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IntermediateWritten15 min

Soldering a bead that stays put

A good solder bead is a rounded, even line that looks like it was poured. Almost everything separating that from a lumpy one comes down to three variables: temperature, flux and how fast you move. They are easy to isolate once you know they are the only three.

You will need

  • Temperature-controlled iron, 60/40 solder, flux and a brush
  • A damp sponge or brass wool for the tip
  • Ventilation: a fan moving air across the bench, away from you

Get the three variables right

  • Temperature: around 700–750°F (370–400°C) for 60/40. Too cold and the solder sits in lumps; too hot and it runs flat and starts scorching flux and crazing foil adhesive.
  • Flux: enough to wet the seam, no more. Flux is what lets solder stick to the foil at all. A flooded seam spits and leaves residue that corrodes the panel later.
  • Speed: the bead height is set by how fast you travel. Slower means taller and wider. Consistency matters more than which speed you pick.

The order of operations

  1. 1Tack firstA small blob at each junction, just enough to stop the panel moving. Do the whole panel before running any bead: a panel that shifts mid-bead will never fit its frame.
  2. 2Flat-solder every seamFlux a seam and run solder along it to fill and cover the foil, without trying to build height. This layer is the structure. Do the whole side.
  3. 3Then run the beadFlux again, and travel steadily along the seam feeding solder into the puddle just ahead of the tip. Let the surface tension make the curve. Do not push the solder about with the tip.
  4. 4Turn it over and repeatBoth faces get the same treatment. Let the panel cool before flipping it: solder stays soft for longer than it looks and a warm bead will flatten against the bench.
  5. 5Clean the flux off nowNot tomorrow. Flux residue keeps working and will bloom white across your solder within days. Warm water, washing-up liquid, soft brush, dry thoroughly.
foil, burnished downflat fill: the structurebead: surface only
Flat fill first, then the bead on top of it. The bead is decoration and weatherproofing; the fill underneath is what holds the panel together.

Reading a bad line

  • Lumpy, with distinct blobs

    Usually
    Iron too cool, or moving too fast for the heat available.
    Fix
    Turn up 25°F (15°C) and slow down. Re-flux and go back over the seam in one pass rather than trying to melt individual lumps.

  • Flat and wide, will not build

    Usually
    Iron too hot. The solder is too fluid to hold a curve.
    Fix
    Turn down and let it settle for a couple of minutes. Irons take longer to come down than to go up.

  • Solder beads up and will not stick

    Usually
    No flux, or flux burned off before the solder arrived.
    Fix
    Re-flux immediately ahead of the iron, not the whole seam at once.

  • Copper showing through at the edges

    Usually
    Not enough solder, or the foil was not burnished down.
    Fix
    Add solder over the gap. If the foil is lifting, it needs burnishing before soldering, not more heat.

  • Pitted or grainy surface

    Usually
    Dirty tip, contaminated solder, or the panel moved while the bead set.
    Fix
    Clean and re-tin the tip, then re-flux and reflow the seam.

  • Glass cracked while soldering

    Usually
    Localised heat. Usually dwelling too long in one place, often at a junction.
    Fix
    Nothing to do but replace the piece. Prevent it by keeping moving and never resting a hot tip against glass.

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