Transmission Electron Microscopy of Spontaneous Tin (Sn) Whisker Growth under High Temperature/Humidity Storage

The present work is devoted to characterize the microstructures of Sn/Cu lead-frame attached on the printed circuit board (PCB) and of tin-whiskers formed on the Sn-surface after exposure under 85°C/85 %RH for 1000 and 2000 h in order to verify the whisker-growth behavior. Whiskers were observed on the un-corroded and corroded Sn-surfaces and confirmed to be of Sn ß-phase. The number of whiskers was relatively small. Within the corroded area Sn was oxidized to be SnO₂. The SnO₂ domain is made of nanocrystalline grains about 10-20 nm in size for specimen exposed for 1000 h, whereas in another one exposed for 2000 h an amorphous phase is included in the SnO₂ domain. Intermetallic compound (IMC) of Cu₃Sn and Cu₆Sn₅ is formed at the Sn/Cu interface. The results suggest that there is competitive stress generation according to formation of the Cu-Sn IMC and the Sn-oxide phase during oxidation and corrosion process. The stress-driven diffusion of Sn-atoms from the higher-stress area into the lower-stress area to form whiskers is discussed.