TRANSMISSION ELECTRON MICROSCOPY OF SPONTANEOUS TIN (SN) WHISKER GROWTH UNDER HIGH TEMPERATURE/HUMIDITY STORAGE
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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 85C/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 –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.