The massive spalling of Cu3Sn in the soldering reaction between high-Pb solders and Cu substrates was studied to identify the mechanism behind this rather interesting and frequently observed phenomenon. Four different alloys (99.5 Pb 0.5 Sn, 99 Pb 1S n, 97 Pb 3 Sn, and 95 Pb 5 Sn, in wt%) were soldered at 350 °C for durations ranging from 10 s to 600 min. At low Sn concentration (0.5 or 1 Sn), massive spalling occurred as early as 20 min. However, at high Sn concentration (3 or 5 Sn), massive spalling was not completed even after 600 min. To the best of our knowledge, these results are the most detailed observations ever reported on the sequence of events that occur during massive spalling. The Pb–Sn–Cu phase diagram is used to rationalize the phenomenon.