(A) study on microstructures of Sn-3.0Ag-0.5Cu/Sn-57Bi-1Ag combination structure

Abstract

For 3-D multi packaging, step soldering is the key packaging technology because solder bump which connects chip to chip should not melt at next reflow. For step soldering, high melting temperature and low melting temperature solder are needed. $97Pb-3Sn(320^\circ C)$ and $63Pb-37Sn(183^\circ C)$ were used for step soldering. However, environmental and health concerns over the use of lead have led to investigation of the alternative Pb-free solders to replace commonly used Pb-Sn solders in microelectronic packaging application. In this study, Sn-3.0Ag-0.5Cu $(217^\circ C)$ and Sn-57Bi-1Ag $(140^\circ C)$ were selected for combination structure. Sn-57Bi-1Ag $(140^\circ C)$ becomes liquid over $140^\circ C$. Then, the composition of Sn in Sn-3.0Ag-0.5Cu diffuses to Sn-57Bi-1Ag and the composition of Bi in Sn-57Bi-1Ag diffuses to Sn-3.0Ag-0.5Cu. By diffusion, new composition which has high re-melting temperature is formed. Therefore, this combination structure will not melt at next reflow. This study can be divided by 2 parts. First, the microstructure was investigated as reflow temperature, reflow cycle and the amount of Sn-57Bi-1Ag changed. The proportion of Sn-Bi eutectic decreased as reflow temperature increased. However, there was little difference by changing reflow cycle. And continuously grown β-Sn at interface interrupted diffusion between Sn-3.0Ag-0.5Cu and Sn-57Bi-1Ag. IMC formed between UBM and Sn-57Bi-1Ag was $(Ni,Cu)_3Sn_4$ and the thickness was from 0.8μm to 1.3μm as reflow temperature increased. Second, ball shear test and lab shear test was experimented for mechanical property. The result of ball shear strength and lab shear strength increased as temperature increased. And there were two types of fracture surfaces. First fracture surface was observed between Sn-Bi eutectic and $(Ni,Cu)_3Sn_4$. Second, fracture was observed at UBM. Optimized conditions are selected based on the results of microstructure analysis and mechanical test.