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December 2007

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Subject:
From:
Paul Reid <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Paul Reid <[log in to unmask]>
Date:
Thu, 13 Dec 2007 09:20:46 -0500
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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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