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April 2006

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Subject:
From:
John Burke <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, John Burke <[log in to unmask]>
Date:
Wed, 19 Apr 2006 09:58:17 -0700
Content-Type:
text/plain
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I would think that the people with most experience with that (which I
wouldn't do either) would be the mobile telephone guys.

Any of them on the forum?

John

John Burke
Senior Manager - Operations , Optichron
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W: 510 249 5233
M: 408 515 4992
http://www.optichron.com
Profile: https://www.linkedin.com/e/fps/2665502/
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Valerie St.Cyr
Sent: Wednesday, April 19, 2006 8:33 AM
To: [log in to unmask]
Subject: Re: [TN] Question re Test Probe and blind microvia

I don't want to allow it, the Test development engineer doesn't want to
allow it.
But we have no data at all one way or the other, and it would take some
time
to make up a DOE, get samples fabricated, make a test fixture for this
experiment etc etc...  They want an answer and preferably one with some
amount of experience informing the decision..... so, just trying to see
if
anyone
has tried it.

Valerie



Rex Waygood <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
04/19/2006 10:28 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Rex Waygood <[log in to unmask]>


To
[log in to unmask]
cc

Subject
Re: [TN] Question re Test Probe and blind microvia




I will be interested to read the replies of others.
We've never probed microvias but as vias in general are probably the
least reliable part of a pcb and pcbs often the least reliable part of
an assembly.............do you want to be "hitting it with a LOT of
force"????
Our approach is to avoid probe testing on vias, never yet contemplated a
microvia.
Rex

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Valerie St.Cyr
Sent: 19 April 2006 15:10
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,

Valerie

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