Effect of Voiding on Solder Joint Shock and Thermal Cycle Reliability

摘要

Solder joint void has been considered a typical phenomenon in electronics assembly. Voids are caused by entrapped gases produced during flux volatilization during SMT, air entrapment in plated through holes and flux reaction with metal contaminates during the assembly reflow process. The factors affecting void formation are complex and subjective. It has been observed that BGAs with 2%Ag (62Sn36Pb2Ag) ball tend to create larger voids in reflow solder joints than eutectic (63Sn37Pb) ball. This study investigates the effect of different process conditions on void formation in the eutectic and 2%Ag solder joints and their effect on long term (2nd level) reliability. A series of test vehicle PCBs were populated for 2nd level reliability testing using three different BGA packages (165PBGA, 144PBGA, and 1657 FCBGA). A DOE was designed using three factorials; 1) BGAs were acquired with eutectic balls and 2%Ag balls, 2) two reflow profiles were established, 245C peak volcano-type profile with 3C/sec ramp and long soak profile with 204C peak, 3) two package preconditioning, 60C/60%RH 10days and non-aged. After SMT assembly test vehicles were x-ray inspected for void distribution and void size. X-ray results showed that large voids, greater than 35% of the solder joint diameter according to IPC-7095A for class 2, were observed in both eutectic and 2%Ag solder jqints assembled using large solder paste volume and reflowed using fast ramp rate to 245C peak temperature. Approximately 60-250% higher distribution of voids was observed in 2%Ag ball packages than eutectic ball packages. It is believed that the lower melting of 2%Ag (179C) solder ball can reflow slightly before the eutectic (183C) solder paste, thus the solder ball collapses or encapsulates more solder paste/flux thus entrapping more gases within the bulk solder and consequently promoting void formation. The effect of voiding on solder joint shock and thermal reliability was investigated and discussed. It showed voids generated during board level assembly process do not degrade solder joint reliability.