Abstract A Monte Carlo simulation based algorithm is developed to compute the effective diffusivity of lead free solder alloys. Simulations are performed to determine the vacancy diffusivity for 95.5Sn–4.0Ag–0.5Cu (SAC405) solder alloy for different paths. Temperature and micro-structural influence on diffusivity are studied. A map of diffusivity versus temperature and average grain size is developed. Significant differences between diffusivity values reported in the literature for the same solder alloy is also discussed.