SMD LED Troubleshooting: A Practical Engineer Checklist
Quick answer: troubleshoot SMD LEDs in this order — polarity, joints, in-circuit voltage, off-board diode test, then process history (reflow, ESD, moisture). Most faults are found within the first three checks.
1. Polarity and Placement
Confirm the cathode mark against the PCB silkscreen and the datasheet pad drawing. Rotated placements are the top assembly defect for 0603/0805 chips.
2. Solder Joints
Look for tombstoning, head-in-pillow, insufficient wetting. Measure continuity from pad to LED terminal under magnification.
3. In-Circuit Voltages
With power on: Vf present but dark → dead die. Zero volts → upstream fault (resistor, driver, trace). Full supply across LED → open joint.
4. Off-Board Diode Test
Desolder and test in diode mode. OL both ways = open die; near-zero both ways = shorted die. ESD and over-current are the usual root causes.
5. Process History
- Reflow peak and time within the datasheet profile?
- Floor life tracked, baking done per MSL?
- ESD controls (wrist straps, ionizers) in place?
6. When Many LEDs Look Different
Uniformity problems (brightness or color) are binning or circuit issues, not failures — see our guides on brightness mismatch and white color mismatch.
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