Effect of environmental condition and nickel underlayer on whiskers formation in tin surface finish

Abstract

The ban on lead in electronic industry caused manufacturers to search for alternative ways to replace lead without affecting the performance of electronic products. Among lead-free alternative surface finishes, pure tin plating has attracted greater attention as potential candidate to replace hot air solder levelling (HASL) in electronic application. However, tin whiskers were reported to form on tin surface finishes and have caused the failure to electronic components. The study regarding whiskers phenomenon is important especially with the miniaturisation of electronic components in the electronic industry because whiskers from adjacent area may touch each other, causing short circuit. The main objective of this research is to study tin whiskering behaviour of immersion tin plating after being exposed under two different temperatures and relative humidity conditions: normal condition (30oC/60%RH) and severe condition (55oC/85%RH). Tin layer was deposited by using immersion process. The effect of external stress on whiskers growth was investigated by applying indentation at 2N load on tin surface. In addition, a nickel underlayer on copper substrate was deposited by electroless plating prior to tin plating to investigate the effectiveness of nickel underlayer in mitigating whiskers formation. Field emission scanning electron microscopy (FESEM) was used to observe the behaviour of tin whiskers and intermetallic compound (IMCs), Energy dispersive x-ray (EDX) was used to determine chemical composition and image analyser was used to measure coating thickness of tin, nickel and whiskers length based on micrographs obtained from FESEM. The results showed that whiskers have formed on immersion tin surface finish exposed under normal and severe conditions. These whiskers formed in various types including straight, bent, kinked, and spiral with striations along their circumference. The tin whiskers length was directly proportional to the exposure time for both conditions; normal and severe. However, under normal condition exposure, whiskers have grown longer than severe condition due to formation of small and irregular shape of IMCs. Indented area of tin surface showed shorter whiskers formed than non-indented area indicating that the whiskers grow longer at lower external stress concentration. The deposition of nickel underlayer on copper was effective in mitigating whiskers formation as no whiskers were observed under both conditions up to 12 weeks.