Diagnosing and repairing a cold joint
A cold joint is not a joint with too little solder. It is a joint where the alloy froze while the surfaces underneath were still below wetting temperature, so the bond is mechanical contact rather than an intermetallic layer. Repair means removing the joint entirely, not adding solder on top of it.
Procedure
- 1
Photograph the suspect joint at 10x next to a known-good neighbour so the difference in surface texture is unambiguous.
- 2
Confirm under load or with thermal stimulus before deciding to rework; do not rework on appearance alone in production.
- 3
Wick all existing solder out of the joint and clean the pad and lead with alcohol.
- 4
Re-flux with ROL0 and rebuild the joint with fresh SAC305 at 340 degrees C for two seconds.
- 5
Let the joint cool undisturbed for three seconds, then re-measure the voltage drop under the same load.
- 6
Record the joint location in the rework log - repeat failures at the same reference designator usually indicate a layout or thermal relief problem.
Workshop notes
The four visible signatures
Look for a matte or grainy surface where neighbouring joints are shiny; a convex, dome-shaped fillet instead of a concave one; a dark ring where the solder meets the lead, indicating the alloy pulled away on cooling; and a fillet that wets the lead but stands proud of the pad. Any one of these justifies a rework pass. Two or more and the joint is electrically suspect, not just cosmetically imperfect.
Confirming electrically before touching anything
A cold joint often reads fine on a DC continuity test and fails under load. Measure the voltage drop across the joint with a 1 A load applied: a good through-hole joint drops under 3 mV, a cold joint typically 20 to 200 mV and unstable. Thermal testing is faster in practice - press a cold spray or a chilled probe on the joint while monitoring the circuit, and an intermittent joint will often reveal itself immediately.
Repair means removal first
Adding solder to a cold joint produces a larger cold joint. Wicked the alloy out completely, clean the pad and lead with alcohol, re-flux, and rebuild the joint from zero with a two-second dwell. Bumping the temperature up by 20 degrees C for the repair pass is standard practice because the pad has already been thermally cycled once and its surface is rougher.
Practice drill
Deliberately make ten cold joints by soldering with the iron set to 250 degrees C, then diagnose and repair each one using the four signatures as your only guide. Check your hit rate; experienced operators classify correctly on 9 out of 10.
Failure modes
| Symptom | Cause | Treatment |
|---|---|---|
| Joint reads good on a meter but fails at temperature | Mechanical contact with no intermetallic layer; thermal expansion opens the contact | Remove the joint completely and rebuild it; never attempt to top up a cold joint |
| Repaired joint fails again within hours | Underlying cause was thermal mass or a contaminated pad, and the repair repeated the original mistake | Measure the voltage drop before and after, log the reference designator, and if the same joint fails twice, change the tip geometry rather than the temperature |
| Dark ring around the lead after repair | Board or lead moved during the cooling window and the alloy tore away from the lead | Support the lead with tweezers until the joint is fully solid, and do not test the circuit for at least five seconds |
Supplies
- 0.8 mm SAC305 solder wire
- ROL0 no-clean flux pen
- 2.5 mm desoldering braid
- Isopropyl alcohol and swabs
Tools
- Station at 340 degrees C
- 2.4 mm chisel tip
- 10x loupe or USB microscope
- Bench supply and millivoltmeter for the load test
Safety
The load test that reveals a cold joint puts real current through a suspect connection, which can arc or heat unpredictably. Use a current-limited supply, start at 100 mA and ramp up, and never probe a live mains-derived rail with both hands.
Neighbouring cards
- Through-hole fillet baseline 340 °C / 2.0 s
- Hook splice on stranded wire 330 °C / 2.5 s
- Wick cleaning a through-hole pad without lifting it 350 °C / 1.5 s
- Drag soldering a 0.1 in header 360 °C / 1.2 s