Soldering Sheet Metal

How soldering works

Soldering joins metal parts using a filler metal (solder) that melts at a lower temperature than the parts themselves. The parts never melt. Heated metal and flux let the molten solder wet the surface, and capillary action draws it into a tight gap. When it cools, it forms a bond. This is different from welding, where the base metals are fused together.

On sheet metal, soldering is commonly used for seams on copper, brass, tin-plated and galvanized steel items such as gutters, flashing, boxes, lamps and decorative work. Solder joints are not as strong as welds, so they suit sealing and light structural jobs rather than heavy loads.

Which sheet metals solder well

• Copper and brass. Generally easy to solder.
• Tin-plated steel (tinplate). Solders readily.
• Galvanized steel and zinc. Can be soldered with a suitable acid flux, though galvanized coatings can release harmful fumes when heated, so ventilate well.
• Mild steel. Possible with proper cleaning and flux, but less forgiving.
• Stainless steel and aluminum. Harder, because of tenacious surface oxides. They need special fluxes and solders made for those metals, and many people choose other joining methods instead.

Tools and materials

• Solder. Traditional sheet metal work often used tin-lead alloys, but lead-free solder is safer and often required for anything touching food, drinking water or consumer products. Choose solder rated for your metal and application.
• Flux. Flux removes oxides and keeps them from forming as you heat. Sheet metal commonly uses acid-type (corrosive) flux, which works well but must be cleaned off afterward. Do not use acid flux on electronics, where rosin-based flux is used instead. Leftover flux is also a known cause of corrosion on circuit boards, as described in circuit board failure causes and diagnosis.
• Heat source. A heavy soldering iron (larger tips with high wattage so the metal does not draw heat away too fast) or a propane torch.
• Cleaning supplies. Steel wool, abrasive pads or sandpaper, and a degreaser.
• Clamps or helpers to hold parts tightly together.
• Safety gear: eye protection, gloves, ventilation and a heat-safe work surface.

Step-by-step

1. Clean the metal. Remove oil, paint and oxidation until bright metal shows at the joint. Avoid touching the cleaned area with bare hands.
2. Fit the joint. Parts should overlap or sit tightly together with a minimal gap, because solder flows best in thin gaps.
3. Apply flux. Brush flux onto the joint surfaces.
4. Heat the metal, not the solder. Bring the joint to temperature with the iron or torch.
5. Feed solder. Touch the solder to the heated joint (not directly to the flame or iron) and let the metal melt it. It should flow into the seam.
6. Hold still while it cools. Moving the joint before the solder sets can weaken it.
7. Clean up. Remove flux residue with water or the cleaner recommended for your flux. Residue left on can cause corrosion.

Pre-tinning, meaning coating the joint surfaces with a thin layer of solder before joining, makes a clean seam easier to sweat together.

Why solder will not stick

The most common causes:

• Dirty or oxidized metal. Clean to bright metal again.
• Insufficient or wrong flux. Use a flux suited to the metal.
• Not enough heat. Sheet metal conducts heat away quickly. Use a larger iron or a torch, and let the parts reach temperature.
• Overheating, which burns the flux and oxidizes the surface so solder beads up. Clean and start again.
• Wrong solder for the metal. Check compatibility.
• Gap too wide for capillary action.

Propane torch or iron?

You can use a propane torch for soldering sheet metal, and it is common for thicker pieces and larger seams. The risk is overheating, burning the flux, or discoloring and warping thin metal, and a torch brings fire hazards. A heavy iron gives more control on thin sheet and small seams but may lack power for large pieces. Choose based on the thickness and size of your work.

Soldering versus brazing and welding

• Soldering uses low-temperature filler and gives modest strength.
• Brazing uses higher-temperature filler for stronger joints, still without melting the base metal.
• Welding fuses the base metals and gives the strongest joints. For sheet metal, TIG and MIG welding are common choices, though thin material takes skill to avoid burn-through.

If the joint needs to carry real structural load, consider brazing or welding rather than soldering.

Safety

Flux fumes, galvanized coatings and lead-containing solder can all be hazardous. Work in a well-ventilated area or use fume extraction, wear eye protection, wash hands after handling solder (especially leaded), and keep it away from food areas. Use solder certified for potable water if the joint will contact drinking water. Check the product’s safety data sheet for details.