concur.  that being said, the frequency and signal integrity requirements 
of the app may not tolerate the additional inductance of a staggered uVia.

At 06:20 AM 12/13/2007, Paul Reid wrote:
>Hi Mark,
>
>Given that you have a well fabricated microvias...
>
>We have found that staggered microvia out perform stacked microvias all 
>things being equal (which they never are). Also buried vias filled with 
>the flow from the "b stage" material out perform other fills generally.
>
>As for reliability, the best thing to do is test them. We feel robust 
>microvias should not fail tested to 190°C in 500 cycles.  Most microvia 
>will survive 1000 cycles and after that testing becomes a bit academic.
>
>CTE is only part of the story. My sense is that the gradual loss of 
>elasticity as a material changes from elastic to plastic due thermal 
>cycling is as important. An elastic material relieves stress between 
>thermal cycles while a most dielectric materials loses elasticity and 
>becomes more plastic causing an accumulation stress between thermal 
>cycles. With this in mind, is this a lead free application?
>
>Paul
>
>PWB Inc.
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Mark F. Focht
>Sent: Wednesday, December 12, 2007 7:37 AM
>To: [log in to unmask]
>Subject: [TN] Stacked Via Help
>
>I have a design that,due to space constraints, requires stacking microvias 
>(L1-
>L2) directly on top of (in line with) drilled buried vias (L2-L7, 
>L2-L11).  I am receiving conflicting information about this strategy being 
>reliable (storage temp. range -50C to 110C, operating temp. range -40C to 
>85C) because of the different CTE's of the via fill material used in the 
>L2-L7/L2-L11 vias and the board material (FR406, 170Tg). While the board 
>shops that will fab this admit they prefer the vias be staggered (not very 
>pratical in this design), they claim they have produced boards of this 
>type without negative feedback or issues.  The CM that will be assembling 
>this is extremely hesitant about using this via strategy.  Does anyone 
>have experience using a stack-up similar to this?  Would this be 
>reliable?  Is there a plugging paste material that very closely matches 
>the CTE of FR406, 170Tg?  Any and all comments are welcome.
>
>Thanks,
>
>Mark Focht C.I.D.+
>Pole/Zero Corporation
>5530 Union Centre Dr.
>West Chester, Oh  45069-4821
>
>Voice:  513.870.9060 X173
>FAX:    513.870.9064
>Email:  [log in to unmask]
>
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