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.

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