  is properly secured on the PCB  105 . Over time, when the gap  125  is closed due to minor creeping of the solder columns  107  ( FIG. 4B ), the shim  101  will be completely immobilized. It should be noted, however, that the shim  101  can be removed at any time and reinstalled at the same or different location. 
     FIG. 5  depicts a method  500  for mechanically supporting an IC package having a CGA interconnection with a PCB. The method  500  preferably comprises the steps of inserting shims between the IC package and the PCB (step  503 ), and fastening the shims to the PCB to secure the support to the IC package and prevent damage to the solder columns of the IC package by undesired movement of the shims (step  505 ). Preferably, the shims are inserted after the IC package is solder attached to the PCB, so that the presence of the shims will not interfere with the soldering process. Shims  101  may be removed for rework of the ASIC  103  and reinstalled after the rework of the ASIC  103 . 
   The preferred embodiments of the supporting device of the present invention are intended to be illustrative and not limiting. It should be understood that modifications and variations can be made by persons skilled in the art in light of the above teachings. Therefore, changes may be made in the particular embodiments disclosed which are within the scope of what is described as defined by the appended claims.