Susan
     
     Here's my opinions:
     
     1) REAL ESTATE. When holes only go halfway or part way through a 
     board, the hole and associated pads don't take up space on all layers. 
     This gives more routing room to designers to run traces, where a 
     normal through via would not allow.
     
     2) & 3) Blind and buried via's "typically" get filled with resin 
     during the lamination step(s). I can't imagine a case where you 
     prevent the holes from becoming filled.
     
     4) I wouldn't say they are required to be capped with plating and or 
     soldermask. As part of the natural process after lamination, boards 
     see an e'less deposition (direct platers, no offense) and electroplate 
     of coppers. This tends to "cap" the blind via's. I have seen cases of 
     "thermal via's" actually being depth drilled after lamination to 
     remove some of the resin. During thermal cycling, the resin will 
     soften and "move" and can cause the plating on the surface to break 
     away or lift.

        I would offer some pictures of the above conditions, but I'm not very 
skilled in ASCII art, as are some of our fellow `netters. Please feel free to 
call me to discuss or perhaps I could FAX you something!

Regards,

Tom Bresnan
Product Eng'g Mgr
MULTEK
16 Hammond 
Irvine, CA 92718
714.951.3388
714.951.8794 FAX
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Date:    2/18/96 12:09 PM



     
ding Blind Vias in multilayer
printed boards.
1)  Why would someone want to use them? 
2)  If used are they always filled?
3)  When does the filling operation take place? 
4)  Are they required to be capped by plating 
    or solder mask?
     
I am looking at a lot of multilayers with blind vias and 
need some help in understanding the manuf process.
     
Thanks,
Susan Mansilla
Robisan Laboratory