Effect of Pb free Alloy Composition on Drop/Impact Reliability of 0.4, 0.5 & 0.8mm Pitch Chip Scale Packages with NiAu Pad Finish

Recent industry data show that the drop/impact reliability of Chip Scale Packages has been detrimentally affected by switching to SAC alloys with Ag percent equal to or greater than 3%. The primary cause of this is attributed to the higher strength and lower ductility of SAC305 and SAC405 alloys compared to SnPb solder, resulting in higher stresses at IMC layers and causing interfacial failures. While previous study [1] focused on Cu OSP finish for the packages, the study reported here is a comprehensive collection of data for NiAu pad finish using various solder alloys. The alloys considered had Ag percent varying from 1.0 to 3.0%, Cu % from 0.1 to 0.7%, and small amount of Ni, In, Co, and Ge addition to the main SAC system. Metallurgical studies were performed on identifying the IMCs and their growth with time and temperature for these alloys. Additionally, a number of board level drop tests (JESD22-B111) were performed on 0.4, 0.5, and 0.8 mm pitch CSP devices using different ball sizes and solder alloys. Finally, the data was compared against the optimum Pb free alloy solution for Cu OSP substrates. The results show that while three element SAC alloy is sufficient of NiAu surface finish, the Ag content has to reduce to gain appreciable increase in drop performance. The effect of Cu % alone in solder alloy was not found to be significant for drop performance improvement. Also, the Sn1.2Ag0.5Cu0.05Ni alloy that worked best for Cu OSP finish [1] did not perform as good as Sn1.0Ag0.5Cu alloy for NiAu finish.