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Soldering a Through-Hole Joint: Heat the Joint, Not the Solder
Almost every bad solder joint comes from the same mistake: melting solder onto the iron and wiping it onto the work. That makes a grey blob sitting on top of the pad, held by nothing, which passes a visual glance and fails a month later.
A solder joint is not glue. The solder alloys with the copper of the pad and the metal of the lead, and it can only do that if BOTH are hot enough to melt it themselves. So the whole technique is one sentence: heat the joint, then feed solder into the joint.
Everything else — tip choice, flux, timing, what a good joint looks like — is detail that follows from that. Fifteen SIK circuits are published here and the whole series lives on a breadboard; this is how you make one permanent.
Anfänger
2 hours
Anweisungen
1
1
Tin the tip, and keep it tinned
Tin the tip, and keep it tinned
Bring the iron to temperature, wipe the tip, and immediately coat it with a thin layer of fresh solder so it looks bright and wet. Re-tin it every time you put the iron down.
A bright tinned tip transfers heat; a dull oxidised one does not, however hot the iron claims to be. Most complaints that an iron is 'not hot enough' are an oxidised tip, and the fix takes three seconds.
Around 320-350 degrees Celsius suits leaded solder, a little higher for lead-free. Hotter is not better: above about 400 the flux burns off before it can clean the joint, and the tip oxidises while you work.
Materialien für diesen Schritt:
Lötdraht1 StückBenötigte Werkzeuge:
Lötstation
Baumwolltuch2
2
Heat both metals, then feed the solder
Heat both metals, then feed the solder
Lay the tip so it touches the PAD and the LEAD at once — the flat of the tip against both, not the point against one. Count about two seconds, then touch the solder to the far side of the joint, away from the iron.
Feeding the solder to the opposite side is the test. If the joint is hot enough, solder melts on contact with the metal and flows around the lead by itself. If it only melts where it touches the iron, the joint is cold and you must stop and let it heat.
Then remove the solder first, the iron second, and do not move the work while it cools. A joint disturbed during freezing comes out crystalline and grey and has a fraction of the strength.
Materialien für diesen Schritt:
Lötdraht1 StückBenötigte Werkzeuge:
Lötstation
Dritte Hand
Schutzbrille3
3
Read the joint
Read the joint
Look at the finished joint under a lens. A good one is a shiny concave FILLET, like a tiny volcano hugging the lead, with the outline of the lead visible through the solder.
A ball or a dome that sits proud with a sharp edge where it meets the pad is a COLD joint — the solder never wet the copper. Dull grey and rough means it moved while cooling, or the alloy was overheated and the flux long gone. A joint that covers the pad but not the lead, or the lead but not the pad, only wet one of the two metals.
The shape is the whole inspection. Concave and shiny is sound; convex or dull is not, and no amount of adding more solder fixes it — it has to be reheated so it flows, or removed and redone.
Benötigte Werkzeuge:
LupeLötstation4
4
Trim the lead, and why flush matters
Trim the lead, and why flush matters
Cut the protruding lead off close to the joint with flush cutters, holding the offcut so it does not fly. Cut with the flat face of the cutters toward the board so the stub is left level.
A protruding lead is a short waiting to happen once the board is in a case, and a sharp stub will cut you every time you handle it. Flush cutters exist for exactly this: ordinary side cutters leave a pointed spike.
Cut AFTER soldering, not before. A lead trimmed first has nothing for the solder to climb, and the joint ends up relying on the pad alone. Wear the glasses — offcuts leave at speed and they are aimed at eye height.
Benötigte Werkzeuge:
SeitenschneiderSchutzbrille5
5
Flux, and cleaning up after it
Flux, and cleaning up after it
Most electronics solder has a rosin flux core, which is usually enough. Add extra flux when a joint refuses to wet, when reworking an old board, or on anything tarnished.
Flux is what removes the thin oxide layer from the copper so the solder can actually alloy with it. It is not optional chemistry; it is the reason the solder sticks at all. That is also why an overheated joint fails — the flux is spent before the metals are ready.
Clean rosin residue off with isopropyl alcohol and a brush once the board is cool. It is mildly conductive and slightly hygroscopic, so on a high-impedance circuit it can leak enough current to matter, and it makes later fault-finding harder because probes skate on it.
Materialien für diesen Schritt:
Lötflussmittel-Paste (auf Kolophoniumbasis)1 Stück
Isopropylalkohol 99 %1 StückBenötigte Werkzeuge:
LötstationBaumwolltuch6
6
Ventilate, and wash your hands
Ventilate, and wash your hands
Work where the air moves and keep your head out of the plume. What rises off the joint is vaporised flux, not lead — lead does not boil at soldering temperature — and rosin fumes are a respiratory irritant and a recognised cause of occupational asthma.
The lead risk is by HAND, not by air: wash your hands before eating or touching your face, and do not eat at the bench. Lead-free solder removes that concern but runs hotter and wets less readily, which is a real trade for a beginner.
A small fan drawing the plume sideways away from you costs almost nothing and removes the whole issue. This is the one genuine hazard on this bench, and it is easy to manage once named.
Benötigte Werkzeuge:
SchutzbrilleBaumwolltuchMaterialien
3- 2 StückPlatzhalter
- Platzhalter
- 1 StückPlatzhalter
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