Guide · Solder and flux
63/37 vs 60/40 vs SAC305 solder
Melting points from Kester, what eutectic means at the bench, why lead-free is harder to work, and the lead hygiene that comes with 63/37 and 60/40.
Short answer
63/37 melts at a single 183 °C, 60/40 melts over 183–190 °C, and SAC305 melts at 217–220 °C.[1] Leaded 63/37 wets well and sets at one temperature; lead-free SAC305 is harder to work with and needs a hotter iron.[2] Choose a leaded alloy only if you can keep lead away from your food, your hands and your family. Choose SAC305 to avoid lead altogether.
The numbers
Kester's alloy chart gives these melting points:[1]
| Alloy | Composition | Melts at | Melting behavior |
|---|---|---|---|
| 63/37 | 63% tin, 37% lead (Sn63Pb37) | 183 °C (361 °F) | Eutectic: a single melting point |
| 60/40 | 60% tin, 40% lead (Sn60Pb40) | 183–190 °C (361–374 °F) | A 7 °C pasty range |
| SAC305 | 96.5% tin, 3% silver, 0.5% copper (Sn96.5Ag3Cu0.5) | 217–220 °C (422–428 °F) | A narrow range, well above tin-lead |
You may also see 62/36/2, a leaded alloy with 2% silver that Kester lists at 179 °C (354 °F), and tin-copper Sn99.3Cu0.7 at 227 °C (441 °F).[1]
What "eutectic" means at the bench
A eutectic alloy goes from liquid to solid at one temperature. 63/37 does it at 183 °C:[1] the joint is either molten or set, with nothing in between.
60/40 passes through a pasty range between 183 and 190 °C, where it is part solid and part liquid.[1] In practice, hold the joint still until the solder has fully set. With 60/40, that takes a moment longer.
SAC alloys melt over a narrow range too, but much hotter. Kester notes that the Solder Products Value Council standardized on SnAg3.0Cu0.5, and that tin-silver-copper alloys with 3–4% silver and 0.5–0.7% copper melt within 217–220 °C.[3] So the SAC variants you will find melt in the same window as SAC305.
Why lead-free is harder
- It melts higher: 217–220 °C for SAC305, against 183 °C for 63/37.[1]
- It wets less readily. iFixit describes lead-free tin-silver-copper as harder to work with because of poorer wetting and a wider melting range than 63/37.[2]
- It needs a hotter iron and more flux. Adafruit suggests about 400 °C (750 °F) for lead-free and 370 °C (700 °F) for 60/40,[4] and says lead-free may need more flux.[5]
The gap matters most on heavy joints, where a small iron is already struggling. See why ground planes are hard to solder.
Which to choose
| Your situation | Choose |
|---|---|
| Learning, kits and general repair, and you can keep strict lead hygiene | 63/37 |
| You already own 60/40 | Use it, with the same lead precautions, and hold joints still a moment longer |
| Children at home, or a bench in a shared living space | SAC305: EPA says young children are the main concern with lead brought home from jobs and hobbies[6] |
| You want no lead in the house at all | SAC305 |
Whichever you pick, the listing or data sheet should name the alloy by composition. "Lead-free solder" without an alloy is not enough to go on. The solder buying guide covers diameter and flux core.
Lead: what the precautions are
- How it gets in. HSE says the body absorbs lead when you breathe lead dust, fume or vapor, or swallow it, for example by eating, drinking, smoking or nail-biting without washing first. It lists soldering with lead alloys as higher-risk work.[7]
- Take-home dust. OSHA notes that lead enters the body mainly by inhalation and ingestion, and that workers can carry lead dust home on clothing and shoes.[8] Its limits apply to workplaces, not hobby benches, but the routes are the same.
- At home. EPA advises changing clothes and shoes that might carry lead dust, washing hobby clothes separately, and keeping hobby materials away from living areas.[6]
Flux fumes come with every alloy
Switching to lead-free does not remove the flux hazard. The same rosin-based flux cores are sold on both kinds of wire: Kester's 275 core, rated ROL0, comes on Sn63Pb37, Sn60Pb40 and SAC305 alloys.[9] HSE names rosin flux fume as a top cause of occupational asthma.[10]
Next
Questions
Is 63/37 solder better than 60/40?
For hand soldering it is easier to control: 63/37 is eutectic and sets at a single 183 °C, while 60/40 passes through a 183–190 °C pasty range. Both contain lead and need the same hygiene.[1]
What temperature should I use for SAC305?
Adafruit suggests about 400 °C (750 °F) for lead-free solder on a temperature-controlled iron. Kester's recommendations depend on the flux: about 400 °C for its rosin-flux wires and 315–345 °C for its no-clean wires.[4][3]
Is lead-free solder safer to use?
It removes lead exposure, not flux fumes. The same rosin-based flux cores are sold on lead-free wire, and HSE names rosin flux fume as a top cause of occupational asthma, so extraction still matters.[9][10]
Is it safe to touch leaded solder?
HSE lists breathing lead dust or fume and swallowing lead as the ways it is absorbed, so the risk from handling solder is lead reaching your mouth. Wash your hands and face and scrub your nails before eating, drinking or smoking.[7]