Microstructural Analysis Of The Sn-Bi-Zn-Ag Lead-Free Solders And The Soldered Interfaces

Most of the developed Sn-Ag based lead-free solders contain high Ag content which leads to the high cost of the solders. It is necessary to develop a low Ag content lead-free solder with good properties. Therefore, based on the Sn-3.7Ag-0.9Zn (in weight percent, here after) eutectic alloy, the Ag content is decreased from 3.7 to 0.3% and Bi is introduced as the fourth component. The Sn-Bi-Zn-Ag solder alloy has been systematically studied in the paper. Firstly, effects of Bi content and different cooling rate on the microstructure of the Sn-Bi-Zn-Ag solder alloy were studied. Furthermore, the influencing factor, hardening theory and failure mechanism were explained based on the experimental results of properties. Then, the interfacial structures between Sn-Bi-Zn-Ag solder alloys and Cu pad were observed and the growth and formation of the intermetallic compound (IMC) layer were explained. At last, the effect of Ni-plating layer on the formation of IMC layer, and the evolution of the IMC layer after aging were discussed. More details were given as follows. The microstructures of Sn-xBi-0.9Zn-0.3Ag(x=1, 2, 3 and 4)solder alloys were composed ofβ-Sn phase, AgZn3 phase, Zn-rich phase and Bi segregation (except 1% Bi addition) under slowly-cooled condition. The coexisting of Zn-rich phase and Bi segregation caused the local melting at low temperature, which could be suppressed by increasing the cooling rate. After aging, Bi segregations were refined and Bi particles distributed along the grain boundaries to decrease the system free energy. Because of the solution strengthening, precipitation strengthening and grain refinement, the microhardness of the Sn-Bi-Zn-Ag solder alloy increased as the Bi content and cooling rate increased. However, Bi solution and more grain boundaries increased the scattering of electron, so the electrical conductivity of the solder decreased. In addition, though the Bi addition increased the ultimate tensile strength (UTS) of the solder, the elongation decreased dramatically and the failure mechanism turned from ductile fracture to brittle fracture with Bi content increasing.The layer spalling happened between the Sn-Bi-Zn-Ag solder alloy and Cu pad. The interface contained two IMC layers, the CuZn layer near the solder and the Cu6Sn5 layer near the Cu pad. Both the Bi addition and Ni-plating layer could suppress the spalling phenomenon. Moreover, only one Sn-Ni-Zn-Cu layer formed between the Sn-Bi-Zn-Ag solder and Ni/Cu pad. When the interface was aged, the thickness of IMC layer increased firstly and then decreased which due to the internal stress in the layer. The Bi and Ni-plating layer could hinder the growth of IMC layer during the aging process, but when the Bi content was high, the growth of IMC layer between the Sn-Bi-Zn-Ag solder and Ni/Cu pad increased abnormally, owing to the existence of local melting.