Address,
     
     Be careful when identifying the cause of defect, since blowholes can 
     be attributed to either fabrication or assembly processes.  
     Identifying this type of defect is quite difficult.
     
     FAB:
     
     1. Plating pockets caused by a dull drill bit pulling laminate/prepreg 
        from the hole wall versus cutting.
     2. Closed or open pockets where the plating solutions are          
        encapsulated.
     3. Thin plating in hole wall
     
     ASSY:
     
     1. A prebake (1 to 2 hrs @ 110°C to 120°C) is sometimes recommended 
        prior to assembling (caution is recommend at the elevated 
        temperatures, due to material Tg).  Doing so will eliminate any 
        moisture the laminate has absorbed.  Remember the laminate if 
        hygroscopic and if left exposed, no desiccant or proper packing in 
        a controlled environment, will absorb any moisture.
     
        The moisture typically escapes through the PTH and there are enough 
        cavities present, will work its way to the surface.
     
        However, the baking will not eliminate the existing cavities 
        created at the fab level, which can not be removed until the solder 
        is brought to its melting point.
     
     2. Monitor your wave process carefully.  I typically found that the 
        preheat was too high, the conveyor too slow, insufficient flux or 
        any combination of these variables would present blowholes.  If    
        there is not enough flux present at the wave, then the solder will 
        solidify prematurely, hence, blowholes or cavities.  Play with the 
        above variables and sizzle the flux in wave and you may be able to 
        minimize if not eliminate its occurrences.