Buying task · Solder and flux
Solder and flux for electronics
Leaded or lead-free, which flux, and how to handle residue, fumes and lead: the solder and flux decisions for electronics repair, with melting points from Kester.
Short answer
Use electronics solder wire with a rosin or no-clean flux core, never acid-core plumbing solder. Leaded 63/37 melts at 183°C and wets easily; lead-free SAC305 melts at 217-220°C and is harder to work with.[1][2] Keep a separate flux for rework, and control fumes and lead with extraction, ventilation and hand washing.
What solder and flux do
Solder is the filler metal that forms the joint. Flux clears oxide from the metal surfaces so that molten solder can wet them and bond, instead of beading up on top. Electronics solder wire usually carries flux in its core.
Plumbing solder is a different product. Canfield, a solder maker, says acid-core solder is not recommended for electronics because its residue is corrosive.[3]
Leaded or lead-free
| Alloy | Melting point, per Kester | What it means at the bench |
|---|---|---|
| Sn63Pb37 (63/37) | 183°C (361°F) | Eutectic: melts and freezes at a single temperature |
| Sn60Pb40 (60/40) | 183-190°C (361-374°F) | A short pasty range between solid and liquid |
| SAC305 (Sn96.5Ag3Cu0.5) | 217-220°C (422-428°F) | Common lead-free alloy; needs more heat |
The melting figures come from Kester's alloy chart.[1] iFixit adds that 63/37 wets well, while tin-silver-copper alloys such as SAC305 are harder to work with because of poorer wetting and a wider melting range.[2] Adafruit notes that lead-free solder may need more heat and more flux.[4] The 63/37 vs 60/40 vs SAC305 comparison goes further.
There is no single correct iron temperature. SparkFun teaches a medium heat of 325-375°C, Adafruit suggests 370°C for 60/40 and 400°C for lead-free, and Kester ties its recommendations to specific products.[5][4][6]
Choosing flux
| Flux | What to do with the residue |
|---|---|
| Rosin | Kester says the residue is normally non-conductive and non-corrosive, so removal is cosmetic, unless the assembly runs above 200°F[6] |
| No-clean | SparkFun says cleaning is not required; isopropyl alcohol removes it if you want[7] |
| Water-soluble | SparkFun says to remove any remaining residue from the board[7] |
Flux labels often use IPC J-STD-004 codes, which combine the flux base, its activity and its halide content: ROL0 means rosin, low activity, no detectable halides.[8] Kester's 185 liquid flux, sold as mildly activated rosin, is classified ROL0, for example.[9] Prefer a flux whose maker publishes its classification, and see the flux types comparison and how to clean flux residue.
Wire diameter is a dosing choice. Kester's 275 no-clean wire, for example, is made in diameters from 0.25 to 2.5 mm.[10] Thin wire is easier to feed onto small pads; thick wire suits large joints.
Fumes and lead
The fume extraction overview covers extractors and filters.
How to choose
- First spool: a rosin or no-clean cored wire from a maker that publishes the alloy and the flux classification. Leaded 63/37 is fully molten at the lowest temperature of the three alloys above, and it wets well. Choose lead-free when your school, workplace or the board itself calls for it.
- A separate flux, as a pen or syringe, for rework and desoldering.
- Cleanup that matches the flux, if the flux needs cleaning at all.
How to choose solder for electronics walks through the choice step by step.
Everything in this topic
Guides
- 63/37 vs 60/40 vs SAC305 solderLets you choose between the three common solder alloys on handling, iron temperature and lead exposure, rather than on habit.
- How to choose solder for electronicsTurns a confusing solder listing into four checks, so you buy one spool that suits your work and your household.
- How to clean flux residueTells you whether your board needs cleaning at all, and how to clean it without leaving a film or powering up a wet board.
- Rosin vs no-clean vs water-soluble fluxHelps you pick a flux whose residue you can either leave on the board or clean off reliably, instead of guessing from a syringe label.
Common 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. More in 63/37 vs 60/40 vs SAC305 solder
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. More in 63/37 vs 60/40 vs SAC305 solder
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. More in 63/37 vs 60/40 vs SAC305 solder
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. More in 63/37 vs 60/40 vs SAC305 solder
What is SAC305 solder?
A lead-free tin-silver-copper alloy, Sn96.5Ag3Cu0.5. Kester lists its melting range as 217–220 °C and notes that the Solder Products Value Council standardized on this composition. More in 63/37 vs 60/40 vs SAC305 solder
What solder should a beginner buy for electronics?
A flux-cored wire with a stated alloy and flux type, about 0.5–0.8 mm thick by our rule of thumb. Leaded 63/37 melts at 183 °C and is easier to work; lead-free SAC305 melts at 217–220 °C and suits homes where you want no lead. More in How to choose solder for electronics
Can I use acid-core solder on a circuit board?
No. Canfield, a solder maker, says acid-core solder is not recommended for electrical or electronic work because its residue is corrosive. More in How to choose solder for electronics
Is lead-free solder harder to use?
Yes. iFixit describes lead-free tin-silver-copper solder as harder to work with because of poorer wetting, and Adafruit says it may need more heat and more flux than leaded solder. More in How to choose solder for electronics
Questions
What is the difference between 63/37 and 60/40 solder?
Kester lists 63/37 as melting at a single temperature, 183°C (361°F), because it is eutectic, while 60/40 melts over a range from 183 to 190°C (361-374°F).[1]
Is lead-free solder harder to use?
Usually, yes. iFixit says tin-silver-copper alloys such as SAC305 are harder to work with because of poorer wetting and a wider melting range, and Kester lists SAC305 at 217-220°C against 183°C for 63/37.[2][1]
Do I need to clean flux residue off the board?
It depends on the flux. SparkFun says to remove water-soluble flux residue and that no-clean residue does not need removing. Kester says rosin residue can normally stay, unless the assembly runs above 200°F.[7][6]
Can I use plumbing or acid-core solder on electronics?
No. Canfield, a solder maker, says acid-core solder is not recommended for electrical or electronic work because its residue is corrosive.[3]
What temperature should I set my iron to?
There is no single answer. SparkFun teaches 325-375°C as a starting range, and Adafruit suggests 370°C for 60/40 solder and 400°C for lead-free. Check your solder maker's data sheet as well.[5][4]
Are solder fumes harmful?
Rosin flux fume can be. The UK's Health and Safety Executive lists rosin-based solder flux fume as a top cause of occupational asthma and advises extraction at the source and keeping your head out of the plume.[11]