Guide · Solder and flux
How to choose solder for electronics
Choose electronics solder in four steps: leaded or lead-free alloy, flux core, wire diameter and a maker with a data sheet. Plus why to skip acid core.
Short answer
Buy flux-cored solder wire whose listing states the alloy, the flux type and the diameter. Leaded 63/37 melts at 183 °C and lead-free SAC305 at 217–220 °C,[1] and iFixit notes that lead-free is harder to work with.[2] A no-clean flux core is the simplest default, and about 0.5–0.8 mm is our rule of thumb for general work. Never use acid-core solder on electronics.[3]
Choosing solder comes down to four decisions: the alloy, the flux core, the diameter and the maker.
1. Leaded or lead-free
| Alloy | 63/37 | 60/40 | SAC305 |
|---|---|---|---|
| Composition | Sn63Pb37 | Sn60Pb40 | Sn96.5Ag3Cu0.5 |
| Melts at (Kester) | 183 °C (361 °F) | 183–190 °C (361–374 °F) | 217–220 °C (422–428 °F)[1] |
| How it behaves | Wets well and sets at a single temperature | Passes through a short pasty range as it sets | Harder to work: poorer wetting and a wider melting range than 63/37[2] |
| Lead precautions needed | Yes | Yes | No lead |
Lead-free solder asks more of your iron. Adafruit suggests about 400 °C for lead-free against 370 °C for 60/40,[4] and says lead-free may need more flux as well as more heat.[5]
Leaded solder is easier to learn on, but it comes with hygiene rules (see the safety note below). EPA's guidance on lead from jobs and hobbies says young children are the main concern,[6] so if children share your space, lead-free is the simpler choice. The alloy comparison goes deeper.
2. The flux core
The flux inside the wire decides what is left on the board afterward.
- No-clean: made to leave residue you do not have to remove. SparkFun says cleaning is not required.[7] Kester's 275 no-clean wire, for example, is rated ROL0 and leaves what Kester calls an extremely clear residue.[8]
- Rosin: Kester says rosin residue is non-conductive and non-corrosive, and normally only needs removing for looks, unless the board runs above 200 °F (about 93 °C).[9]
- Water-soluble: SparkFun says any remaining residue must be removed.[7]
A code such as ROL0 on the data sheet is the reliable label. Under IPC J-STD-004 it means rosin, low activity and no detectable halides.[10] The flux guide explains how to read the codes.
3. Diameter
Solder wire comes in many diameters. Kester's 275 wire alone runs from 0.25 to 2.5 mm (0.010 to 0.093 in), including 0.4, 0.5, 0.8, 1.0 and 1.5 mm.[8] No standard maps diameter to a job, so these are our starting points, not rules:
| Work | Our starting point |
|---|---|
| Small surface-mount parts and fine pads | About 0.3–0.5 mm |
| Through-hole kits, headers, wires and general repair | About 0.5–0.8 mm |
| Large connectors, heavy wire and ground planes | About 1 mm or more |
Thin wire lets you feed a small, controlled amount. Thick wire delivers a lot of solder quickly, which is more than a small joint needs.
Makers also vary how much flux is in the core: Kester offers its 275 wire with flux cores from 1.1 to 3.3 percent.[8] Since lead-free may need more flux, a fuller core can help with lead-free wire.
4. A listing you can trust
Before you buy, check that the listing gives:
- The alloy by composition, such as Sn63Pb37 or Sn96.5Ag3Cu0.5. A listing that says only "solder wire" or "tin wire" tells you nothing.
- The flux type, ideally with its J-STD-004 code.
- The diameter in millimeters or inches.
- A maker's data sheet you can read before you order.
Old labels are a weak guide. Kester sells its 185 liquid flux as mildly activated rosin (RMA) and classifies it ROL0,[11] but that mapping belongs to that one product. An unbranded "RMA" syringe tells you much less.
What not to buy
- Acid-core solder. Canfield, a solder maker, says it is not recommended for electrical or electronic work because the residue is corrosive.[3]
- Solder and flux sold for plumbing. Buy products labeled for electronics.
- Unlabeled alloys. If the listing will not say what the wire is made of, skip it.
Next
Questions
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.[1][2]
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.[3]
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.[2][5]
Do I need separate flux if my solder has a flux core?
For simple joints, the core may be all you need. For rework, desoldering and heavy joints, extra flux helps: SparkFun's fix for ground planes and large through-holes includes adding flux.[14]