Solder and flux explained: alloys, types and how to choose

Joint quality is decided by three things: the iron, the technique and the consumables — and the third gets the least attention while causing a remarkable share of the problems. The solder alloy sets the melting point, the strength and even what a good joint looks like; the flux decides whether the solder wets the metal at all. Buy those two well and an average iron produces good work; buy them badly and no soldering station can save the joint.

This guide covers the leaded versus lead-free decision and the compliance reality behind it, the common alloys by name, what flux actually does and which type to use, when separate flux earns its place on the bench, and the fume-safety facts that are widely assumed and mostly wrong.

Leaded or lead-free?

The regulatory position first, because it's widely misunderstood: RoHS restricts lead in electrical products placed on the market, which is why commercial manufacturing moved to lead-free — but leaded solder itself remains legal to sell and use in the UK for repair, prototyping, education and the applications RoHS exempts. A production line building products for sale needs lead-free; a repair bench matching the alloy already on a board, or a workshop building one-offs, has a genuine choice.

The working difference is temperature and behaviour. Traditional Sn63/Pb37 melts at 183°C and flows beautifully; the standard lead-free alloy SAC305 melts around 217–219°C, needs the iron run hotter, and is less forgiving of hesitant technique. And one fact saves endless false alarms: a healthy lead-free joint looks duller and slightly grainier than a leaded one. Generations were taught that a dull joint is a cold joint; with lead-free, matte is normal, and rejecting good joints on leaded-era appearance standards is one of the most common inspection errors in the transition.

Which alloy should you buy?

The eutectic point deserves its footnote: Sn63/Pb37's single melting temperature means the joint goes from liquid to solid instantly, so a tremor during cooling matters little. Alloys with a pasty range can form disturbed, unreliable joints if moved while part-set — the original "dry joint". Beyond alloy: wire diameter should suit the work — around 0.7–1 mm for general through-hole, finer for surface-mount — and cored wire carries its flux inside, typically a few per cent by weight, which is what makes wire soldering a one-handed-feed operation at all. And if the table leaves you between options, our own-brand R-TECH solder covers the common alloys and diameters — we've written separately about why we put our name on it.

What does flux actually do?

Metal surfaces oxidise the moment air touches them, and solder will not bond to oxide — it beads up and sits on the surface like water on wax. Flux is the chemistry that strips that oxide at soldering temperature and shields the cleaned metal while the solder wets it. Every soldering fault described as "the solder won't take" or "it just balls up" is a flux story: exhausted flux, insufficient flux, or a surface too oxidised for the flux grade in use. It's also why re-melting a bad joint with the leftover solder rarely fixes it — the flux burned off the first time — and why the fix is fresh cored wire or a dab of separate flux, not more heat.

Which flux type should you use?

The naming trap sits in the last row: "water-soluble" sounds like the gentle option and is precisely the opposite — its residues are corrosive and hygroscopic, and a board soldered with it and not washed will grow faults over weeks as the residue eats tracks in humid air. Meanwhile "no-clean" carries its own asterisk: the residues are safe to leave for general work, but fine-pitch RF circuits, high-impedance analogue and anything due for conformal coating still wants cleaning, because even benign residue has electrical presence and coatings adhere badly over it.

When do you need separate flux?

Cored wire feeds flux automatically for straightforward joints; the bottle, pen or syringe earns its place the moment work gets harder. Rework and repair top the list — old joints re-melt far better with fresh flux applied first. Surface-mount drag soldering is essentially a flux technique with solder in a supporting role. Desoldering with solder wick transforms with added flux: braid that seems useless dry becomes startlingly effective wet, which is the single most useful desoldering tip in existence, and pairs with the desoldering tools for anything the wick can't lift. Tinning stripped wire, reviving oxidised component legs on old stock, and preparing large pads all go the same way: when the surface is doubtful, flux first. The wider bench kit — from tip cleaning to accessories — supports the process, but flux is the consumable that changes outcomes.

What about the fumes?

The widely believed hazard is the wrong one. Lead does not meaningfully vaporise at soldering temperatures — the smoke rising from a joint is not lead fume. The real airborne hazard is the flux: rosin (colophony) fume is a recognised respiratory sensitiser and a leading occupational cause of asthma, and it's just as present when soldering lead-free. The practical consequences: fume extraction — from a simple filtered fan to a proper extractor — matters for every soldering bench regardless of alloy, and matters most in classrooms and production settings where exposure is daily. Lead's genuine route into the body is hands, not lungs: handle leaded solder, then wash before eating — a hygiene habit, not a respirator problem. Getting the hazard right means spending the safety budget where it works.

Featured solder and flux

Frequently asked questions

Is leaded solder still legal in the UK?

Yes — to sell and to use. RoHS restricts lead in products placed on the market, which is why manufacturing is lead-free, but repair, prototyping, education and exempted applications may still use leaded solder legitimately.

Why does my lead-free joint look dull?

Because healthy lead-free joints do — SAC-family alloys freeze with a matte, slightly grainy surface. The shiny-equals-good rule belongs to the leaded era; judge lead-free joints by wetting and fillet shape, not gloss.

What temperature should I solder lead-free at?

Typically 350–380°C for standard PCB work, against roughly 315–350°C for leaded — the alloy melts some 35°C higher and needs the extra headroom. Larger joints and thermal planes justify more; use the lowest setting that wets a joint within a couple of seconds.

Do I need to clean no-clean flux?

Usually not — that's the point of it. Clean anyway for fine-pitch RF, high-impedance analogue circuits, and any board due for conformal coating, where even benign residue affects performance or adhesion.

What's the difference between 60/40 and 63/37 solder?

Sn63/Pb37 is eutectic: it melts and freezes at a single 183°C point, so joints set instantly. Sn60/Pb40 passes through a brief pasty range while cooling, during which movement can produce a disturbed joint. For a small premium, 63/37 is the more forgiving wire.

Why doesn't my solder wick work?

It's dry. Wick relies on flux to make removed solder flow into the braid — add liquid or paste flux to the braid first and it works dramatically better. Also use a wide enough braid for the joint and a tip with enough thermal mass to heat both.

Stock the bench properly

Alloys for every compliance situation, flux in every format, and the wick, cleaners and extraction that finish the job safely.