When these defects occur, check the flux and conveyor speed.
Last month, we discussed insufficient solder topside fillets. This month, we look at insufficient solder bottom-side fillets (Figure 1). Per IPC-A-610, an acceptable condition is a 100% solder fillet and circumferential wetting present on the secondary (solder source) side of the solder joint. The minimum acceptable condition is a 330° circumferential fillet and wetting present for Class 3 boards, and 270° for Class 1 and 2 boards.
Primary process setup areas to check are:
- Conveyor speed too slow.
- Excessive time over preheat, causing the flux to be burned off.
- Excessive dwell time, causing flux to be destroyed before exiting the wave.
- Excessive bottom-side board temperature, causing flux to be burned off before the wave.
- Insufficient flux or flux activity.
- Wave height too low on one or both waves.
Other things to look for in the process:
- Insufficient solder temperature.
- Excessive preheat.
- Excess flux blow-off.
- Board not seated properly.
- Insufficient preheat.
- Insufficient flux blow-off.
- Solder wave height too low.
- Contaminated flux.
- Flux not making contact.
- Solder wave uneven.
- Flux SP GR too low.
- Conveyor speed high.
- Solder contaminated.
- Flux SP GR too high.
- Flux applied unevenly.
Other things to look for with the assembly:
- Board contamination.
- Improper board handling.
- Component contamination.
- Component leads too short.
Other things to look for with the bare board:
- Board oxidized.
- Mask in hole.
- Board warped.
- Board contaminated.
- Poor plating in the hole.
- Component contamination.
- Misregistration of the mask.
- Hole and pad misregistered.
Other things to look for with the board design:
- Poor pallet design.
- Internal ground plane.
- Pad size mismatched.
- Large ground plane on component side.
- Lead-to-hole ratio too large.
- Weight distribution.
- Large ground plane on solder side.
Paul Lotosky is global director - customer technical support at Cookson Electronics (cooksonelectronics.com) plotosky@cooksonelectronics.com.