Dramatic morphological change of interfacial prism-type Cu6Sn5 in the Sn3.5Ag/Cu joints reflowed by induction heating

Ball grid array (BGA) solder interconnecting is one of the key technologies in electronic packaging and assembly. Legislation of lead-free process has made the application of lead-free solder become wider in electronic products. Compared to the lead-tin solder, the relatively higher melting points of most lead-free solders call for a higher reflow temperature. Thus conventional integral-heating process at an elevated temperature would induce severe warpage of components and substrates and reduce the in-service reliability of solder joint. In this dissertation, a new concept of selective heating was proposed to address the reliability issues of integral heating process in lead-free BGA interconnection. By using this concept a novel BGA interconnection technology, named as induction heating reflow, is developed. The significance of this research lies in the potentials of acceleration of the lead-free process in electronic productions, promotion in the applicability of lead-free BGA packaging, and the enhancement of the reliability of BGA components. Compared with conventional hot-air reflow, the thermal characteristics of liquid-solid interfacial reaction and solid-solid interfacial reaction, the formation of interfacial intermetallic compound (IMC) and their growth kinetics were all studied. The result shows that with the increasing of temperature, Cu₆Sn₅ grains change from scallop-type into prism-type. In the solid-state aging process the evolution of interfacial IMC is merely controlled by the diffusion of Cu atoms from pad, and its thickness grows linearly with the square root of aging time.