Effects of Reflow Peak Temperature and Reflow Times on Microstructure of the Mixed Lead-free BGA/Sn62Pb36Ag2 Solder Joints

Abstract

The microstructure changes of Sn62Pb36Ag2 solder and lead-free BGA Solder Joints under different reflow peak temperatures and reflow times were analyzed. The test results show that: 1) with the reflow soldering peak temperature reaching or exceeding the melting point of SnAgCu (217°C), and the time above the initial melting point temperature (204°C) of the mixed Pb-free BGA/Sn62Pb36Ag2 solder joints exceeds 60s, the AgSn compound becomes a network structure with large size distribution along the grain boundary and gradually becomes a large particle with dispersed distribution; The white Pb phase diffuses uniformly, and gradually changes from discontinuous linear aggregation distribution to island granular. At the same time, the flake Ni-Cu-Sn compounds that are slightly larger and darker than AgSn compounds appear more obviously. The final mixed solder joint is marked by the uniform distribution of Pb, Ag and Ni elements. 2) With the increase of reflow times, the thickness of IMC layer at the PCB side interface increased significantly at the second reflows, from 2.4µm to 3.9µm, and the bulge of intermetallic compound at the interface became obviously thicker; from the second to the fifth time, the IMC thickness increased slowly; After the fifth reflows at the component side, the thickness of IMC layer at the interface gradually increases from 0.9µm to 1.7µm and continues gradually, some areas reached 4.7µm. In the actual production process, the reflow times shall be controlled below 2 times as far as possible.