Soldering: Types, Process, Flux, Machines and Applications

Soldering joins metal parts by melting a filler metal (solder) that flows into the joint and wets both surfaces, without melting the parts themselves. Classic electronics solder is tin-lead 63/37, which melts at 183 °C; the lead-free alloy that replaced it in most products, SAC305, melts at about 217-220 °C. A flux cleans the surfaces so the solder can bond. Soldering makes the electrical joints in every printed circuit board, and the leak-tight joints in copper water pipes and radiators.

If you are comparing soldering with brazing, including the 450 °C dividing line and joint strength, see soldering vs brazing; for the higher-temperature process itself, see the brazing process. Neither is welding, because the base metal does not melt (compare the types of welding). This page covers how soldering is actually done.

How does soldering work?

A solder joint is not glue. When molten solder wets clean copper, tin in the solder reacts with the copper to form a very thin intermetallic layer (mainly Cu₆Sn₅). That layer is the actual bond. For it to form, three things are needed:

1. Clean metal: oxide and grease stop wetting. Flux removes thin oxide during heating and keeps air off.
2. Enough heat in both parts: the pad and the lead must both be above the solder’s melting point, or the solder sits on one of them as a blob.
3. Capillary flow: in a well-designed joint the solder is drawn into the gap and forms a smooth, concave fillet.

Types of soldering: soft and hard

• Soft soldering uses tin-based alloys that melt at roughly 180-230 °C for electronics and plumbing. This is what “soldering” normally means.
• Hard (silver) soldering uses silver-bearing alloys that melt far higher, usually above 450 °C, giving much stronger joints for jewellery, refrigeration pipework and tools. By the modern definition it is really brazing, which is why it is covered on the brazing pages linked above.

Soldering is also grouped by heating method: iron (hand) soldering, torch soldering (plumbing), wave soldering, reflow soldering, selective soldering, induction soldering and hot-air rework.

Solder alloys: 63/37 tin-lead vs lead-free SAC305

RoHS (the EU Restriction of Hazardous Substances directive) limits lead in most electrical and electronic products, which drove the change to lead-free solder worldwide. Tin-lead is still used in some aerospace, defence and repair work. The soldering vs brazing page has more on lead-free alloys and RoHS. For plumbing, always use lead-free solder on drinking-water lines.

Flux types

Electronics fluxes are classified under IPC J-STD-004 with codes such as ROL0: RO = rosin base, L = low activity, 0 = essentially halide-free (a typical no-clean paste’s datasheet quotes halides below 0.05%).

Soldering iron temperature and tip care

A temperature-controlled soldering station is worth far more than an uncontrolled plug-in iron. Typical tip settings are around 320-350 °C for tin-lead and 350-380 °C for lead-free solder (typical figures; follow the solder maker’s advice). Use the lowest setting that makes a good joint in about 2-4 seconds; a hotter tip burns flux, oxidises faster, and can lift pads.

• Pick the tip for the job: a chisel or bevel tip transfers heat far better than a fine conical tip. Use the largest tip that fits the joint.
• Keep it tinned: a thin coat of solder on the tip conducts heat and stops oxidation. Re-tin before putting the iron in its stand.
• Clean on brass wool or a damp (not wet) sponge. Never file or sand a plated tip.
• Turn it down or off when idle: a hot, untinned tip turns black and stops wetting.

How to hand-solder a through-hole component on a PCB

1. Clean the pad and lead if they are dull or oxidised; insert the component and bend the leads slightly to hold it.
2. Touch the tinned tip so it heats both the pad and the lead at the same time.
3. After a second or two, feed solder wire into the joint on the side opposite the tip, not onto the tip.
4. Stop feeding when the solder forms a smooth, concave cone around the lead, covering the pad.
5. Take the solder away first, then the iron. Hold the part still for a few seconds while the joint freezes.
6. Trim the lead just above the fillet. Inspect: a good joint is smooth and concave (shiny for tin-lead, satin for lead-free).

Surface-mount parts are hand-soldered by tinning one pad, tacking one end of the part, then soldering the other end and re-flowing the first.

Soldering machines: wave vs reflow

Most modern boards are mainly SMD and use reflow, with selective soldering (a small moving solder fountain) for the few through-hole parts. Hot-air rework stations are used to replace SMD parts by hand. For board design basics, see this overview of the printed circuit board.

Worked example: timing a lead-free reflow profile

A reflow profile for SAC305 paste is set by a few limits. The paste datasheet used here says the preheat ramp should not exceed 2.5 °C/s. Typical targets across SAC305 pastes (check your own paste’s datasheet) are a soak of about 150-190 °C, a peak of about 235-250 °C, and 45-90 s time above liquidus (TAL), the time the joint spends above 217 °C.

First attempt:

• Ramp 25 → 150 °C at 1.5 °C/s: 125 / 1.5 = 83.3 s (under the 2.5 °C/s limit)
• Soak 150 → 190 °C in 90 s: 40 / 90 = 0.44 °C/s
• Ramp 190 → 245 °C at 2.0 °C/s: crosses 217 °C after 27 / 2 = 13.5 s, then 217 → 245 °C takes 28 / 2 = 14 s
• No hold at peak; cool at 3 °C/s from 245 back to 217 °C: 28 / 3 = 9.3 s
• TAL = 14 + 9.3 = 23.3 s: too short. Large parts and ground planes may not fully reflow, giving cold, grainy joints.

Revised profile: slow the final ramp to 1.0 °C/s, hold 10 s at the peak, cool at 2.5 °C/s.

• 217 → 245 °C at 1.0 °C/s = 28 s
• Hold at 245 °C = 10 s
• 245 → 217 °C at 2.5 °C/s = 11.2 s
• TAL = 28 + 10 + 11.2 = 49.2 s, inside the 45-90 s window.

Total time to peak = 83.3 + 90 + 55 (190 → 245 °C at 1.0 °C/s) = 228.3 s, about 3.8 minutes, plus hold and cooling. In practice the profile is checked with thermocouples on the real board, because a heavy board lags the oven setting.

Common soldering defects

The electronics industry judges joints against the IPC-A-610 acceptability standard, which has photos of acceptable and reject joints for each case.

Soldering safety

• Fumes: the smoke is mostly burnt flux, and rosin flux fume can cause asthma. Use a fume extractor or work in good ventilation, and keep your face out of the plume.
• Lead: wash hands after handling tin-lead solder, and do not eat or drink at the bench.
• Burns: the tip is at 300-400 °C. Always return the iron to its stand and never catch a falling iron.
• Eyes: wear safety glasses; flux and solder can spit, and clipped leads fly.
• Torches and acid flux: for plumbing, keep the flame away from flammables and wash acid flux residue off hands and pipes.

Applications of soldering

• Electronics: PCB assembly for phones, computers, appliances and vehicles; wire and connector joints.
• Plumbing and HVAC: copper water pipes and fittings (lead-free), sweat joints.
• Automotive: radiators and wiring loom joints.
• Sheet metal: gutters, tin cans, jewellery and stained glass (with specific alloys).

For more on joining processes, see TWI – Welding Job Knowledge and the American Welding Society (AWS).

FAQs

What is soldering?

Soldering joins metal parts by melting a low-melting filler, solder, into the joint so it wets and bonds to both surfaces. The parts themselves do not melt. Common solders melt between about 180 and 230 °C.

What is the melting point of solder?

Tin-lead 63/37 solder melts at 183 °C. Lead-free SAC305 (tin, 3% silver, 0.5% copper) melts at about 217-220 °C. Tin-copper lead-free solder melts at about 227 °C.

What temperature should a soldering iron be?

Typically about 320-350 °C for tin-lead solder and 350-380 °C for lead-free. Use the lowest temperature that makes a good joint in a few seconds with the tip size you are using.

What is the difference between wave and reflow soldering?

Wave soldering passes the board over a wave of molten solder and is used for through-hole parts. Reflow soldering melts solder paste printed on the pads by heating the whole board in an oven, and is used for surface-mount parts.

Why is flux used in soldering?

Flux removes the thin oxide on the metal surfaces during heating and keeps air away, so the molten solder can wet the metal and form the bond. Without flux, solder beads up and does not stick.

Related Topics on EngineeringHulk

• 👉 Difference Between Soldering and Brazing
• 👉 Brazing Process: Filler, Advantages & Applications
• 👉 TIG Welding: Equipment & Techniques
• 👉 MIG Welding
• 👉 Flux-Core Welding