Thermal Cycling Reliability of Newly Developed Lead-Free Solders for Harsh Environments
Authors: Francy John Akkara, Cong Zhao, Sudan Ahmed, Mohammed Abueed, Sinan Su, Sa'd Hamasha, Jeffrey Suhling, Pradeep Lall Company: Auburn University Date Published: 9/22/2019
Abstract: SnAgCu (SAC) alloys that were a replacement for traditional Tin-Lead (SnPb) alloy has been around for about two decades. Since the detrimental effects of aging were observed, several other elements such as Bismuth (Bi), Antimony (Sb), Nickel (Ni), Cobalt (Co) has been micro-alloyed to mitigate the aging effects. Recently, the leading solder manufacturers have put forward new alloys that claim to have high reliability in harsh environments. Several elements have been added in different proportions to make the solder alloys better. However, none of the new materials have been tested in harsh environments. In this study, four alloys with different compositions of SAC-Bi-Sb-Ni-Co and one alloy without Ag are analyzed and compared with SAC305. The test vehicle used in the study consists of three CABGA208s (15mmx15mm), three LGAs (5mm x 5mm) and six surface mount resistors (SMRs) connected in series with SAC305 solder spheres. Two popular surface finishes, namely, electroless Nickel immersion Gold (ENIG) and organic solderability preserve (OSP) were included. The BGAs in the test, with SAC305 solder spheres, represent the ones readily available in the market. The boards are aged at +125oC for a period of twelve months immediately after assembly, after which they are thermally cycled from -40C to +125C with a ramp time of 50 minutes and dwell times of 15 minutes each at high and low temperatures.
Two-parameter Weibull analysis is used for analyzing the solder joint reliability. B10 analysis is also done to investigate the early failures. The results clearly showed that the newly developed alloys with dopants have much better reliability than SAC305. Representative cross-sections are also presented to investigate the intermetallic compound precipitates and the failure mode.