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In the present study, tin (Sn) whisker growth has been observed from SAC0307-ZnO composite solder joints. The commercial SAC0307 solder alloy was reinforced by ZnO nano-particles, which resulted in composite solder joints. The ZnO nano-particles were mixed into the solder paste in a 0.25 wt% volume fraction, using the ball milling process. Solder joints were prepared from the composite solder alloy and from the reference SAC0307, on Ag surface finish by conventional SMT. The solder joints were exposed to an accelerated lifetime test (85 ºC/85 RH% THB, 3000 hours) to enhance the Sn whisker growth. Corrosion spots were observed after 1000 hours of aging, and the first whiskers appeared after 1500 hours of aging on the reference SAC0307 solder joints. In the case of the ZnO composite solder joints, non of the previous phenomena were observed during 3000 hours of THB aging. Furthermore the mechanical strength of the solder joints was investigated, but it was found that the ZnO nano-particle could not increase the mechanical properties of the composite solder joints considerably.