Solder joint attachment reliability and assembly quality of a molded ball grid array socket

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Abstract
This paper summarizes efforts to improve the assembly quality and solder joint reliability of a molded, plastic ball grid array socket. Metallographic analysis shows that a ball grid array type solder interconnect has superior assembly quality compared to a butt type solder interconnect. Ball shear tests, coupled with reflow preconditioning show that the socket ball attachment exceeds industry requirements established for area array packages. Isothermal aging, ball shear tests, and X-ray fluorescence confirm that the electrolytic Ni/Au surface finish on the socket bond pads presents no risk for interfacial, intermetallic embrittlement. The coefficient of thermal expansion (CTE) of the socket is optimized through the proper selection of material and molding process. The physical property improvements are confirmed using bulk CTE measurements on sockets manufactured with and without material and process optimization. Sockets are assembled on PWB test vehicles using typical surface mount manufacturing processes and temperature cycling is used to assess the long-term, solder joint attachment reliability. Failure analysis of the thermally cycled assemblies shows that the reduction in the anisotropy and mismatch of the CTE alters the failure mode of the sockets, which results in a substantial improvement in the overall long-term attachment reliability. The improvements in solder joint reliability are correlated with the improvements in the thermal expansion properties and the enhanced reliability is discussed in terms of lowering the risk of using this socket technology in more demanding use environments.

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