🔌 Soldering & Electronics Repair

What you’ll learn

1. Solder Doesn't Glue — It WetsExplain soldering as wetting and intermetallic bonding rather than gluing, and identify a cold joint from the shape of its fillet.Solder doesn't fill a gap — it wets the metal, and the tin reacts with the copper to grow a thin intermetallic layer that IS the joint. If the copper never reaches temperature the solder melts but no bond forms, producing a cold joint that conducts today and fails later. A wetted joint is concave and pulled in; a cold one is a convex ball sitting on top.
2. The Iron Is a Heat Pump, Not a Hot StickDistinguish temperature from heat delivery, and use contact area and a tinned tip to make joints instead of raising the dial.An iron's tip stores almost no energy, so a copper pad drains it faster than the element can resupply — the tip temperature crashes on contact. Wattage buys recovery rate, not temperature, and the real throttle is contact area: a large flat tip plus a bead of solder bridging the microscopic gaps. A tip must stay tinned to exclude air, and must never be filed through its iron plating.
3. Flux: The Half of the Job You Can't SeeExplain what flux does chemically, choose between rosin, no-clean and water-soluble, and control rosin fume as part of technique.Copper oxidises within minutes and molten solder will not wet an oxide, so flux must strip the oxide, seal the surface from air, and lower surface tension — all in the instant before the solder arrives. Rosin and no-clean leave residues you can ignore; water-soluble is corrosive and demands a wash. The visible smoke is flux, not metal, and rosin fume is a leading cause of occupational asthma by sensitisation, which is why moving the plume away from your face is a technique rather than a precaution.
4. The Alloy You Choose, and the Metal You RespectChoose between leaded and lead-free alloys on melting point and wetting behaviour, and control lead by its actual route of exposure.63/37 tin-lead is eutectic: it melts at 183 °C — below either pure metal — and passes straight from solid to liquid with no pasty range to disturb. Lead-free SAC melts at 217 °C, wets less eagerly, and sets duller, which costs you the 'shiny means good' test. Lead is a neurotoxin with no known safe exposure level, but it cannot vaporise at bench temperatures (it boils at 1749 °C); the route in is hand-to-mouth, so the controls are no food at the bench and real hand-washing.
5. Making the JointExecute the three-second joint — touch, feed, withdraw — and diagnose each common defect from its appearance.Lay a tinned tip flat against pad and lead together and wait for the copper to heat; feed solder into the far side of the joint so the flux lands on the oxide and the joint itself proves it's hot enough; withdraw wire then iron, and leave it undisturbed for two seconds. Every defect has one cause: convex and dull means it never wetted and needs more contact area, grainy means it moved while freezing, and a ring crack means years of thermal fatigue.
6. The Bench That Doesn't Burn YouArrange the bench so that burns, eye injuries and dropped irons are prevented by geometry rather than vigilance.Burns don't happen during the joint — they happen in the gaps, and above all when the reflex to catch a falling iron fires before judgement does. The controls are all layout: a weighted stand always in the same place, the cable routed behind, parking it every time, and deciding in advance to let it fall. Eyes need glasses against spitting flux and a finger over every clipped lead, and the work needs a third hand, because iron-solder-board-component is four things for two hands.
7. Reading the BoardRead a board's through-hole and surface-mount parts, identify polarity marks, and remove solder without lifting pads.Surface-mount parts exist for machines; through-hole parts are bigger, stronger and human-friendly — and by luck they're also the parts that actually fail, since capacitors, connectors and power devices have a mechanical life. Polarised parts announce themselves with stripes, bands, notches and leg lengths, and your phone is the best documentation tool on the bench. Desoldering fails on force, not heat: add fresh flux-cored solder to re-flux an old joint before wick or a sucker can take it away.
8. Finding the FaultDiagnose a dead device outside-in, read a board by eye and nose, and recognise the cracked joint as the repair this course was built for.Work from the socket inward — cable, fuse, connector, switch — because most devices are broken somewhere boring, and the board is roughly the fifth suspect. Then look before measuring: bulged capacitor tops, scorching, corrosion and cracks announce themselves, and burnt or shorted parts have distinctive smells. The commonest repairable fault is a cracked joint at a connector, betrayed by intermittency, and reflowing it is the whole fix.

Questions this course answers

Grounded in trusted sources

• Solder — melting points of 63/37 (183 °C), 60/40 (188 °C), intermetallic compounds, wetting and RoHS: https://en.wikipedia.org/wiki/Solder
• Lead-free solder — Sn-Ag-Cu ternary eutectic 217 °C, Sn-Ag 221 °C, Sn-Cu 227 °C, wetting and flux demand: https://en.wikipedia.org/wiki/Lead-free_solder
• Flux (metallurgy) — oxide removal, wetting, rosin/no-clean/water-soluble, and rosin fume as a cause of occupational asthma: https://en.wikipedia.org/wiki/Flux_(metallurgy)
• Soldering iron — tip temperatures 200–480 °C, iron-plated copper tips, tinning, and why wattage buys heat capacity rather than temperature: https://en.wikipedia.org/wiki/Soldering_iron
• Tin — melting point 231.93 °C: https://en.wikipedia.org/wiki/Tin
• Lead — melting point 327.46 °C, boiling point 1749 °C: https://en.wikipedia.org/wiki/Lead
• WHO, 'Lead poisoning and health' — no level of lead exposure known to be without harmful effects; children absorb 4–5× the ingested dose of adults: https://www.who.int/news-room/fact-sheets/detail/lead-poisoning-and-health
• UK Health and Safety Executive, 'Controlling health risks from rosin (colophony)-based solder flux fume' (INDG249) and 'Solder fume and you' (INDG248) — rosin-based solder flux fume among the leading causes of occupational asthma; LEV controls: https://www.hse.gov.uk/pubns/indg249.htm

Every Wunder lesson is built from real, reputable sources — never invented.

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