Murray State University School of Engineering CourseWiki
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CourseWiki/Engineering Fundamentals/Electrical Systems

Making and Inspecting Solder Joints

Heat both parts, feed solder into the joint, let it cool without movement, and inspect the bond rather than its shine alone.

Heat both parts, then feed solder into the joint

Heat both parts, then feed solder into the joint

Prepare the iron and bench using Soldering Equipment and Setup. The iron heats the parts; the heated parts melt the solder. Touch the tinned working face to both wires, or to both the pad and the component lead. Feed solder into the joint, away from the iron's contact point when access allows.

First the iron contacts both pad and lead. Then solder wire approaches the heated joint from the other side while the iron stays in contact. Finally remove solder, remove iron, and hold still.

A one-second count is a rhythm, not a temperature test

“Apply iron, one-second count, apply solder” reminds you to heat before feeding. The actual time depends on the joint, tip, alloy, and component limits. Look for solder flowing onto both surfaces. If it will not flow, stop and check cleanliness, flux, contact, and tool settings rather than holding the iron there indefinitely.

Work through the joint in this order; Checklist; Practice

Work through the joint in this order

  • 1. Strip and clean. Expose only the conductor needed for the connection without nicking it. Clean the surfaces to be joined.
  • 2. Support the parts. Form the connection and secure it so it cannot move during soldering.
  • 3. Flux if needed. Use flux compatible with the solder and electrical work.
  • 4. Heat BOTH parts. Put the tinned working face of the iron against both parts at the joint.
  • 5. Feed solder into the heated joint. Let it wet both surfaces. Do not melt a blob on the iron and wipe it onto cold parts.
  • 6. Remove the solder, then the iron. Stop feeding once the connection has enough solder and both surfaces are wetted.
  • 7. Hold still while it cools. Let the solder solidify without moving the connection.
  • 8. Inspect. Check wetting, coverage, clearance from adjacent connections, and damage before powering the circuit.

Practice the sequence

Look for wetting and shape; shine alone is not a verdict; Practice

Look for wetting and shape; shine alone is not a verdict

A good joint flows onto both surfaces. On a typical through-hole pad, look for a smooth transition from pad to lead, often a concave fillet (an inward-curving slope). Tin-lead joints are often shiny. A sound lead-free joint may be dull; dullness alone does not diagnose a cold joint.

Why lead-free solder may look different

Kester's soldering FAQ, under lead-free joint appearance, explains that tin-silver-copper joints can have a dull surface without being poor joints. Judge the connection, not shine alone.

Conceptual cross-sections: wetted solder slopes from pad to lead; a cold poorly wetted bead sits on the pad; a solder bridge connects two pads that should be separate.

Cold or poorly wetted
Solder balls up or fails to bond to one surface, often because the parts were not heated adequately. A rough or dull appearance can be a clue, but inspect the contact and shape too.
Disturbed
The connection moved while solidifying, leaving a cracked or disrupted joint. Secure it before reworking.
Too much solder
A large mound hides the connection and makes bridges easier to miss. Remove excess so the joint can be inspected.
Too little solder
The intended connection lacks coverage or a continuous bond. Reheat both surfaces and add only what is needed.
Bridge
Solder joins connections that should be electrically separate. Remove it before applying power.

Practice joint inspection

Correct the cause before reheating

Correct the cause before reheating

Keep the circuit disconnected. Secure loose parts, clean and re-tin the tip, and use compatible flux if needed. Reheat both surfaces to remake a poorly wetted or disturbed joint. Use wick or a pump to remove excess solder or a bridge; do not pry at a pad while the solder is solid.

Repeated heating can damage components or lift copper pads from the board. If a joint still will not wet, let it cool and get help finding the cause. Inspect again after repair. A continuity check can help find an open connection or an unintended short, but it does not prove mechanical strength.

See Soldering Wire Splices and Through-Hole PCBs for splice support and through-hole work.

Appears in
DET 403 DET Soldering