•Cu substrates with different grain sizes display a different voiding propensity.•Vacancies on or near grain boundaries are defined as the effective vacancies.•The smaller the grain size, the more effective vacancies exist at the interface.•Only when there are plenty of effective vacancies could voids form at the interface.Effect of Cu grain size on void formation at the interface of SAC305 (Sn3.0Ag0.5Cu)/Cu solder joint was studied. The joints with ED CCL (Electrodeposited Copper Clad Laminate) substrates exhibited a strong voiding tendency. But this tendency was significantly reduced after annealing treatment at 150 °C for 1000 h. At the same time, no voids were found at the interface of SAC305/HPOFC (High Purity Oxygen Free Copper, 99.9999 wt%) plate. It was found that the grain size of annealed ED CCL increased by two orders of magnitude than that of ED CCL, but it was just about half of the grain size of HPOFC plate. The smaller grain size of Cu could increase the amount of grain boundaries, thus increasing the amount of effective vacancies (EVs). Under gradient of concentration, these EVs would coalesce to form voids at the interface of Cu3Sn/Cu. Therefore, to lower the tendency of void formation at the interface, increasing the size of grain is an effective way.

•Voiding propensity of electrodeposited Cu was reduced after rapid thermal processing.•Residuals can act as precipitate for heterogeneous nucleation of void formation.•Residuals would be “deactivated” after rapid thermal processing treatment.•Reduction of grain boundary energy can act as the driving force of void formation.Electrodeposited (ED) Cu was prepared and exhibited a strong voiding propensity at the Cu3Sn/Cu interface in the subsequent Sn3.0Ag0.5Cu/Cu solder joint. However, we found that the rapid thermal processing treatment could significantly reduce the voiding propensity of ED Cu. After rapid annealing, the grain size of Cu was obviously increased, which was found to reduce the amount of effective vacancies and lower the driving force of void formation. Meanwhile, the ED Cu before and after rapid annealing was analyzed using X-ray photoelectron spectroscopy, and it can be found that there did exist additives remained in the Cu layer during electroplating process. However, after rapid annealing treatment, some residuals would evaporate, and the others would decompose and react with Cu, which would change the residuals from the free state to the “deactivated” state and make them lose their acting as the precipitate for the heterogeneous nucleation. Thus, the Cu3Sn/Cu interface with few voids was finally formed.