SAC305 Solder Reflow: Identification of Melting and Solidification Using In-Process Resistance Monitoring

The proper control of solder heating and melting leads to successful joining of surface-mount technology devices to the metallization of printed circuit boards. When developing new solder alloys for more miniaturized and lower temperature processes, the improved process control for the heating and melting during solder reflow can be beneficial. To find an optimized temperature profile efficiently, it is helpful to know the exact onset times of the physical mechanisms of soldering including sintering, melting, and solidification. In particular, to exactly know the time, the solder remains in the molten state can help to minimize the temperature exposure of sensitive substrates. To this end, we used standard lead-free SAC305 solder and a real-time resistance monitoring setup to observe the in-process resistance during the soldering of a 0805-type chip resistor to find out exact points in time when the soldering mechanisms occur. Crosssectioning studies and microscopic observations were performed in parallel to explain three distinct resistance drops observed in the resistance signals. Sintering of solder balls resulted in drop 1, the collapse and melting of all the solder balls resulted in drop 2, and the solidification of solder resulted in drop 3, which actually consisted of two separate drops, one for each solder joint of the chip resistor. The observation of the resistance drop due to solder solidification was achieved because our newly improved setup reduced the signal noise to less than 0.012 mQ (root mean square value). On average, the solder was in a liquid state for 17.6 min ± 0.18 min (seven samples). The solidification drop was 0.2 mQ and happened at approximately 200 °C, indicating not more than 28 K of undercooling. Such an accurate measurement of melting and solidification times may prove useful in the future when optimizing the reflow temperature profiles for advanced soldering applications, e.g., to minimize the melting-solidification interval on heat-sensitive thin flexible substrates for wearables.