Soldering joins metals with a filler that melts at a lower temperature than the parts you bond. From printed-circuit boards to copper pipes and jewelry, this age-old process delivers electrical contact, fluid seals, and structural support with basic tools. The guide below strips away jargon, explains best practices, and lists real shop parameters so you can solder clean joints on the first try.
Table des matières
- Why Understanding Soldering Matters
- Core Theory Of Soldering
- Choose The Right Solder Alloy
- Match Flux To The Job
- Set Tools And Temperatures
- Step-By-Step Hand Soldering
- Wave And Reflow Basics For Production
- Inspect Joints For Quality
- Common Defects And Fixes
- Soldering Safety And Fumes
- Cost Comparison Of Methods
- Need Precision Fabrication?
- Quick Recap And Next Steps
Why Understanding Soldering Matters
Good solder joints hold circuits alive, stop roof leaks, and secure sensor wires in harsh engines. Poor joints crack, corrode, or overheat. By learning basic metallurgy and process control, you cut rejects and save time.
Core Theory Of Soldering
Solder flows by wetting. Heat raises the filler above its liquidus, flux removes oxides, and surface tension draws metal into gaps as narrow as 0.05 mm. Because base metals stay below their melting point, parts keep shape and temper.
Melting Range Chart For Popular Alloys
Tin-Lead 60/40 183–191 °C ███████████ Tin-Silver-Copper (SAC305) 217 °C ████████████ Tin-Bismuth 42/58 138 °C ██████ Tin-Copper 99/1 227 °C █████████████
Lower bars mean lower soldering temperature and less thermal stress on parts.
Choose The Right Solder Alloy
The table summarizes key alloys and where they fit best.
| Alliage | Melting °C | Lead-Free? | Main Advantage | Typical Use |
|---|---|---|---|---|
| Sn60 / Pb40 | 183–191 | Non | Wide pasty range, easy hand work | Vintage electronics repair |
| SAC305 (Sn96.5 Ag3 Cu0.5) | 217 | Oui | Strong joint, RoHS compliant | SMT boards |
| Sn42 Bi58 | 138 | Oui | Low heat, good for sensors | Heat-sensitive components |
| Sn97 Cu3 | 227 | Oui | Faible coût | Plumbing fittings |
| Sn96.5 Ag3.5 | 221 | Oui | Superior fatigue life | Aerospace circuits |
Match Flux To The Job
Flux cleans and protects surfaces. Pick one that activates at your solder temperature and suits downstream washing.
| Flux type | Activation temp °C | Residue cleaning | Best on |
|---|---|---|---|
| Rosin RMA | 160 | Optional (isopropyl) | Electronics hand joints |
| No-clean | 180 | Aucun | High-volume SMT |
| Water-soluble organic | 120 | Hot DI water | High-reliability boards |
| Inorganic acid (HZnCl) | 110 | Hot wash + neutralize | Plumbing copper pipe |
Set Tools And Temperatures
Tool choice affects joint quality more than alloy costs. Use the table for a quick match.
| Tool | Power rating | Tip temp setpoint | Ideal joint type |
|---|---|---|---|
| Soldering iron (analog) | 25–40 W | 370 °C | Through-hole leads |
| Temperature-controlled station | 60–90 W | 320–350 °C | Fine-pitch SMT |
| Hot-air rework gun | 400 W | 280–360 °C air | BGA reflow |
| Propane torch | NA | Flame ~600 °C | Brass fittings, roof flashing |
Step-By-Step Hand Soldering
- Prep. Cut leads, deburr edges, and wipe with alcohol.
- Tin tip. Melt a small solder bead to wet the iron face.
- Heat joint. Press the tip to both parts for 2–3 s.
- Add solder. Feed wire to the joint, not the iron, until a smooth fillet forms.
- Remove heat. Hold still until shiny. A dull finish hints at cold solder.
- Inspect. Use 3× magnifier; look for cone shape, full wetting, no voids.
Joint Dimension Targets For Through-Hole Pins
Pin Ø 0.8 mm → Fillet height 0.4–0.6 mm Pin Ø 1.0 mm → Fillet height 0.6–0.8 mm Pin Ø 1.3 mm → Fillet height 0.8–1.0 mm
Wave And Reflow Basics For Production
In high-volume boards, solder paste is printed, parts placed, and the assembly rides a conveyor through a reflow oven. Each zone matters.
| Zone | Temp °C | Time s | But |
|---|---|---|---|
| Pre-heat | 120–150 | 60–90 | Remove solvents |
| Soak | 150–180 | 60–120 | Activate flux |
| Reflow peak | SAC305: 240 | 30–45 above liquidus | Wet all pads |
| Cool | <3 °C/s ramp | - | Freeze grain-fine joint |
Inspect Joints For Quality
Check visual cues first, then sample destructive pulls or X-ray for hidden voids.
- Shiny, concave fillet indicates good wetting.
- Fillet angle between 40° and 70° gives strongest shear.
- Solder thickness should be 0.5–0.8 × lead diameter.
Common Defects And Fixes
The table pairs each defect with a quick remedy.
| Defect | Visual clue | Main cause | Fix |
|---|---|---|---|
| Cold joint | Dull, grainy surface | Low temp or short heat | Raise tip 20 °C |
| Solder bridge | Tin between pads | Too much paste | Reduce stencil aperture |
| Void | BGA X-ray shadow | Trapped flux gases | Slow soak ramp |
| Black pad | Dark copper finish | Sulfur tarnish + heat | Use ENIG boards |
Soldering Safety And Fumes
- Ventilate with fan and carbon filter; flux fumes irritate lungs.
- Wear eye protection—molten solder can spatter.
- Avoid bare skin contact: Tin–lead alloy is toxic if ingested.
- Keep tip cleaners wet with rosin, not water, to stop thermal shock.
Cost Comparison Of Methods
Evaluate per-joint cost for prototypes versus mass runs.
| Méthode | Cap-ex USD | Joints/hour | Labor cost/joint |
|---|---|---|---|
| Hand iron | 150 | 150 | $0.12 |
| Hot-air rework | 900 | 80 | $0.28 |
| Batch wave | 35 000 | 15 000 | $0.005 |
| Inline reflow | 70 000 | 25 000 | $0.003 |
Need Precision Fabrication?
Beyond solder work, our shop provides CNC machining, sheet-metal, and injection solutions under one roof. Send your drawings for a single-source quote.
Quick Recap And Next Steps
- Pick a RoHS-compliant alloy unless legacy repair demands Sn-Pb.
- Match flux and temperature to metals and cleanliness goals.
- Control heating profile—wet surfaces, avoid overshoot.
- Inspect joints visually and by pull or X-ray if critical.
- Store solder wire dry, sealed, and reel off slowly to stop oxide build-up.
Master these points and your solder joints will shine—electrically, mechanically, and under audit.
