If solder isn’t reaching the components, check these areas.

The condition known as solder skips occurs where a component on the board has not been soldered during the soldering process.

Primary process setup areas to check include:

Conveyor speed; if too fast, the dwell could be too short in the wave.
The chip or turbulent wave, to ensure it is turned on.
Insufficient flux.
Wave height too low on one or both waves.

Other things to look for in the process:
Solder wave height low.    
Preheat too high.    
Board not seated properly.
Solder wave uneven.    
Flux applied unevenly.    
Insufficient flux blow-off.
Flux SP GR too high.    
Contaminated flux.    
Board pallet too hot.
Flux not making contact.
Conveyor fingers bent.
Conveyor speed high.

Other things to look for with the assembly:
Board contamination.
Component contamination.
Improper board handling.

Things to look for with the bare fabrication:
Board oxidation.    
Defective mask material.    
Board warpage.
Board contamination.    
Mask in hole.    
Component contamination.
Misregistration of mask.    
Hole and pad misregistered.

Things to look for with the board design:
Poor pallet design.    
Internal ground plane.    
Component orientation.
Pad size mismatched.    
Component shadowing.    
Weight distribution. 

Paul Lotosky is global director - customer technical support at Cookson Electronics (cooksonelectronics.com); plotosky@cooksonelectronics.com.