SMTA International Conference Proceedings


MECHANICAL FAILURES IN Pb-FREE PROCESSING: SELECTED MITIGATION TECHNIQUES FOR PAD CRATER DEFECTS

Authors: Brian Gray, P. Eng., John McMahon, P. Eng.
Company: Celestica Inc.
Date Published: 10/24/2010   Conference: SMTA International


Abstract: The higher temperatures required for lead-free processing have resulted in changes to laminates, including the addition of micro clay (to reduce z-axis expansion) and replacing dicyandiamide based epoxies with phenolic based epoxies, among others. These changes in combination with stronger lead-free SAC (Sn-Ag-Cu) alloys have changed the failure mode from predominantly interfacial fracture in complex intermetallic compound at the board / package interface to adhesive and cohesive failure of the buttercoat layer in the laminate. This new failure mode pad crater [1] presents a significant risk to product reliability because it does not necessarily result in electrical failure and there is currently no repair process. The manifestation of this new failure mode highlighted an electronics industry test standards shortcoming and in response to this, IPC-9707 Spherical Bend Method for Characterization of Board Level Interconnects is being developed to better measure resistance of laminates to pad crater.

Multiple pad crater mitigation paths are currently being pursued, which include changes to resin formulations and determination of working process strain limits. Initial determination of working strain limits for ball grid arrays (BGAs) has been completed at multiple strain rates and pad crater was found to be the predominant failure mode [2]. The pad crater occurred in the area of highest strain, the BGA package corners. Investigation of pad crater mitigation through changes to footprint design including larger solder masked defined (SMD) pads at BGA corners is investigated.

Key words: pad crater, spherical bend test, mechanical failure mode, process strain, solder mask defined (SMD) non-solder mask defined (NSMD), ball grid array (BGA)



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