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