Hand Soldering Electronics

Hand soldering connects components to circuit boards and wires to terminals. Each joint is small, but one bad joint can stop a whole product from working, or make it fail later. Assemblers and rework technicians make these joints all day, often under a magnifier, to a written standard.

What You Will Be Able to Do

• Set up and care for a soldering iron.
• Choose a solder and a flux for the job.
• Make a through-hole joint and a wire joint.
• Spot cold joints, bridges and other defects.
• Protect parts from heat and static electricity.

Setting Up Your Station

Common iron settings are about 650 to 750 F for tin-lead solder and 700 to 800 F for lead-free. Your work instruction may give a set temperature. Hotter is not better: too much heat lifts pads and damages parts.

ESD means electrostatic discharge, the static shock that can damage a component without leaving a mark. Wear a wrist strap, work on a grounded mat, and keep parts in protective packaging until you need them.

Solder and Flux

Electronics solder comes in two main groups.

SAC305 is 96.5 percent tin, 3 percent silver and 0.5 percent copper. Lead-free joints have a duller, grainier look, so do not judge them the same way as tin-lead joints. Do not mix solder types on one joint.

Flux for electronics must not corrode the board.

Flux-core wire usually has enough flux for a basic joint. Add liquid flux for rework or drag soldering. Never use plumbing or acid flux on a circuit board. It keeps eating metal after you are done.

Tinning and Caring for the Tip

Tinning means coating the tip with a thin layer of solder. A shiny, tinned tip passes heat into the joint quickly. A dry, oxidized tip turns dark and acts as an insulator, so heat cannot get through it, and solder will not stick. Then people tend to press harder and hold it longer, which damages the board.

1. Heat the iron.
2. Wipe the tip on brass wool or a damp sponge.
3. Melt a small amount of fresh solder on the tip.
4. Wipe off any excess. The tip should stay bright and silver.
5. Tin it again before you put the iron in the stand, and before you turn it off.

If a tip has oxidized, a tip tinner or a mild tip cleaner can restore it. Do not file it, because that removes the plating.

Making a Through-Hole Joint

A through-hole part has leads that pass through holes in the board.

1. Insert the part and bend the leads slightly to hold it in place.
2. Clean the tip and add a tiny bit of fresh solder. This small bead helps transfer heat.
3. Touch the tip to the pad and the lead at the same time.
4. After about a second, feed solder into the joint, where the pad meets the lead, and not onto the iron.
5. Let solder flow around the lead and fill the hole.
6. Remove the solder first, then the iron.
7. Hold the part still until the solder freezes.
8. Trim the lead, as your work instruction says.

The whole joint should take about 2 to 3 seconds. If it takes much longer, the tip may be too small, or the iron may need attention. On a large ground plane, a bigger tip works better than more time.

Making a Wire Joint

1. Strip the wire to the right length with a stripper that will not nick the strands.
2. Twist the strands and tin the wire with a little solder.
3. Tin the terminal or pad.
4. Hold the wire in place and heat it until both tinned surfaces melt together.
5. Remove the iron, and hold it still until the solder sets.

Keep solder from wicking up under the insulation, because it makes the wire stiff and it can break there.

Judging a Joint

A good tin-lead joint is smooth, bright and concave, and it wets both the pad and the lead. A lead-free joint looks duller but has the same shape. You should see the outline of the lead through the solder.

Cold joints can pass a quick test and then fail later, because their resistance is high and they crack over time.

Workmanship Standards

Commercial electronics are built to written standards. IPC J-STD-001 gives the requirements for soldered electrical and electronic assemblies, and IPC-A-610 gives the visual acceptance criteria. Each has classes for general, dedicated service and high-reliability products. Your employer may require training and certification to those standards. A guide cannot certify you. Your work instruction and customer requirements are the rule.

Removing Solder

• Solder wick is braided copper that soaks up molten solder. Press it to the joint with the iron, then lift them away together.
• A desoldering pump sucks up molten solder.
• A desoldering station does the same with a heated tip and a vacuum.

Add flux first, and do not rub the pad. Pads are thin and lift easily when hot.

Staying Safe

• Burns: The iron tip runs hot enough to give an instant burn. Always return it to the stand.
• Fume: Soldering smoke comes mostly from the flux. Long-term exposure to rosin flux fume can cause occupational asthma, so use fume extraction and keep your face out of the smoke.
• Lead: The main risk from tin-lead solder is from your hands, not the smoke. Wash your hands before you eat, drink or smoke, and keep food away from your bench.
• Eye protection: Solder can spit, and clipped leads can fly. Wear safety glasses.
• Fire: Turn the iron off at the end of a shift.

Common Mistakes

• Melting solder on the tip and carrying it to the joint. The flux burns off, and the joint comes out cold.
• Pressing harder when heat will not transfer. You risk lifting the pad. Clean and tin the tip instead.
• Using too much solder. It hides bad wetting and bridges pads.
• Letting the part move while it freezes. The joint will be disturbed.
• Skipping ESD protection. Damage may not show until the product is in use.

Check Your Understanding

Your iron won't melt solder quickly, and the tip has turned dark. What is wrong, and what do you do?

The tip has oxidized, so it is insulating the joint. Clean and tin it. Do not simply raise the heat.

A through-hole joint looks dull and lumpy. It is a tin-lead joint. What probably happened?

It is likely a cold joint from too little heat, or the part moved. Add flux and reheat until the solder flows.

You find solder connecting two neighboring pads. What is it, and how do you fix it?

That is a solder bridge. Add flux, and drag the solder away with a clean tip, or remove it with wick.

Why is rosin flux acceptable on a board when plumbing flux is not?

Rosin residue is non-corrosive and non-conductive. Plumbing flux residue corrodes metal and can conduct electricity.

Key Takeaways

• Heat the pad and the lead together, and feed solder to the joint, not the iron.
• Keep the tip clean and tinned, because a dry tip will not pass heat.
• Use rosin or no-clean flux on electronics, never acid flux.
• SAC305 is the common lead-free alloy, and it starts to melt near 422 F.
• Learn what a good joint looks like for the solder you are using.
• Protect parts from heat and static, and protect yourself from fume and lead.