LED Not Working After Reflow: Causes and SMT Process Fixes
Quick answer: if LEDs work before reflow and fail after, suspect three things — a reflow peak above the rated limit, moisture-popcorning from unbaked parts, or lens pressure during handling.
Cause 1: Profile Over the Limit
Most SMD LEDs allow a 245-260 °C peak for 10 s max, two reflow passes at most. Verify the real profile with a thermocouple on the LED pads — oven setpoints are not board temperatures.
Cause 2: Moisture (MSL) Popcorning
SMD packages absorb moisture. If an opened reel exceeded its floor life, steam pressure during reflow delaminates the package and breaks wire bonds. Fix: store dry, track floor time, and bake per the MSL label before use.
Cause 3: Mechanical Stress
- Pick-and-place nozzle pressing the epoxy lens.
- Board depaneling flex near the LED.
- Tweezers gripping the optical surface during rework.
Diagnostic Clues
- Failure rate highest near board edges → depaneling stress.
- First-run boards fine, failures after humid weekend → MSL issue.
- Fails only on double-sided reflow → second pass exceeded limits.
The rated profile, MSL level and baking conditions for every model are in our datasheet center.
Need datasheets, samples or a bulk quote? Browse our LED datasheet center or product center. Reflow-rated parts: SMD LED series. Hongcheng Optoelectronics, a member of Guangyu Group, has 20+ years of LED packaging experience with A-grade chips and 99.99% pure gold wire bonding.
