Influence of systematic coefficient of thermal expansion (CTE) variations on Sn whiskering

Large variations in the coefficient of thermal expansion (CTE) between Sn and growth substrates provide a highly reliable and fast method to grow whiskers. The CTE effect can be enhanced by thermal cycling during whisker incubation, which is able to reverse reductions in whiskering due to small amounts of dopant atoms in the Sn (e.g., Pb). Investigated substrates with CTE close in value to Sn (0 < %ΔCTE < 25) were Al, Ag, and brass; intermediate in value to Sn (25 < %ΔCTE < 75) were Zn, Ni, and Ta; and far in value to Sn (75 < %ΔCTE < 100) were semiconductors Si, GaAs, and InP. A thickness of 0.5 micron of sputtered Sn was deposited on each coupon. The thermal cycling range was −40°C to 125°C, with 2 hour ramps and 4 hour dwells, for a total of 12 hours per cycle. A second, comparative set of specimens underwent isothermal annealing at 100°C ± 5°C. All samples were incubated for 37 days (74 cycles) before observation. Samples with a %ΔCTE > 75% resulted in drastically higher whisker densities (when cycled) compared with those with %ΔCTE < 75%. There appears to be a critical activation (or nucleation) threshold CTE mismatch which “turns on” fast whisker growth when using semiconductors as substrates.