SMTA International Conference Proceedings

Metallization and Surface Finish Effects on Pb-Free WLCSP Thermal Cycle Reliability

Authors: Michael Meilunas and Jim Wilcox
Company: Universal Instruments Corporation
Date Published: 9/27/2015   Conference: SMTA International

Abstract: The reliability of solder interconnects often depends on the surface finishes of the two opposing interfaces of the joint. For the case of a wafer level chip scale package (WLCSP), reliability may be further influenced by the geometry of the CSP metallization structure. This study explores several of these contributing effects.

WLCSP packages having several different under ball metallization (UBM) structures were assembled to NSMD pads on test boards with six different surface finishes. Board finishes include two OSP formulations, two Pd bearing finishes, an electroless Ni finish and a Pb-free HASL finish. The resulting 0.4 mm pitch SAC305 solder ball interconnects were electrically stitched through the board to monitor continuity during reliability testing. A -40 to 125°C thermal cycle reliability stress was used.

Nearly all package metallization/board finish combinations exhibited component side solder fatigue failures in thermal cycle. A notable exception was the case of WLCSP assembly to boards with a direct Pd finish, which failed on the board side. Single layer wafer metallization structures were found to systematically reduce the thermal cycle reliability of the WLCSP solder interconnect.

Key Words: 

wafer level CSP, surface finish, SAC305 bump

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