63/37 vs 60/40 Solder
The numbers look almost identical — 63/37 and 60/40 tin-lead — yet the difference between eutectic and near-eutectic changes how a joint freezes, and that matters on a high-mix line running tight profiles.
Eutectic matters
63/37 tin-lead is eutectic: it melts and freezes at a single temperature (183 °C) with no mushy paste range. 60/40 is near-eutectic, with a small plastic range between solidus and liquidus. That small window is enough to cause cold-joint shading and 'paste' movement under vibration during cooling.
- 63/37 — single freezing point, sharp transition, lower cold-joint risk
- 60/40 — small plastic range, slightly more sensitive to cooling rate
- Both leaded; for RoHS designs use SAC305 and manage the higher reflow peak
Reliability and rework
For most commercial products the two are interchangeable in reliability. The practical edge of 63/37 is rework: a single sharp melt point makes hand and hot-air removal cleaner. On lead-free lines, SAC305's higher peak (around 245 °C) stresses components more, so profile tuning is essential.
Which to specify
If you are leaded and value clean rework and a forgiving profile, 63/37 is the safer default. Reserve 60/40 for legacy BOMs. For new designs, plan around lead-free SAC305 and its thermal budget from day one.
Key takeaways
Key Takeaways
- 63/37 is truly eutectic; 60/40 has a small plastic range.
- 63/37 gives cleaner rework and a more forgiving profile.
- New designs should plan around lead-free SAC305 thermal budget.
