The effect of cooling rate on growth kinetics of interfacial IMCs during multiple reflows

An layer of Cu
₆
Sn
₅
intermetallic compound (IMC) is formed during the interfacial reactions between lead-free solder and Cu substrate, and it plays a pivotal role in the reliability of solder joint. There are two different growth morphologies of Cu
₆
Sn
₅
grains-scalloped grain and prism grain. Scallop grains are commonly formed at the interface during the heating up and isothermal heating. Its growth behavior can be explained by interfacial reaction and ripening mechanism. Meanwhile, during cooling, prism grains are often formed, and its growth is driven by the flux of the interfacial reaction. Moreover, in device manufacturing procedure, several reflows are required; the solder joints have to be remelted many times and this introduces new problems related to altered morphology of Cu
₆
Sn
₅
grains. Several researchers have found that the two morphologies of Cu
₆
Sn
₅
grains would interconvert in different ways. Nevertheless, it is still hard to give a full and detail description of the evolution behavior of Cu
₆
Sn
₅
morphology during multiple reflows due to the lack of direct observation on interfacial reaction. In situ study on the growth behavior of Cu
₆
Sn
₅
during a single reflow has been reported. But for multiple reflows, the change in solubility of Cu in Sn during every heating and cooling stage could have great influence in the formation of two differing morphologies. On the other hand, due to the high content of reactive species of Sn and the increasing content of Cu element with reflow cycles, the growth kinetics of interfacial compounds on solder/substrate become more complicated. In this work, growth behavior of interfacial intermetallic compound in multiple reflow process for air cooling (AC), half cooling (HC, cooling in furnace without window) and furnace cooling (FC) was investigated by Shanghai Synchrotron Radiation Facility (SSRF) and scanning electron microscope (SEM). Pure Sn (99.95%) foils were used as solder and pure Cu (99.99%) sheets were employed as substrates. It was found that scallop Cu
₆
Sn
₅
formed at the interface during the heating stage of a reflow and converted to prism type during subsequent cooling, then transformed back to the scalloped grains of larger size during heating stage of the next reflow and so on. Furthermore, SSRF images illustrated that IMC dissolution phenomenon appeared and prism-type grains gradually sharpened into scallops during the heating stage. In addition, time exponent value obtained from the slop of ln(Δy) versus lnt increased with cooling rate slower.