For small blind via, an aspect ratio of 0.5-1 (depth/diameter) is good.  We 
     typically recommend .005"-.006" (drill, not finished) hole diameter for 
     laser drilling.  I believe this is also a good number for other 
     technologies, such as plasma and photoimageable dielectric.
     
     For large, deep blind vias (typically go down several layers), sequential 
     lamination might be necessary.  This means you build a multilayer 
     subassembly, drilled, plated and then laminated with another subassembly.  
     In this case, the aspect ratio limitation of the "finished blind via" 
     should be the same as the through vias.
     
     I think I may have repeated Dougal's comments.  If so, please take it as a 
     confirmation.
     
     Mason Hu
     Zycon Corporation


______________________________ Reply Separator _________________________________
Subject: FAB: BLIND VIAS                                             
Author:  [log in to unmask] at corp
Date:    11/7/96 4:43 AM


Jim Marsico wrote:
Can anyone tell me the standard aspect ratio for blind vias?
  What is the minimum and maximum hole diameters?
     
I think that I can only answer from experience rather than any 
design guidelines.
We manufacture sequentially bonded blind vias with hole diameters 
of 0.3mm in 007" laminate, 0.35mm in 028" laminate, and 0.6mm in 
070" laminate, as well as lots of others in between.
The aspect ratio on sequentially bonded (resin filled) blind vias
is not as important as the length of the hole. We are aware of issues 
with long holes due to the resin expansion during SMD assembly, and
it can be predicted that the maximum hole depth should be 0.6mm (024") 
to avoid this issue.
I am currently evaluating the reliability of different aspect 
ratio blind vias, in different thicknesses and types of material 
with different plating thicknesses, and hopefully will be able to 
publish a paper in the new year.
     
As far as depth drilled blind vias (access blind via holes), the 
hole size is determined by the technique used (mechanical/laser etc) 
but the rule of thumb is to keep the aspect ratio to 1 or less.
     
Dougal Stewart
Exacta Circuits
Scotland
     
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