Tombstoning and Rework on a 0805 LED Board
A small assembly unit produces 200 boards a week carrying 0805 LEDs, 0603 resistors and one QFN regulator, using SAC305 solder paste and a stainless stencil. After reflow, about 6 per cent of boards show LEDs standing on one end, and many joints look dull and grainy. A technician repairs them with an iron set to 420 °C, touching each joint for 5-8 seconds without flux, and reports that the solder does not flow. The bench has an ESD mat and a wrist strap, but the strap lies coiled on the bench and is not worn. The reflow profile was copied from a supplier's tin-lead process; the oven has a short preheat and a single hot zone. Boards that pass are packed without cleaning, and some have small solder balls trapped under the LEDs. The unit wants to ship the current batch on time and is considering simply reflowing every board a second time.
1. Explain the likely causes of the tombstoning and the dull, grainy joints, and state what evidence you would collect before changing the process.
Read the worked answer
Tombstoning is a force imbalance during melting: one termination wets and pulls the part upright before the other end wets. Likely contributors here are unequal paste volume or pad heating, off-centre placement, a via or trace heating one pad faster, and a profile with too little soak and too fast a ramp. The solder balls under the LEDs point to paste slumping, poor stencil release or a profile that boils off flux too quickly.
Dull, grainy SAC305 joints are not automatically defective, because lead-free joints are often less shiny than tin-lead. Judge them by wetting angle and fillet shape, not shine alone. However, combined with the copied tin-lead profile, the short preheat and the technician's report that solder will not flow, insufficient heat and flux exhaustion are likely.
Before changing anything, collect the profile with a thermocouple on a representative board, check stencil apertures and paste volume, inspect placement offset, review pad and via design, and record the failure rate by board position. Change one variable at a time and re-inspect.
2. Evaluate the technician's rework method and describe a correct repair procedure for one tombstoned 0805 LED.
Read the worked answer
The method is unsafe for the board and the part. A 420 °C iron held for 5-8 seconds without flux causes excessive intermetallic growth, lifts pads, cracks the LED's epoxy and can destroy the part. The 'solder does not flow' observation usually means the joint is oxidised, the tip is dirty or undersized, or the board is being heated from one side only.
Correct procedure:
- Wear the ESD wrist strap, connected and tested; use a grounded iron and a fume extractor.
- Apply gel flux to both joints, set the iron to about 330-350 °C with a fine chisel tip, and preheat the board if possible.
- Remove the LED with tweezers or hot air, clean the pads with braid, and check for lifted pads.
- Apply fresh paste or solder, place the LED with the polarity mark correct, and reflow both joints evenly.
- Inspect under magnification, clean flux residue, and test the LED with a current-limited source.
Reject any board with a lifted pad or damaged LED rather than reworking it repeatedly.
3. The unit proposes running every board through reflow a second time to fix the defects. Give your decision and the process and inspection changes you would require.
Read the worked answer
A blanket second reflow is not acceptable. It doubles thermal exposure, grows intermetallics, increases warpage and may move good parts, while doing nothing for boards with paste-volume or placement faults. It also risks trapping more flux and solder balls. Only reflow boards that have a specific, correctable defect, and only after the process is fixed.
Required changes:
- Build a proper SAC305 profile with a soak zone, a ramp below about 3 °C/s, a peak near 235-245 °C and adequate time above liquidus, verified with a thermocouple.
- Check stencil apertures and paste release, and control paste volume.
- Correct placement offsets and review pad and via design for thermal imbalance.
- Enforce ESD discipline: the strap must be worn and tested daily.
- Clean boards where flux residue or solder balls are present, and inspect with magnification against defined acceptance criteria.
Contain the current batch, sort it, and record the defect rate before and after the changes.