The morphology variation of IMC on the solder/bubble interface under different cooling rates and temperatures

As bubbles on the solder/copper interface can seriously decrease the joint reliability of lead-free soldering in electronic packaging industry, the research of interfacial bubbles is extremely imperative. In the local area around an interfacial bubble, the Cu atoms tend to migrate towards the gas/solid interface, providing the copper element to form the interfacial intermetallic compounds (IMCs). Because of the different thermal conductivity inside and outside the void (inside gas lower than outside solid), the bubble interface will have a disparate temperature gradient comparing with the solid area around the bubble. Based on this, the morphology variation of IMCs on the solder/bubble interface under various soldering temperatures and cooling rates was investigated in this study, using the scanning electron microscope (SEM) for the cross section each time after the soldering reaction part carried out in furnace with the soldering temperature of 250°C, 300°C, 350°C and holding time of 60s, followed by water, air, furnace cooling (WC, AC, FC). It was found that, IMCs are much more easily to exist on the bubble interface, especially the position including some small holes due to the higher concentration of metal atoms brought by them; On the other hand, most of the bubble interface will have no well-formed IMCs if no small voids sticking to when the cooling rate is high. In a certain soldering temperature, with the cooling rate declining (WC>AC>FC), the morphology of the interfacial IMCs gradually change from solid to hollow structure due to different growth rates of two vertical growing orientation inside the new formed phase. In addition, when the soldering temperature improved, the volume ratio of the cavity part in the hollow structure will reduce gradually.