In a previous lifetime, we were doing some sequentialy laminated
panels with blind vias.  The holes were drilled .015"
and the outer core was .005" h/h.  After plating, but before
black oxide, we found the best method to plug the vias, was
to single side screen print resist on the "inside" side of the 
panel with a dot pattern about double the dia of the hole. This 
packed ink into the via holes and left a retaining pad on the 
"inside" side of the panel.  We used clear photoimagable soldermask
(Enthone DSR-3241-clear).  The clear ink allowed the UV to go 
very deep.  Tak dry, and expose from the "inside" side of the 
finished panel only.  That way you don't get any cured on the
"outside" side of the panel.  Develop, cure-bake, Black oxide,
layup, press, trim-edge, strip the black oxide, touch-up.  
After a little "learning curve" at the black oxide microetch 
you should get zero leaking plugs, but if you do have any leakers,
they can easly be seen (clear resist over black oxide).  A rotery 
drafting eraser does wonders for touchups.  I don't remember which 
eraser compound worked best. We tried plasma, but the cycle 
time was longer, and more $$$ than the eraser trick.

	BTW, The solder resist takes Electroless very nicly, so
you end up with no dishdown, and the via hole plug plates up
and "disapears" after outerlayer processing.

Good luck..., and feel free to email me direct if you have
any questions.
                               Doug Alcoe
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>> We are currently producing large quanties of blind via panels on standard
>> FR4 materials. This is causing a bottleneck at our resin removing operation,
>> prior to drilling, and I have been asked look at a more efficient method.
>> 
>> Our current procedure is Plasma desmear after bonding followed by a light
>> brush prior to drilling.
>> 
>> We have evaluated PTFE and other proprietary blocking films during the
>> bonding operation, but have found them to be too expensive,or they create
>> other problems due to 'dish down' at the via enterance. Allowing the resin
>> to flow onto the surface has proved the most effective method at this time.
>> 
>> I would be intrested to hear how other manufacturers remove resin from blind
>> via panels, and if they have any comments or suggestions on the procedure
>> stated above.    
>> 
>> yours in anticipation
>> 
>> Billy Shaw
>> 
>> Process Engineering Manager
>> 
>> Thomas Walter, Marlow, England, UK
>>      
>> 
------------------done--------------------

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