Estimation of Liquidus Temperature when SnAgCu BGA/CSP Components are Soldered with SnPb Paste

Recently, the soldering of lead-free components with SnPb paste, or lead-free backward compatibility, is becoming a hot topic. One of the major challenges in backward compatibility assembly is the development of a right reflow profile for the soldering of SnAgCu ball grid array (BGA)/chip scale package (CSP) components with SnPb paste. If the SnAgCu reflow profile is used, the reflow temperature may be too high for other SnPb components in the same board during assembly according to the component rating per IPC/JEDEC J-STD-020C. In addition, the flux in SnPb solder paste may not function properly in such a high reflow temperature. On the other hand, if the SnPb reflow profile is used, SnAgCu solder ball may only partially melt. The incomplete mixing of the solder paste with the BGA/CSP ball raises serious reliability concern. Therefore, it is important to know the minimum reflow peak temperature that is able to achieve complete mixing of SnPb paste with lead-free components. This paper presents a method to estimate the liquidus temperature of mixed compositions when SnAgCu BGA/CSP components are soldered with SnPb paste. The liquidus temperature is the minimum reflow peak temperature able to achieve complete mixing of SnPb paste with lead-free components. It is shown that the liquidus temperature depends on the Pb ratio in the mixed composition and the liquidus temperature is below 217degC, which is the liquidus temperature of SnAg3.0Cu0.5 solder. The liquidus temperatures of several experimental studies in literature are estimated and it is found that the estimated temperatures are consistent with experimental results. A user interface is designed using visual basic for application in the Microsoft Excel environment to facilitate the estimation of the liquidus temperature. It is expected that the estimation of the mixed compositions liquidus temperature are able to guide process engineers to develop a right reflow profile in backward compatibility assembly