Blowholes I have seen have been caused for various reasons.  I'll try to explain some instances that may help you.

1) Moisture blow holes:  appears to have had a mini explosion the edges are very sharp.  Cured by prebaking PCB's.

2) Heavy ground layers on terminations:  small blowholes, good positive wetting in all joints.  Cured by re-adjusting wave angle.  We normally ran at a 6 degree angle.  With this angle the PCB was moving faster than it was soldering by changing the angle we started soldering faster than the PCB was moving.  This helped with the thermal loss due to the ground layers.  This helped compensate for the ramp up speed we weren't able to change at all.

3) Lead to hole ratios.  We had this issue with small diameter leads in big old through holes.  We were able to slow down the product enough to allow the barrels to fill. This was a hit and miss but slowing it down really minimized the situation.  We then averaged how much rework per board and gave the customer a choice, either pay for the design related defects we weren't able to remove form the process or change the lead to barrel ratios so the defect won't reoccur.

 

Have a day......

 

Kathy