Lifted Pad and Solder Bridge on a Single-Sided LED Driver Board
A trainee is repairing a 12 V LED driver board from a shop sign. The 1000 µF/25 V electrolytic capacitor is bulged and must be replaced. The board is single-sided phenolic with narrow copper pads. The trainee's 25 W iron has a chisel tip at about 380 °C; the tip is slightly loose and the temperature dial is not calibrated. Solder is 60/40 with rosin core. While desoldering the capacitor, one pad lifts from the board and a solder bridge forms between the capacitor's negative pad and a nearby trace. The driver has a 230 V mains input and a transformer, but the capacitor is on the 12 V secondary side. The trainee has desoldering braid, a manual solder sucker, isopropyl alcohol, a replacement 1000 µF/35 V capacitor and thin tinned copper wire. The supervisor is away, and the shop needs the sign working that evening.
1. Explain the most likely reasons the pad lifted, and describe a safer desoldering sequence for this capacitor. Include the iron temperature and the role of the loose tip.
Read the worked answer
Likely causes 380 °C is high for a single-sided phenolic board. The loose tip increases thermal resistance, so the trainee holds the iron longer. Phenolic resin softens and the copper adhesive weakens. Lifting also occurs if the lead is pulled while solder is only partly molten, or if the pad rather than the joint is heated.
Safer sequence
- Isolate and discharge the 12 V rail; remove the board.
- Tighten or replace the tip and set about 330–350 °C. Tin the tip.
- Add fresh 60/40 solder and flux to the old joint to improve heat transfer.
- Heat the pad and lead together briefly, then use the sucker; finish with braid. Do not lever the capacitor.
- Alternate the two leads and allow cooling between attempts. If a lead remains stuck, cut the capacitor lead and remove it in two steps.
- Clean with isopropyl alcohol and inspect for barrel or pad damage.
The key is short dwell and heat transfer to the joint, not brute force.
2. The pad has lifted and there is a bridge to an adjacent trace. Decide whether to repair the board or replace it, and describe a repair that would be acceptable on the 12 V secondary side. State the tests you would perform.
Read the worked answer
First remove the bridge with braid and use a continuity test to confirm whether the negative pad net and the adjacent trace are actually separate. If they are the same net, the bridge may be electrically harmless but still needs cleaning.
For a 12 V secondary-side pad, a repair can be acceptable. Scrape solder mask from the connected trace, tin it, solder a short length of thin tinned copper wire from the capacitor lead to the trace, and secure it with a small amount of epoxy so it cannot flex. Keep the wire short and clear of neighbouring traces. Do not use this method on the mains side or where the trace carries high current.
Tests: continuity from the capacitor pad to the next intended node; no continuity to adjacent traces; correct polarity; no solder splash; then current-limited power-up and check the 12 V rail and ripple. If the pad is on the mains side or the copper is badly burned, replace the board rather than risk a repair.
3. The replacement is 1000 µF/35 V instead of 25 V. Is it suitable? Discuss voltage rating, ESR/ripple, size and polarity.
Read the worked answer
It is electrically suitable if:
- capacitance is still 1000 µF;
- the 35 V rating is greater than 25 V, which is fine on a 12 V rail and gives more margin;
- ripple-current rating and ESR are at least as good as the original;
- diameter, height and lead spacing fit.
A higher-voltage capacitor is often larger and may have slightly higher ESR, so check the driver's ripple and temperature. Observe polarity: the stripe or negative lead goes to the negative pad. Do not fit it if it is physically strained or if its ripple rating is lower. After fitting, check output ripple and case temperature.