I could not agree more on the cost issue commented by Jeff Seeger.  We are 
     frequently asked by our customers "XXX is building this for us, why can't 
     you?"  It really isn't an issue of capability.  It's a matter of whether 
     the customer is willing to pay that kind of money and whether we can make 
     money out of it.
     
     The way we settle for now is to build blind vias only from layer 1 to 2 and 
     from layer n to n-1.  We will use laser to drill through aramid laminate 
     and create the blind vias.  The rest of the board is still the normal 
     epoxy/glass laminate.  Aramid laminate is used to speed up laser drilling.  
     Our cost analysis shows that (1) blind via produced this way is cheaper 
     than the conventional approach (drill innerlayer, plate and then laminate; 
     or control-depth mechanical drilling), (2) IF DESIGNED PROPERLY, there will 
     be cost saving comparing to the higher layer count board.  I said this with 
     caution because there are some constraints in aramid laminate.  For one, it 
     is only available in 18"x24".  If the board does not fully utilize this 
     panel size the cost saving may not be justifiable.
     
     Also, when laser is used to drill vias, the smaller the diameter the faster 
     the speed.  Don't ask for a .012" via and think you are doing your supplier 
     a favor.  It may take longer time to drill that than a .006" via, depending 
     upon the type of laser you use.  0.006" is generally considered to be a 
     good size at this time.
     
     You may ask how much layer reduction can be made with only blind vias.  We 
     have successfully reduce a 12 layer to 8.  For higher layer count boards, 
     after you removed 2 signal layers, generally you can take out 2 planes as 
     well.  The design rules definitely play a big part in how much space you 
     can save.  For instance, via in pad saves you more space then the 
     conventional design.  The blind vias only go down 1 layer such that it does 
     not deplete solder paste during assembly like their through hole 
     counterpart.  Therefore, via in pad is a recommended practice when you go 
     blind.
     
     I agree with Jeff's comment that small blind vias is more of an enabling 
     technology than a cost saving solution.  But if the designers can work with 
     their board supplier on the design limitations, blind vias can be cost 
     effective.
     
     Mason Hu
     Zycon Corporation
     408-243-1976x5547
     [log in to unmask]

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