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Frame Support for Reliable Surface Mount Technolgy Array PackageTerminations

IP.com Disclosure Number: IPCOM000125187D
Original Publication Date: 2005-May-23
Included in the Prior Art Database: 2005-May-23
Document File: 5 page(s) / 222K

Publishing Venue

IBM

Abstract

Low cost plastic injection molded external frame stops have been developed to provide optimum reliability of high performance, surface mount solder ball or solder column array packages that are coupled to large mass scalable cooling solutions. Specifically, the frame stops are used to reduce or eliminate detrimental long term creep and deformation of solder interconnections. Once installed, the frame stops offset high mechanical loads present on solder joints from the attachment of large heatsinks, and also restrict lateral motion of the module package structures relative to the externally applied thermal cooling solutions. The use of the frame stops therefore inhibits both large scale solder joint array connection creep induced from compressive stresses present due to mechanical loading of large heatsinks or thermal masses, and also reduce lateral strain induced solder creep that results on the array interconnections during in service operation. The frame stops are physically mounted along module edges, and are affixed between opposing module and card surfaces after the module has been soldered to the board surface. After insertion, the rigidly affixed frame stops possess physical "downstops" which offset applied loads used to affix thermal masses away from solder joints to module component perimeters To facilitate manufacturing consistency and leverage the beneficial effects of this packaging structure, the frame stops also possess built in crush ribs that accommodate gap tolerances of as soldered, module- to- board height differences. Finally, the two-piece design of the frame stops, accommodate easy installation and removal under and around portions of the module perimeter, and are compatible with all conventional assembly processes.

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Frame Support for Reliable Surface Mount Technolgy Array PackageTerminations

    In order to enable high interconnection density, excellent electrical performance, and enhanced interconnect reliability, many processor and ASIC module applications organic or inorganic solder ball, or solder column array packages for attachment to printed circuit boards. Since high performance chip devices are commonly used on solder ball and solder column array packages, their high power requirements typically mandate the attachment of large, heavy, externally applied cooling solutions such as heatsinks or evaporator structures which can be affixed to bare chip device surfaces, or to integral heat spreaders, or lids present on the array packages. To affix large externally applied heatsinks, a mechanical attach configuration which applies a compressive load through the module and solder joints is usually provided to secure the cooling solution hardware, and to ensure consistent thermal interface materials function between module and heatsink.

    Within this realm of packaging requirements, testing of column grid and ball grid array package technologies soldered to printed boards indicate that external compressive mechanical load restrictions must be placed on externally applied heatsink members that are rigidly affixed to soldered array packages to avoid solder joint cracking, solder column crushing, or solder ball deformation under steady state intended long term field use temperatures and power on/off cyclic field conditions. Failure to work within these guidelines can result in packaging interconnects which fail relatively short term due to tilt and crushing phenomenon as shown in Figure 1.

    Compressive load conditions and their impact on joint cracking or interconnect crushing phenomenon are a strong function of multiple variables including solder material creep properties, system/customer field use temperatures, cyclic power conditions, applied mechanical loads, and the ultimate load distributions and stresses that result on solder joints from effects of heatsink geometry, spreader structures used, spreader geometries used, load configurations used, and overall flatness, planarity, and mechanical tolerances of the module, the circuit board and all mechanical components involved in the overall board assembly packaging. In most circumstances, these aggregate factors restrict the allowable compressive load on any given soldered column array package to approximately 10 gm/soldered contact, while for ball grid array packages, loads are typically restricted to approximately 15-20gm/solder contact to avoid significant solder creep and associated array connection solder deformation.

    However, as module sizes, power densities, and performance requirements increase, large thermal cooling solution masses or enhanced spreader structures coupled with larger, advanced cooling technology heatsinks are also required to ensure efficient cooling performance. Because thes...