Soldering Fundamentals
TECHNIQUE // QUALITY // TOOLS // TROUBLESHOOTING
Why a technician needs to solder: Terminal repair, sensor wire splicing, PCB component replacement, connector rework, relay socket retrofits. A bad solder joint looks fine visually but fails under vibration or heat — which is always when you need it most.
⚡ The 4-Step Process
Click each step to expand details →
1
Heat Part & Pad
Touch the iron tip to BOTH the component lead AND the copper pad simultaneously. Heat the joint, not the solder. Hold for 2–3 seconds before adding solder. If you don't heat the pad — the solder balls up and won't wet the surface. Most cold joints are caused by only heating the lead.
Tip temp: 320–370°C for leaded, 350–400°C for lead-free
Tip temp: 320–370°C for leaded, 350–400°C for lead-free
2
Add Solder
Feed solder into the joint, NOT onto the iron tip. Touch the solder wire to the pad/lead junction — let the heated joint melt it. The solder should flow smoothly and wick into the joint. A small amount goes a long way — resist the urge to add more.
Common mistake: applying solder to the iron tip first (creates cold joints)
Common mistake: applying solder to the iron tip first (creates cold joints)
3
Continue Heating
Keep heat on for 1 second after solder flows. This ensures the solder fully wets the pad and flows into the barrel (for through-hole). Then remove the iron smoothly. Don't yank it — a sudden pull can disturb the joint before it solidifies.
Total contact time typically 3–5 seconds. Too long = lifted pad risk.
Total contact time typically 3–5 seconds. Too long = lifted pad risk.
4
Let Cool
Do NOT blow on it. Do NOT move the component. The joint needs 5–10 seconds to fully solidify. Blowing causes premature quench = grainy structure = cold joint. Moving the part mid-solidification = fractured joint. Let gravity and ambient air do the work.
Inspect: shiny (leaded) or slightly grainy (lead-free) — both OK if shape is correct.
Inspect: shiny (leaded) or slightly grainy (lead-free) — both OK if shape is correct.
🔍 Joint Quality Reference
Click any joint to see diagnosis →
Perfect
✔ GOOD
Too Much
✘ FAIL
Too Little
✘ FAIL
Cold Joint
✘ FAIL
Too Much Heat
✘ FAIL
Short / Bridge
✘ FAIL
Select a joint above to see diagnosis
Click any joint image →
🛠 Essential Tools
Soldering Iron
Temperature-controlled station preferred. Pencil iron for field work. 25–65W adjustable. Keep tip tinned.
Solder Wire
60/40 Sn/Pb (leaded) easiest for learning. 63/37 = eutectic, best flow. Lead-free for RoHS. 0.8–1mm dia for most work.
Tip Cleaner
Brass wool (preferred) or damp sponge. Clean before each joint. Oxidized tip = poor heat transfer = bad joints.
Solder Wick
Braided copper braid for desoldering. Press on joint with iron — solder wicks up. Works on SMD too.
Flux
Rosin flux promotes solder flow, removes oxidation. No-clean flux leaves little residue. Essential for rework.
Magnifier / Loupe
Inspect joints at 5–10x. Bridging and cold joints often invisible to naked eye. 10x loupe is cheap and lives in your kit.
Third Hand / PCB Vise
Holds board still while you work. Frees both hands. Critical for quality work. Binder clips on a scrap block = field version.
IPA / Flux Remover
99% isopropyl alcohol to clean flux residue after soldering. Use brush, wipe dry. Flux left on board can cause corrosion or leakage current.
🌡 Temperature Reference
| Solder Type | Tip Temp | Notes |
|---|---|---|
| 60/40 Leaded | 320–370°C | Easiest. Shiny finish. Not used in new EU products (RoHS). |
| 63/37 Eutectic | 320–360°C | Best flow. Transitions solid↔liquid instantly. No plastic phase. |
| Lead-Free (SAC305) | 350–400°C | Higher melting point. Slightly duller finish. Mandatory for RoHS work. |
| Silver solder (high %) | 700°C+ | Torch required. NOT electronic solder. Used for pipe/HVAC. |
⚠ Common Mistakes & Fixes
❌ Mistakes That Kill Joints
| Mistake | Result |
|---|---|
| Heating solder, not joint | Cold joint, poor adhesion |
| Moving joint before cool | Fractured crystalline joint |
| Blowing to cool | Grainy, weak joint |
| Dirty / oxidized tip | Heat doesn't transfer |
| Too much heat / time | Lifted pad, burnt flux |
| No flux on rework | Solder won't wet surface |
✅ Field Pro Tips
- Always tin your iron tip before starting and before putting it away
- Flux is your friend — add it liberally on any rework
- Let the joint tell you when it's hot enough (solder flows in immediately)
- Through-hole should show solder on BOTH sides of the board
- Use the right chisel or bevel tip for the pad size — conical tip loses heat fast
- When desoldering IC pins, work fast — 3 seconds per pin maximum
- Clean flux residue within an hour — it hardens and becomes harder to remove
🦺 Safety Notes
FUMES Lead solder produces lead particulates when flux burns — ventilate or use fume extractor
BURNS Solder tip runs 350°C+ — never set iron face-down
IPA 99% isopropyl is flammable — no open flame nearby
EYES Solder can spit — safety glasses when snipping leads