Tin whisker growth from micro-alloyed SAC solders in corrosive climate

In this paper the tin whisker growth properties of lead-free SAC (SnAgCu) solder alloys were investigated. There different alloys were studied: two micro-alloyed SAC (SnAgCu + Bi + Sb) and the widely applied SAC305 as reference. FR4 test board was developed for 0805 chip resistor components. Solder joints were fabricated on imm-Sn surface finish. The samples were aged in elevated temperature and different humidity levels to induce the tin whisker formation: at 85 °C/85RH% and 85 °C/20RH%. The test duration was 3000 h. Whisker growth was checked after every 500 h by scanning electron microscope. The developed whiskers were etched by focused ion beam and the cross-sections were observed by scanning ion microscope and transmission electron microscope. It was shown that the lead-free SAC solder alloys can also develop tin whiskers and the whiskering ability depends on the corrosion behavior of the SAC solders. Copper and bismuth precipitations were found at the whisker roots and within the whiskers which could effect on the number, length and type of the developed whiskers.