Strategies for inducing heredity of β-Sn texture in micro solder joints during multi-reflow process

The strategies for inducing heredity of β-Sn texture and thus controlling β-Sn orientation in micro solder joints during multi-reflow process were investigated. The temperature gradient (TG) induced gene clusters that carried the information similar to the solid-state crystals and conical cavities that provided preferred nucleation sites at one side of the joints were identified as the keys for generating heredity effect. There was non-heredity effect in Cu/Sn/Cu joints whether the joints were reflowed under TG or not for the first time due to the absence of gene clusters and in Cu/Sn-3.0Ag-0.5Cu(SAC305)/Cu joints during isothermal multi-reflow due to the absence of deep and narrow conical cavities. However, the Cu/SAC305/Cu joints that had orientated β-Sn grains after the first reflow under TG exhibited remarkable heredity effect during the subsequent second and third reflow. The critical peak reflow temperature was confirmed to be 255 °C over which the heredity effect disappeared. A general guidance of the strategies for inducing heredity effect in Cu/SAC305/Cu joints during multi-reflow process was demonstrated focusing on reflow temperature, times, duration, cooling rate and TG. The novel reflow method is believed to be beneficial to improve the reliability and anisotropy of micro solder joints containing few β-Sn grains.