How Soldering Works
Wetting, flux, heat and time, the joint and its intermetallic layer, and the energy a joint needs.
What soldering is
Soldering joins two metal parts with a filler metal (the solder) that melts below 450 °C and below the melting point of the parts. The parts themselves never melt. The solder wets the clean surfaces, flows into the joint by capillary action and freezes, forming a thin alloy layer on each surface and a bond that carries both current and a modest mechanical load. Above 450 °C the same process is called brazing (stronger, hotter, used for pipework and tools); welding melts the parts themselves.
A soldered electrical joint is mostly an electrical connection with a light mechanical job; the part should be held by its shape (a bent lead, a clinch, a board hole) rather than by the solder alone, and a heavy part needs a clamp, screw or adhesive as well (see Wires, Connectors and Cables and Through-Hole Soldering).
Wetting and flux
Wetting is the spreading of molten solder over a solid surface. It happens only when the solder can touch bare metal, and that is why flux exists: it dissolves the oxide film that forms in seconds on hot copper, shields the surface from the air while it is hot and lowers the solder’s surface tension. The result is judged by the contact angle at the edge of the solder: under 30° is excellent, 30° to 90° is acceptable, over 90° means the solder has not wet and sits as a ball.
cos θ = (γsv − γsl) ÷ γlv
Worked example: what flux does to the angle
- Take typical values for molten Sn63 on copper: γlv = 0.46 N/m. On a clean, fluxed pad assume γsv − γsl = 0.65 N/m.
- cos θ = 0.65 ÷ 0.46 = 1.41, which exceeds 1, so the solder spreads completely (θ ≈ 0°) until it is limited by the amount of metal. That is the “very low angle” feather edge of a good joint.
- An oxidized pad has a far smaller γsv − γsl, for example 0.10 N/m: cos θ = 0.10 ÷ 0.46 = 0.22, so θ = 77°: a poor, high-angle joint.
- The numbers are illustrative: real surface energies depend on temperature, alloy and flux. The pattern, not the values, is the point.
| Without flux | With flux |
|---|---|
| Oxide film blocks the solder from the metal | Oxide is dissolved and carried off |
| Solder balls up (θ over 90°) | Solder spreads and feathers out (θ under 30°) |
| The surface re-oxidizes as it heats | Flux covers the metal while it is hot |
| Cold, dull, weak joint | Bright, concave, strong joint |
Types of flux, their activity and cleaning are on Solder Alloys, Flux and Paste.
Melting, alloys and the pasty range
A pure metal melts at one temperature, but most alloys melt over a range: solid below the solidus, fully liquid above the liquidus and slushy between. An alloy with the eutectic composition is the exception and melts like a pure metal at one temperature, which is why 63/37 tin-lead solder is so popular: it goes from solid to liquid and back at 183 °C with no pasty stage in which a moving joint can crystallize badly.
Worked example: how hot, how far above melting
- Sn63/Pb37 melts at 183 °C. An iron tip at 350 °C is 167 °C above it.
- SAC305 lead-free solder melts at about 219 °C. A tip at 380 °C is 161 °C above it, so the lead-free tip is set about 30 °C hotter but has a smaller margin (161 °C against 167 °C) for the same speed.
- A 60/40 alloy has a pasty range from 183 to about 190 °C; a joint that moves in that range freezes with a grainy, “disturbed” look.
The alloy list, the lead-free choices and the flow-friendly eutectics are on Solder Alloys, Flux and Paste.
The joint: the intermetallic layer
When solder wets copper, tin reacts with the copper and forms a thin intermetallic compound (IMC) layer: Cu6Sn5 next to the solder, and with time and heat a second layer of Cu3Sn next to the copper. This layer is what actually bonds the joint, and it is the sign that the solder has really wet. A good joint has a continuous IMC layer about 1 to 3 µm thick. Too little means the solder only sits on the surface (a cold joint); too much, from long heating or years at high temperature, makes the layer thick and brittle, and cracks run along it under shock or vibration.
The layer grows with the square root of time at temperature (diffusion), so a short joint is both better and stronger.
x = k × √t
Worked example: contact time and layer thickness
- Suppose a 2 s joint grows a 1.0 µm layer (illustrative). Then k = 1.0 ÷ √2 = 0.71 µm/√s.
- An 8 s joint gives 2.0 µm (double, for four times the time). A 30 s joint gives 3.9 µm.
- Quadrupling the heating time doubles the layer, and the damage to the flux, the pad adhesive and the part is far greater than the small gain in alloying. The best joint is the quickest one that wets completely.
Heat: how much a joint needs
Every joint must be brought above the solder’s melting point throughout: the pad, the lead and the solder. The energy is the heat to warm the copper and the solder, plus the latent heat to melt the solder, plus whatever leaks away into the board, the ground plane and the air. A small signal pin is a few joules; a pin tied to a ground plane or a heavy lug is ten to thirty times as much, and that is the usual cause of a joint that “won’t take”.
Worked example: time to solder, by joint and tip temperature
- Energy (ideal, 25 to 250 °C, copper c = 0.385 J/g/K, solder 0.21 J/g/K with 60 J/g latent heat): 7 J for a signal pin, 37 J for a pin through a ground plane, 97 J for a large lug.
- The heat flows through the contact between the tip and the joint: P = G × (Ttip − Tjoint). With a typical contact conductance G = 0.14 W/K and a mean joint temperature of 138 °C, a 300 °C tip gives 23 W, 350 °C gives 30 W and 400 °C gives 37 W.
- Time = energy ÷ power (table below). The small pin is done in about 0.2 s at 350 °C; the lug takes 3 s: too long for the pad, so use a bigger tip, a higher temperature or preheat the board.
| Joint | Energy | Time at 300 °C | Time at 350 °C | Time at 400 °C |
|---|---|---|---|---|
| Signal pin on a small pad | 7 J | 0.3 s | 0.2 s | 0.2 s |
| Pin through a ground plane | 37 J | 1.6 s | 1.3 s | 1.0 s |
| Large power lug or heat sink tab | 97 J | 4.3 s | 3.3 s | 2.6 s |
The practical lessons: a hotter tip with a short contact is better than a cool tip held long; a larger tip raises G; wet the contact with a bead of solder so heat transfers well; and heavy joints need preheat. Iron choice is on Irons, Stations, Tips and Hot Air.
Capillary flow
Molten solder that wets two close surfaces is pulled into the gap by surface tension, even uphill. The rise is h = 2γ cos θ ÷ (ρ g w) between two parallel plates a distance w apart. The narrower the gap, the stronger the pull, which is why solder runs through a plated hole or under a lead but only puddles on a wide gap.
Worked example: capillary rise of Sn63 between plates
- Typical values: γ = 0.46 N/m, density 8 400 kg/m³, θ = 30°.
- For a 0.2 mm gap h = 2 × 0.46 × cos 30° ÷ (8 400 × 9.81 × 0.0002) = 48 mm: far more than any board thickness, so gravity hardly matters in a good joint.
| Gap between surfaces | Rise (ideal) |
|---|---|
| 0.1 mm | 97 mm |
| 0.2 mm | 48 mm |
| 0.5 mm | 19 mm |
| 1.0 mm | 10 mm |
The ideal gap for a wicked joint is 0.05 to 0.15 mm. A hole 0.2 to 0.3 mm larger than the lead on diameter gives about that, and is the rule used on Through-Hole Soldering.
Five rules that follow
- Clean metal and flux. Without oxide-free surfaces nothing else matters.
- Heat the joint, not the solder. Put the tip on the pad and lead together and feed the solder to the joint; the solder melts from the joint’s heat, which proves the joint is hot enough.
- Be quick. One to three seconds; more only damages the flux, the pad and the part.
- Use the right amount. A concave fillet, not a ball (see Joint Quality, Inspection and Failure).
- Hold still. The joint must freeze undisturbed; a moved joint is a cold joint.
Troubleshooting
| Symptom | Likely cause | What to do |
|---|---|---|
| Solder balls up on the pad | Oxide or grease; no flux | Clean with isopropyl alcohol, add flux, retin the tip |
| Solder will not melt on the joint | Not enough heat flow: dirty tip, big thermal mass | Clean and tin the tip, use a larger tip, preheat the board |
| Dull, grainy joint | Disturbed while freezing or too cool | Reheat with fresh flux and hold still |
| Joint looks fine but is open | Cold joint or lifted lead; solder only on the pad | Reheat, add fresh solder, check the lead is in the solder |
| Pad lifts | Heated too long or too hot | Shorter contact, preheat, lower temperature |
| Joint fails after months | Brittle thick intermetallic or fatigue | Faster joints and strain relief; see Joint Quality, Inspection and Failure |
Related
- Soldering: the overview of this guide.
- Solder Alloys, Flux and Paste: alloys, flux and paste.
- Irons, Stations, Tips and Hot Air: irons and tips that deliver the heat.
- Joint Quality, Inspection and Failure: defects and inspection.
FAQ
What is the difference between soldering and welding?
Soldering melts only the filler metal, below 450 °C; welding melts the parts themselves. Soldered joints are weaker but low-temperature and reversible.
Why does solder need flux?
Hot copper oxidizes in seconds and solder cannot wet oxide. Flux dissolves the oxide and shields the surface so that the solder touches bare metal.
What is a cold joint?
A joint that never fully wet or that was disturbed while freezing: dull, grainy or lumpy, with a weak bond and often an intermittent connection.
Why is 63/37 solder special?
It is the eutectic of tin and lead: it melts and freezes at exactly 183 °C with no pasty range, so it forms sound joints even if the work moves slightly.
Why is a hotter iron better?
Heat flows in proportion to the temperature difference, so a hotter tip finishes the joint sooner and gives less total heat damage than a cool tip held for a long time.
References
- Klein Wassink, R. J., Soldering in Electronics, Electrochemical Publications (wetting, flux, intermetallics).
- Lee, N.-C., Reflow Soldering Processes and Troubleshooting, Newnes.
- Manko, H. H., Solders and Soldering, McGraw-Hill (phase diagrams, capillary flow).
- IPC J-STD-001, Requirements for Soldered Electrical and Electronic Assemblies.
Last reviewed: October 2026. Surface energies, contact conductance and layer thicknesses are typical and illustrative.