Effect of laser irradiation on generation and growth of whiskers in tin-electroplated film

In order to suppress generation and growth of whiskers, the effect of laser irradiation on tin-electroplated film on copper was studied by scanning electron microscopy and X-ray diffraction method. In the case of as-plated film, whiskers were generated on the electroplated tin film of 1 mμ thickness within 432 ks. Residual stress of this film and the number of whiskers increased with the amount of copper-tin intermetallic compounds (Cu⁶Sn ⁵) which developed between the plated film and the copper substrate. On the other hand, in electroplated tin film subjected to diode laser of 300 W for 100 to 1000 ms, no whiskers were formed even after 10 Ms. Residual stress of tin after laser irradiation was tensional at first, then, the stress did not change appreciably after 2.6 Ms. Uniform layer of Cu⁶Sn⁵ was formed immediately after laser irradiation, and the morphology showed no meaningful change even after 10 Ms. Whiskers are thought to be suppressed by covering the interface between tin and copper with layer of Cu ⁶Sn⁵ and by reducing nonuniformity of stress field.