AGING EFFECTS ON DYNAMIC BEND TEST PERFORMANCE OF PB-FREE SOLDER JOINTS ON NI/AU FINISH
Authors: Min Ding and Adriana Porras Company: Freescale Semiconductor, Inc. Date Published: 10/1/2006
Abstract: The concern for drop impact reliability of microelectronic ball grid array (BGA) packages has attracted more and more attention due to the prosperity of portable electronic devices. The concern is further accentuated with the introduction of Pb-free solder alloys which are more susceptible to intermetallic compound (IMC) brittle fracture along the solder-pad interface. Previous research has shown that, compared with SnPb alloy, the higher yield strength of Pbfree alloy under high strain rate is one of the major factors that induces the failure. The microstructure of the Pb-free alloy affects the yield strength considerably. In this present work, BGA packages with Pbfree sphere and Ni/Au pad was mounted on to PCB boards and subject to aging tests under different temperatures. The evolution in microstructure of solder alloy after board mounting reflow was examined with scanning electron microscope (SEM) and energy dispersive X-ray (EDX). Significant microstructure change is observed even under room temperature aging. At the same time, the shear strength of the solder joint is found to be decreasing with increasing aging time. The driving force and kinetics of the evolution as well as the correlation between the microstructure and mechanical strength is discussed. Dynamic bend testing was performed on the aged parts. The performance changes considerably with aging. The changes are correlated to the microstructure and mechanical strength change in solder alloy.