Influences Of Minor Elements Addition On The Interfacial Reactions Between Lead-free Solders And Common Substrates

Interfacial reactions between lead-free solders which act as thermal, electronic and mechanical connections in microelectronic packaging process and common substrates affect the reliability of the solder joints directly. In the paper, Sn-Zn and Sn-Ag lead-free solder alloys were selected and the interfacial reactions were explored systematically by the addition of minor thirds elements and nanoparticles and the means of metallographic analysis. In addition, the evolution of phases in the solder matrices, the diffusion of elements and the growth mechanism of IMC (intermetallic compound) layers of the solder joints, which serviced under high-temperature environment were investigated by high-temperature aging treatment and the results are given as follows.Three types of Sn-9Zn, Sn-8Zn-1Bi and Sn-8Zn-3Bi solders were adopted for reflowing on Cu substrate to investigate the influence of minor Bi additions on the evolution of the interfacial morphology in Sn-Zn-xBi/Cu solder joints. The addition of Bi into Sn-Zn solder reduced the tendency of the formation of cracks at the solder joints because alloying of Bi reduced the mismatch of CTE (coefficient of thermal expansion) between the solder alloys and the Cu plate. Moreover, alloying with Bi reduced the melting temperature of the Sn-Zn solder so as to increase the molten period of the Bi-containing solders during reflow. This increased the thicknesses and grain/particle sizes of IMC layers/particles.The microstructural evolution of IMCs in Sn-Ag-X(X = 0, Ni, Zn and In)/Cu solder joints and their growth mechanisms during liquid aging were investigated. The results indicate that compared with the growth of single IMC layer in Sn-Ag/Cu solder joint, there was a two-phase (Ni3Sn4 and Cu6Sn5) IMC layer formed in Sn-Ag-Ni/Cu solder joint during their initial liquid aging stage (in the first 8 minutes). However, after long time aging (more than 24 minutes), the rate of growth of the IMC layer in Sn-Ag-Ni/Cu solder joint decreased due to the phase transformation (from two Ni3Sn4 and Cu6Sn5 phases to a (Cu, Ni)6Sn5 phase). The IMC layers with two Cu6Sn5 and Cu5Zn8 phases formed initially in Sn-Ag-Zn/Cu solder joint during the liquid aging and the rate of growth of these IMC layer was close to that of the layer in Sn-Ag/Cu solder joint. After long time liquid aging, IMC layer with two phases transformed into a Cu-Zn-Sn phase which speeded up its growth. The addition of indium into the Sn-Ag solder alloy to form a Cu6(Snx,In1-x)5 phase speeded up the growth of the IMC layer in Sn-Ag-In/Cu solder joint until the indium became exhausted due to the interfacial reaction.The interfacial reaction between Sn-3.5Ag-1.5In solder and a Au/Ni/Cu pad in BGA package during solid aging were investigated and compared with that between Sn-3.5Ag solder and the Au/Ni/Cu pad. The results indicate that during the solid state aging, the IMC layer in Sn-3.5Ag/Au/Ni/Cu solder joint evolved from a (Ni, Au)Sn4 phase to Ni3Sn4 phase, but the rate of growth did not change greatly. In Sn-3.5Ag-1.5In/Au/Ni/Cu solder joint, the phases evolved from the (Ni, Au)Sn4 and Ni3Sn4 phases into a Ni3(Sn,In)4 phase. The participation of indium atoms in the interfacial reaction accelerated the diffusion of Sn atoms and speeded up the rate of growth of the whole IMC layer, but this effection was reduced gradually after prolonged aging.Ag nano-particles and MWCNT reinforced Sn-8Zn-1Bi composite solders were prepareded and reflowed on a Cu plate to investigate the influence of minor Ag nano-particles and MWCNT additions on the evolution of the interfacial morphology in Sn-Zn-Bi/Cu solder joints. The results indicate that the addition of Ag nano-particles into Sn-8Zn-1Bi solder led to the formation of a AgZn3 phase around the Zn-rich particles, which inhibited the diffusion of water vapor and oxygen along the grain boundaries and restrained the formation of ZnO at the grain boundaries. However, parts of AgZn3 IMC particles were found to adhere on the top of surface of Cu-Zn IMC layer, which increased the entire thickness of the IMC layer. The addition of MWCNT into Sn-Zn-Bi solder refined the microstructure of the Zn-rich particles greatly by reducing the surface energy of the Zn-rich phase due to the Surface Adsorption Effect. In addition, the MWCNT which distributed in the IMC layer inhibited the diffusion of Sn, Cu and Zn atoms effciently and decreased the growth of Sn-Cu-Zn IMC layer.