The 34 °C gap
Tin-lead eutectic melts at 183 °C. SAC305 melts at 217–220 °C, so every thermal step in the process moves up by roughly a third of a hundred degrees: the soak, the ramp, the time above liquidus and the peak.
That is not a detail. It changes what the laminate sees, what the components must survive, how much moisture is driven off during reflow, and how much intermetallic grows at every joint.
Paste and profile
Lead-free paste is specified with its own profile window, and the profile is measured on the assembly rather than assumed from the oven display. Typical peaks sit between 235 and 250 °C, high enough to wet the pads and low enough to stay inside the component ratings.
Three practical consequences:
- Time above liquidus matters more. Longer is not safer: it grows intermetallic and stresses the laminate. The window is bounded on both sides.
- Thermal mass varies. A board with a heavy copper plane or a large connector heats unevenly, so the profile is verified at the coldest and hottest points.
- Mixed technology boards need separate profiles for reflow and selective or wave soldering, because the through-hole parts see the wave after the SMT side is already soldered.
Components and moisture
A lead-free profile heats every package closer to its limit. Plastic packages absorb moisture from the air, and that moisture turns to steam during reflow; the result is cracking or delamination that shows up as a field failure rather than a process alarm.
Moisture-sensitivity level handling therefore becomes part of the build: dry storage, floor-life control, baking before reflow where the level requires it, and reels that are not left on the line overnight.
Finishes and documentation
All the standard lead-free finishes work with SAC pastes — lead-free HASL, ENIG, immersion tin, immersion silver and OSP — and the choice is about planarity and shelf life rather than about lead. What matters commercially is the paperwork: a RoHS declaration, material declarations from the component suppliers and a certificate of compliance with the lot.
Rework under lead-free
Rework is where lead-free is least forgiving. The higher temperature stresses the pad and the laminate, and a second or third reflow keeps growing the intermetallic layer at the joint. Local heating, preheat from below, the right alloy and a documented cycle limit are all necessary — the rules are in the rework note.
Builds run to IPC-A-610 workmanship, and the process limits are on the PCBA manufacturing page. Boards that need a finish decision can be compared on the capability page.
