Good day!
     
     I may have missed the first email on this thread seeing how I just 
     subscribed today, but in reference to the following:
     
     
  >>>If you do not wave solder, I do not know of a reason to tent vias...
     
  > Our boards are done in some kind of convection oven rather than wave    
  > soldered, and we tent our vias because the distance from pad to via is 
  > sometimes too small to get a reliable mask dam: e.g. a via is 8 mils   
  > from the pad it connects to and the board house leaves a 3 mil gap     
  > around vias and pads, and the LPI SM we use won't stick if it's only   
  > 2mils thick. So to prevent solder from getting sucked across the trace 
  > and into the vias we tent them.
     
  > Andy B
  
  Andy does have a point, however, in my experience the amount of solder that 
  wicks and flows down the via is pretty minimal and normally doesn't reduce the 
  volume of solder to the point that causes a problem with the fillet at that 
  joint...unless of course the via's are HUGE!
  
  But the comment before that about not tenting them if you don't wave solder 
  may not be correct all the time. There's assemblies out there that are all 
  surface mount on both sides (double sided reflow) except for maybe a connector 
  or two, and they will be installed using a "Airvac" solder pot or selective 
  wave soldering only the connector...the rest of the PCB is sheilded by the 
  fixture, so the via's won't be filled. If the assembly is ICT'ed, you need to 
  be able to maintain vacuum on the ICT fixture, thus tenting is needed.
  
  There have been cases where I've had my stencil made with the via's imaged 
  along with the surface mount pads to fill them because the fab wouldn't be 
  revised to tent the via's until the 10,000 fabs that are already made, were 
  built.
  
  One last note about tenting, and this is only because I've seen it happen. 
  When specifying the tenting, make sure the bottomside is tented (the side 
  thats going to be hitting the wave), not the top! What happens is that you can 
  get outgassing from the flux and have what look like "little blobs" of solder 
  hanging off the via's, plus if there are any via's that are open beneath a QFP 
  it makes it tough to clean the flux residues that wick-up through the via 
  during wave. This may seem pretty obvious, but I still see it happen now and 
  again.
  
              
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