Yes blow-holes are a defect for class 2 & 3 but not for class 1.

• Blow-holes are caused by poor through hole plating or excessive moisture in the board. If the blow-hole is severe enough it will cause the solder joint to be weak and a cause for failure.
  
  When the board is over the wave the moisture inside the board turns to steam and builds up pressure till it erupts through the wall of the plated through hole.
  
  Poor plating in the holes makes the problem even worse. The only cure is to bake the boards to reduce the amount of moisture in the board.
  
  Bake the boards in a oven at 120C for 4 hours then process them through the wave right away. You may have to increase the bake time if you still see some blow-holes.
  
  
• Blow-holes can also be caused by a component that is capping the top of the plated through hole. This traps the flux and when the PTH is over the wave the flux will out gas and build pressure till it blows the solder out of the PTH.
  
  If this is the cause the component must have a standoff the keep it from capping the PTH.
  

Blow holes are typically caused by moisture or an organic material trapped in the bare PCB that volatilizes rapidly when the board contacts the wave. The escaping gaseous materials will blow the molten metal out forming a void in the solder joint. Baking the PCBs is an option to remove entrapped moisture.

Karthik Vijay Technical Manager, European Operations, Indium Corp. Karthik Vijay is the Technical Manager for Indium Corporation's European Operations. He is based in the UK and is responsible for technology programs and the technical support function for Indium Corporation's customers in Europe. His expertise is focused on solder paste, engineered solders, thermal interface materials, and semiconductor-grade electronics materials.

Sounds like you have a process problem. Are the boards dry going into the wave process?

I am trying to think of what is causing the holes. It could be moisture around the joint area. The moisture turns to steam in the molten solder and hence creates the blow hole.

Have you tried baking board prior to wave soldering, to see if this removes the problem?

Steven Adamson Market Specialist, Nordson ASYMTEK Market Specialist for Nordson ASYMTEK. Mr. Adamson worked for Kodak, Motorola and Plessey, ICL in the UK with 5 US and 2 UK patents. He was awarded a HNC in electrical engineering and was 2008 President of IMAPS. Mr. Adamson was a respected mentor in the electronics industry. He passed away October, 2011. Learn about the Steve Adamson Memorial Annual Scholarship Fund.