Solder Wire, Flux, Solder Paste and Thermal Paste Explained: Alloy and Iron Temperature Selection
Leaded and lead-free solder alloys, what flux does, flux paste vs solder paste, where thermal paste is used, and soldering iron power and temperature settings.
Soldering joins two metal surfaces with an alloy (solder) that melts at a lower temperature than they do. A good joint needs three things: the right solder wire, flux to remove surface oxide and an iron that supplies enough heat. Thermal paste is not a soldering material; it carries heat from an electronic part to a heatsink and is often confused with flux paste.
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1. What each material does
| Material | Job | Where used |
|---|---|---|
| Solder wire | The joining alloy; most have a flux core | Circuit boards, cables, connectors |
| Flux (liquid, gel, pen) | Dissolves oxide and lets solder wet the surface | Rework, surface mount, oxidized surfaces |
| Flux paste | Paste-form flux with no solder in it | Cable and lug soldering; acid-free type for electronics |
| Solder paste (SMD) | Powdered solder mixed with flux | Surface-mount parts, hot air and reflow |
| Acid soldering fluid | Strong acidic cleaner | Sheet metal, gutters, copper pipe; never on electronics |
| Thermal paste | Conducts heat from part to heatsink; does not join | Processors, power transistors, LED modules |
2. Solder wire alloys
| Alloy | Composition | Melting point | Property |
|---|---|---|---|
| Sn63/Pb37 | 63% tin, 37% lead | 183 °C (single point) | Easiest flowing, shiny joints; repair and hobby |
| Sn60/Pb40 | 60% tin, 40% lead | 183–190 °C | General-purpose leaded solder |
| SAC305 | Tin + 3% silver + 0.5% copper | 217–220 °C | Lead-free electronics production standard |
| Sn99.3/Cu0.7 | Tin + 0.7% copper | 227 °C | Economical lead-free solder |
- Lead-free solder needs more heat and looks duller; that is not a defect.
- Commercial electronics must use lead-free solder; leaded solder is for repair and hobby work, with ventilation and hand washing afterwards.
3. Flux types
| Flux type | Activity | Cleaning | Use |
|---|---|---|---|
| Rosin based | Mild to medium | Residue wiped with isopropyl alcohol | General electronics |
| No-clean | Mild | Residue may stay | Production, repair |
| Acid (zinc chloride) | Very strong | Must be fully removed | Plumbing and sheet metal |
Acid fluid or plumber's paste on a circuit board corrodes tracks and pins over time and the joint fails months later. For electronics look for "acid-free" or "electronics grade". Remove residue with cleaners.
4. Iron power and temperature
| Job | Iron type and power | Tip temperature |
|---|---|---|
| Surface mount, fine-pitch ICs | Soldering station, 40–60 W, fine tip | Leaded 300–330 °C, lead-free 340–360 °C |
| General board soldering | Pencil iron 25–40 W or station | Leaded 320–350 °C, lead-free 350–380 °C |
| Cables, connectors, thick pins | Pencil iron 60–80 W | 380–420 °C |
| Heavy cable, lugs, sheet metal | Soldering gun 100 W and above | — |
- Power is not temperature: a powerful iron keeps its heat on a large part; a small iron cannot heat it and gives a cold joint.
- A temperature-controlled soldering station holds the tip steady; an unregulated pencil soldering iron overheats when idle and its tip oxidizes quickly.
- Too high a temperature burns the flux before solder flows and lifts board tracks.
5. How to make a good joint
- Clean the tip on a damp sponge or brass wool and tin it with a thin coat of solder.
- Touch the tip to pad and lead together for 1–2 seconds.
- Feed solder to the heated joint, not to the iron.
- When solder has flowed, remove the wire, then the iron; keep the part still until the joint sets.
- The whole operation should not exceed 3–4 seconds. A good joint is a concave cone; a dull, grainy ball is a cold joint.
Remove excess solder with a desoldering pump. Do not tin stranded wire that goes into a screw terminal; solder creeps and the joint loosens. Use a ferrule instead; see Terminal Block Types and Selection.
6. What is thermal paste?
Thermal paste fills the microscopic air gaps between a semiconductor and its heatsink to improve heat transfer. It has nothing to do with solder and does not bond the parts.
| Type | Thermal conductivity | Electrically conductive? | Use |
|---|---|---|---|
| Silicone based (white) | 0.8–3 W/m·K | No | Power transistors, regulators, LEDs |
| Metal oxide / ceramic filled | 4–9 W/m·K | No | Computer processors, graphics cards |
| Liquid metal | 40–70 W/m·K | Yes | Expert use; attacks aluminium |
- A thin layer is enough; a thick layer acts as an insulator.
- Wipe old paste off with isopropyl alcohol before reapplying; dried paste impairs heat transfer.
- Where electrical insulation is needed (a transistor with a live tab), add a mica or silicone pad.
Frequently asked questions
What is flux and what does it do?
Flux is a chemical that dissolves the oxide layer on the surfaces and lets solder spread over them. Without flux, solder balls up and does not adhere.
Are flux paste and solder paste the same?
No. Flux paste is flux in paste form with no solder in it. Solder paste is powdered solder mixed with flux and makes the joint by itself on surface-mount parts.
Can thermal paste replace solder?
No. Thermal paste gives no electrical or mechanical connection; it only conducts heat from a part to its heatsink.
Leaded or lead-free solder?
Leaded Sn63/Pb37 melts lower and flows more easily, which is practical for repair and hobby work. Commercial products and health considerations call for lead-free solder, which needs 30–40 °C more.
What temperature should a soldering iron be?
For board work, 320–350 °C with leaded solder and 350–380 °C with lead-free. Thick cables and connectors need about 400 °C and a more powerful iron.