Growth Evolution Behavior Of Interfacial Cu₆Sn₅ And Influence Factors During Multiple Reflows

Recently, reflow solder, as the main soldering method used in the electronic packaging has been rapidly developed; moreover, multiple reflows were required in some flip chip package or wafer level chip size package to meet the need of the development of packaging technology, which bring a serious challenge to the reliability of the soldering joint. Appropriate interfacial intermetallic compound (IMC) layer is the guarantee of a good joint. IMC of Cu6Sn5 commonly appeared on the interface, its evolution behavior of growth, including morphology, thickness play important roles on the reliability of the final joints. So it is necessary to study the growth evolution of interfacial Cu6Sn5during multiple reflows and its influence factors.This study mainly observed the growth of interfacial Cu6Sn5 on pure Sn/Cu joints during different stages of multiple reflows. Assisted by real-time synchrotron radiation imaging technology and the high pressure air purging experiment, the growth of Cu6Sn5 at different stages of multiple reflows were analyzed; additionally, the growth behavior of Cu6Sn5 in Sn0.7 Cu/Cu interface at different stages of multiple reflows were observed; Moreover, influence of two important factors:the heat preservation time (10s/30s/60s) and reflow temperature (250℃ /275℃/300℃) on the growth of Cu6Sn5 at different stages of multiple reflows were investigated separately. Followings are conclusions:(1) The morphology of interfacial Cu6Sn5 on Sn/Cu interface during multiple reflows transformed reversibly form scallop type to prism type. In the heat preservation of reflows, scallop Cu6Sn5 formed and transformed to prisms during the following cooling stage; subsequently changed to scallops during the heat preservation of next reflow.(2) With an increasing number of reflows, scalloped Cu6Sn5 formed during heat preservations become bigger in dimension but less in number, scallops grow crawly along the interface controlled by ripening reaction; prismatic Cu6Sn5 formed during cooling vertically grow along the interface, controlled by the interface reaction.(3) With an increasing number of reflows, IMC layers became thicker; as the prisms grew much faster than scallops, the growth of thickness during cooling stage was much more than the heat preservations.(4) During multiple reflows, the morphology of interfacial Cu6Sn5 on Sn0.7Cu/Cu interface transformed from scallops to prisms then to scallops again. Meanwhile, as the limit of interfacial Cu atoms controlled by interface reaction decreased, so did its influence to the growth of prismatic Cu6Sn5, Cu6Sn5. The growth tendency of prismatic Cu6Sn5 became less obvious, enhance, the growth of IMC layers’thickness decrease.(5) During multiple reflows, the increase of heat preservation or the rise of reflow temperate not only promote the grow of scalloped Cu6Sn5 but also make the prismatic Cu6Sn5 formed during cooling stages longer; more facets formed, as well as hollow hexagonal prisms.