A Dead Section Lights Up When I Press the Joint: The Cold Solder Culprit
- 2026-11-02 15:55
- /
- Arraystar LED
A dead section that lights up the moment you press the joint is not a strip problem, it is a dry solder joint. I have stopped chasing strips that measure fine on the bench, and started spending ten seconds re-flowing the suspect joint with a little fresh rosin.
The symptom that almost always means cold solder
The classic report: the run works on the bench, dies on the ceiling, comes back when the user pushes on the solder joint at the feed end, then dies again ten minutes later when the strip expands and cools. Sometimes it is one colour channel on an RGB run, sometimes the whole segment. What it never is, is a controller fault, because the controller is happily sending the same data the whole time.
A cold, or dry, joint looks dull and grainy rather than shiny. On a flexible strip it is the joint where the power wire meets the PCB pad, or the joint where two strip pieces have been buttered together with solder across the copper pads. The solder did not actually wet the pad; it just sat on top of it and held mechanically, not metallurgically.
How a cold joint actually conducts
When solder does not wet, the connection is carried by a handful of tiny point contacts. At room temperature and with a finger pressing down, those points touch and the LED segment lights. As the strip heats up over an hour, the copper and the solder expand at slightly different rates, the tiny points separate by a few micrometres, and the segment drops out. The joint looks fine when you inspect it cold, which is why people blame the strip first.
If pressure changes the symptom, stop replacing parts and fix the joint.
| Behaviour | Pointing to | Quick check |
|---|---|---|
| Works only when pressed | Cold solder joint | Re-flow with rosin |
| Works cold, drops out warm | Dry joint expanding | Feel the joint after warm-up |
| Works at one voltage only | Undersized / loose joint | Wiggle while measuring volts |
| Drops out only on one colour | One leg dry on RGB | Probe each channel |
Re-flowing the joint without lifting the pad
The ten-second fix
- Clean the joint with isopropyl alcohol and let it dry.
- Apply a tiny dot of fresh no-clean rosin flux directly on the dull solder.
- Tin a clean conical tip at 300 C, touch the joint, and feed 1 mm of fresh solder in.
- Pull the iron away first, then hold the strip still for five seconds while it solidifies.
The rosin is the part people skip. A dry joint already has an oxide layer between the old solder and the pad; without fresh flux, the new solder just beads up on top of the same oxide and you repeat the fault. One dot of paste flux, two seconds of heat, and the joint should flow shiny and wet the pad cleanly.
Cracked pads and cracked vias
If re-flowing does not hold, look closer. A cold joint that has been flexed a hundred times often cracks the pad itself or the via that carries the current through the board. On a flexible strip, you will see a hairline crescent around the pad, and when you measure voltage at the pad it reads 12 V one second and 0 V the next as the flex moves the crack open.
That is the point where I stop reworking and cut back. I remove the bad 20 mm section, clean the next intact cut line, and reconnect with a press-fit latch so there is no second joint on a weakened area. Re-soldering a cracked pad is a temporary fix that fails again in a month.
How to stop making cold joints in the first place
Ninety percent of cold joints come from a dirty pad or a cold iron. The pad on a strip arrives with a thin oxide and sometimes a film of the release paper from the silicone backing. Wiping the pad with alcohol before you solder is not optional. The iron temperature matters too: a tip that is too cold takes longer to flow the joint, and that longer dwell time is exactly what makes the solder ball up instead of wet.
- Wipe every pad with isopropyl alcohol before soldering.
- Set the iron to 300 C, not 250 C; cold joints come from cold irons.
- Keep the strip flat on a table, not bent in your hand, while the solder solidifies.
- For joins you may need to take apart later, use a solder-free latch instead of solder.
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Frequently Asked Questions
How do I tell a cold solder joint from a loose wire?
Wiggle the wire, not the joint. If the segment cuts out when you move the wire at the insulation, it is a loose crimp or a broken strand. If it cuts out only when you press the solder blob itself, and the wire stays put, it is a cold joint. Measure voltage at the pad: a dry joint reads intermittent even with the wire held still.
What does a cold solder joint actually look like?
Dull, grainy, and sometimes slightly domed, instead of shiny and concave around the lead. A wet joint wraps the solder around the component leg and clings to the pad at an angle of less than 90 degrees. A cold joint sits on top of the pad like a ball of mashed potato, and you can often break it off with a fingernail.
Can I just add more solder on top of a dry joint?
No. Adding fresh solder on top of an oxide-coated dry joint just makes a bigger ball that still does not wet. You need fresh rosin flux on the old surface, then re-flow the whole joint so the old and new solder mix. If you cannot get it shiny after two attempts, remove the old solder with a braid and start clean.
Why does the segment work when cold but fail after warm-up?
Because copper and solder expand at different rates. A dry joint holds only at a few microscopic points. When the strip warms under load, usually after ten to twenty minutes at full white, the copper moves a few micrometres more than the solder, the points separate, and the segment drops. It is a thermal cycle problem, not an over-current one.
Should I use flux paste or just rosin-core solder?
For strip rework, paste flux. Rosin-core solder has flux mixed inside, but on an existing dry joint you need flux on the oxide surface before heat arrives. A tiny dot of no-clean paste flux on the dull joint, then re-flow, gives a proper wet. Leave the no-clean residue in place; do not scrub it off, because the scrub can lift the pad.
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