When the solder doesn’t stick, first check the pad.

Dewetting is a condition that results when molten solder coats a surface and then recedes, leaving irregularly shaped mound(s) of solder separated by areas that are recovered with a thin film of solder and with the basis metal unexposed.

Non-wetting is a condition in which there is partial adherence of molten solder to a surface it has contacted, and the basis metal remains exposed.

While we usually list in this space the primary process setup areas to check, dewetting and non-wetting typically are board-related due to pad surface contamination.

Other things to look for in the process include:

• Solder temperature too low.     
• Preheat too high or low.    
• Excess or insufficient flux blow-off.
• Solder wave height low.     
• Flux not making contact.    
• Flux contamination.    
• Board pallet too hot.
• Flux applied unevenly.    
• Flux SP GR too low.    
• Conveyor speed too fast or slow.
• Board not seated properly.    
• Flux SP GR too high.    
• Solder contamination.    
• Other things to look for with the assembly include:
• Board or component contamination.    
• Improper board handling.    
• Other things to look for with the board design include:
• Oxidation.    
• Contamination.

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