The last In-Circuit test strategy created by an outsourced Vendor was
like the" blind leading the blind", so maybe I dew.
Dewey 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Valerie St.Cyr
Sent: Wednesday, April 19, 2006 7:10 AM
To: [log in to unmask]
Subject: [TN] Question re Test Probe and blind microvia

All,

I have a question: have any of you first hand experience with allowing
blind microvias to be used as test points for In-circuit or Functional
Test?

The Engineer would like to probe a microvia to eliminate the stub we
would normally have from a through hole via used as a test point. I
don't know if that should be avoided. The probes we used are either
"spear" or "blade" tips; I am talking about probing the loaded board, so
this means probing after the board has seen 3 or maybe 4 reflows; the
microvia starting laser ablated diameter is typically .006"; if it is a
Layer 1 to Layer 2 it is .003" or .004" deep (we used prepreg in that
opening, no RCC); if it is Layer 1 to Layer 3, the depth could increase
by anywhere from another .0035" to .0055"ish.

Our plating specification is IPC6012A Class 3. Our intention would be to
drop that blind via in a very large pad, much larger than normal for a
blind via; then paste it so it is closed up when we go to probe. That
will keep the probe from directly piercing the sidewall plating or the
hole knee. Which leaves, I think, the question of how well the plated
copper is adhered to the copper foil capture pad - we will be hitting it
with a LOT of force - will that mechanical stress cause a de-bonding or
weaken that bond ? What if the board needs to be retested and retested
if there is a fault and we have to do repetitive debug?

So, anyone with experience probing to microvias, your  knowledge would
help us a lot.

Thank you,


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Valerie

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