Gary, 

I have struggled with this issue often.  Will the assembly ever see a 
wave solder machine?  Then tent vias which are located under components, 
where solder ball could be formed.  With the wave, you can cap the 
solder side and leave the top side open, assuming you do not top-side 
flux.

Tenting both sides backs you into Dryfilm soldermask, or a 100% plug (as 
recently 'advertised' by John Gulley).

If you do not wave solder, I do not know of a reason to tent vias... No, 
there was one..  Consideration for a vacuum test machine, and sticking 
test probes.  (Do the test probes really get stuck in a via holes, held 
there by the vacuum?  Or have I been sold a bill of goods?)


--- Forwarded mail from [log in to unmask]

Resent-Date: Wed, 4 Dec 1996 13:34:05 -0800
From: [log in to unmask]
Date: Wed, 4 Dec 96 13:14:10 EST
To: <[log in to unmask]>
Subject: Tented vias???
Resent-From: [log in to unmask]

Colleagues:

Our in house SMT assembly facility has asked that we tent our vias on 
top and 
bottom sides to aid in solder bridge reduction.  If bottom tenting is 
not 
possible becaues vias may double as test points, they ask that we still 
tent 
the top (component) side.  They use a no clean process.  

I am concerned that contaminants will be trapped in vias tented only on 
one 
side and cause premature failure down the road.  Normal via size is 
.012.  
What are your thoughts on tented vias???  One side?  Both sides?  Not 
tented 
ever?  Why???  What other problems may we encounter?

Thanks in advance for your opinions.


gary ...


-- 


George Franck Jr
Raytheon E-Systems
Product Assurance Engineer
Falls Church Va

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