Melting behavior is usually the first consideration in designing new solder materials. The melting point can determine the actual soldering temperature. The pasty range, which is the difference between solidus and liquidus temperatures, where the alloy is part solid and part liquid, should be narrow since alloys with wide pasty ranges are more susceptible to fillet lifting and tombstoning. The solidus temperature should be significantly higher than the solder joint’s maximum operating temperature. Knowledge on the effect of alloying elements on the melting behavior of lead-free solders is of great importance to the development of new solder alloys and the improvement of existing alloys.
This figure shows a comparison of calculated and experimentally determined liquidus and solidus temperatures for a large number of commercial solders. As you can see, the predicted temperatures show good agreement with the experimental data.