Soldering Guide: Iron Selection, Flux, Technique, and Joint Types
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Soldering joins metals with a filler metal (solder) that melts at a lower temperature than the pieces being joined. Electronics soldering and plumbing soldering use different tools, different solder, and different techniques, but the underlying principle is the same: heat the joint, not the solder, and let capillary action pull molten solder into the connection. This guide covers both domains so you know what you need for the work you are doing.
Soldering Irons for Electronics
A temperature-controlled soldering station is worth the investment over a fixed-wattage iron. Temperature control lets you dial in the right heat for different components — lower temperatures for sensitive ICs and higher temperatures for large ground planes and thick leads. A 60 to 75-watt station with adjustable temperature from 200 to 480 degrees Celsius covers all common electronics work.
Tip shape matters more than most beginners realize. A chisel tip (flat, angled face) is the best general-purpose shape — it makes good contact with both pads and leads, transfers heat efficiently, and works for both through-hole and surface-mount soldering. A conical (pointed) tip is useful for fine-pitch work but transfers heat poorly on larger joints. Start with a chisel tip.
Lead-free solder (required for commercial electronics since 2006) melts at higher temperatures and does not flow as smoothly as leaded solder. For hobby and repair work, 60/40 or 63/37 tin-lead solder is easier to work with and produces shinier, more reliable joints. Use lead-free solder only when required by regulation or preference — and wash your hands after working with leaded solder.
Solder diameter should match the work. 0.8mm (0.031-inch) solder is the standard for most through-hole electronics. 0.5mm works better for surface-mount components. 1.0mm or larger is only useful for heavy-gauge wire connections and is too much solder for typical circuit board work.
Flux and Its Role
Flux removes oxides from the metal surface so solder can bond. Without flux, solder beads up and rolls off the joint instead of flowing into it. Most electronics solder has a rosin flux core built in — you do not need to apply additional flux for typical through-hole work.
For surface-mount rework, desoldering, and difficult joints, additional flux paste or flux pen applied to the joint improves wetting and flow. The extra flux helps aged or oxidized pads accept solder that the core flux alone cannot clean. Apply it before heating the joint.
Plumbing flux (also called paste flux or acid flux) is much more aggressive than electronics rosin flux. It cleans copper pipe and fittings for strong solder joints. Never use plumbing flux on electronics — the acidic residue corrodes circuit board traces and causes failures over time. Keep plumbing flux and electronics flux separate.
Clean flux residue after soldering electronics. Rosin flux residue is mildly corrosive over time and can cause leakage paths between closely spaced traces on circuit boards. Isopropyl alcohol (90% or higher) and a toothbrush remove rosin flux effectively. No-clean flux exists but still benefits from cleaning on high-reliability work.
Electronics Soldering Technique
Heat the pad and the component lead simultaneously with the soldering iron tip. Then touch the solder wire to the junction of the iron tip and the pad — not directly to the iron tip. The solder should melt and flow into the joint by capillary action. If you melt solder onto the iron and try to carry it to the joint, you get a cold joint.
A good solder joint is shiny, concave (slightly scooped between the pad and the lead), and smooth. A dull, lumpy, or blobby joint is a cold joint — the solder solidified before it bonded to the pad. Reheat cold joints with fresh flux and let the solder reflow. Do not add more solder to a cold joint without reflowing what is already there.
Contact time matters. Touch the iron to the joint, feed solder for 1 to 3 seconds, then remove the solder wire and then the iron. Total contact time should be under 5 seconds for most components. Longer heating risks damaging heat-sensitive components, lifting pads from the circuit board, and damaging the board substrate.
Tin the soldering iron tip before each joint by melting a small amount of solder onto it. A tinned tip transfers heat more efficiently than a dry, oxidized tip. If the tip turns dark and solder will not stick to it, clean it on a wet sponge or brass wool and re-tin immediately.
Plumbing Soldering (Sweating Copper Pipe)
Plumbing soldering joins copper pipe to fittings using a propane torch and lead-free solder (lead solder is prohibited for potable water lines). The process is called sweating because the solder is drawn into the joint by capillary action — you do not see it flowing; it seeps into the gap between pipe and fitting.
Preparation is everything. Sand the outside of the pipe and the inside of the fitting with emery cloth or a fitting brush until they are bright copper. Apply plumbing flux paste to both surfaces. Assemble the joint. The flux keeps the surfaces clean during heating and promotes solder flow.
Heat the fitting evenly with the propane torch, not the pipe. The fitting is thicker and takes longer to reach temperature. Move the flame around the fitting. When the flux sizzles and turns clear, touch the solder to the joint on the opposite side from the flame — if the fitting is hot enough, the solder will melt on contact and wick into the joint by capillary action.
Remove the flame and let the joint cool undisturbed. A properly sweated joint shows a thin, even ring of solder around the fitting edge. Wipe excess solder and flux with a wet rag while still warm. Test for leaks by pressurizing the system after all joints have cooled. A joint that leaks needs to be heated, pulled apart, cleaned, re-fluxed, and resoldered.
Frequently Asked Questions
What soldering iron should I buy for electronics?
A temperature-controlled station in the 60 to 75-watt range with interchangeable tips. Hakko FX-888D and Weller WE1010 are both reliable mid-range stations that handle everything from basic through-hole to surface-mount work. Avoid cheap fixed-temperature irons — the lack of temperature control makes consistent joints difficult and increases the risk of damaging components.
Why are my solder joints dull and lumpy?
Dull, lumpy joints are cold joints — the solder solidified before making a proper bond. Causes: not enough heat on the pad, moving the joint before the solder solidified, or insufficient flux. The fix is to add fresh flux, reheat the joint until the solder reflows and becomes shiny, then hold everything still until it solidifies.
Is lead solder safe to use for hobby electronics?
Leaded solder (60/40 or 63/37 tin-lead) is widely used for hobby and repair work because it flows better and produces more reliable joints than lead-free solder. The risk is from ingesting lead, not from breathing fumes (solder fumes are from the flux, not the metal). Wash your hands after soldering, do not eat while working, and you will be fine. Use lead-free if you prefer to avoid lead entirely.