Plainwire / Reference / Solder alloys and flux
Solder alloys and flux
Alloys
| Alloy | Melts | Tip temperature | Use |
|---|---|---|---|
| Sn63 Pb37 | 183 °C eutectic | 320–340 °C | The easiest alloy to work with. Goes from liquid to solid at a single temperature with no plastic phase, so a disturbed joint is rare. |
| Sn60 Pb40 | 183–190 °C | 320–340 °C | Practically identical in use. Has a narrow pasty range where a knocked joint can go grainy. |
| SAC305 (Sn96.5 Ag3 Cu0.5) | 217–220 °C | 350–370 °C | The standard lead-free alloy. Wets more slowly, looks duller when correct, and is harder on tips. |
| Sn99 Cu1 | 227 °C | 360–380 °C | Cheap lead-free. Noticeably worse wetting. Avoid for fine work. |
| Bi57 Sn42 Ag1 | 138 °C | 200 °C | Low-temperature alloy for removing parts from boards that will not tolerate heat. Mechanically weak; do not leave it in a finished joint. |
Leaded solder is legal for repair and hobby use in most places and is easier to work with, which is why it appears first in the table. The exposure that matters is ingestion, not fumes — the alloy does not vaporise at soldering temperatures. Do not eat at the bench, and wash your hands before you do. What comes off the joint as smoke is flux, and that is a separate concern below.
Mixing old and new
A lead-free joint reworked with leaded solder ends up as a tin-silver-lead mixture with a lower melting point and a wider pasty range than either parent alloy. It is mechanically acceptable for a repair and it is what most bench work ends up being. What you should not do is add lead-free to a leaded joint and expect the original tip temperature to melt it.
Flux
Solder does not stick to oxide, and every surface you meet has oxide on it. Flux removes the oxide chemically at soldering temperature and shields the surface while the joint forms. The flux inside cored solder is enough for a clean new joint and never enough for a rework.
| Type | Residue | Clean-up | Notes |
|---|---|---|---|
| Rosin, activated (RA) | Sticky, mildly corrosive | Isopropyl and a brush | Excellent activity. Must be cleaned off. |
| No-clean | Hard, clear, benign | Optional | The sane default. Clean it anyway where a residue would bridge fine pitch or sit under a connector. |
| Water-soluble (OA) | Aggressive | Mandatory, with water | Never on a board that cannot be washed and dried thoroughly. It will eat traces over months. |
| Plumbing flux | Strongly acidic | Not applicable | Never on electronics, under any circumstances, no matter what is to hand. |
Fumes
The visible smoke is vaporised rosin and its activators. It is a recognised respiratory sensitiser: repeated exposure can produce occupational asthma, and having tolerated it for years is not protection. Move the air away from your face. A small fan on the bench pointing away from you does more good than any mask.
Tips
A tip that will not wet is oxidised, not worn out. Clean it on brass wool rather than a wet sponge, keep it tinned whenever it is hot, and turn the iron down between joints rather than leaving it at working temperature for an hour. Tip life on lead-free work is a fraction of what it is on leaded, mostly because of the higher temperature rather than the alloy itself.
Never file a plated tip. The iron plating is what holds solder; underneath it is copper, which dissolves into the solder and disappears.
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