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September 2002

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Subject:
From:
Dave Chapman <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 30 Sep 2002 05:02:22 -0500
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One thing we just completed was a pilot where we put apertures in our
topside SMT stencil and pasted over all vias to prevent Conformal Coat
leaking through the board. We opened the apertures +10% and got good solder
fill this also prevented corrosion or having trapped mask inside filled vias
possible "volcano" during reflow.
Dave Chapman
Manufacturing Engineer
Circuit Service Inc.


-----Original Message-----
From: Willie Duersch [mailto:[log in to unmask]]
Sent: Friday, September 27, 2002 2:01 PM
To: [log in to unmask]
Subject: [TN] PCB design with mask over vias.


Hello Peers:
We are a CM who recently fabricated a PCBA which had the vias covered by
solder mask film design on the PCB. This is not the first time we have seen
this done in PCBA. However, in this example, the PCBA ended up inside a
refrigerator. This customer adds few notes with the PCB and nothing was
specified associated with the fab on plugging or capping of the vias. The
PCBA did not require wave solder. Aqueous clean paste was used and PCBA
washed after SMT.

The PCBA was supplied to the customer who then selectively applied
Conformal coating over the top side components on the board with a brush,
but did not coat other through holes or vias on the component side or any
area of the back side.

Over a period of time, The vias on the back side showed evidence of
corrosion and subsequent cross section analysis showed copper corrosion
from the pad part way into the barrel even under soldermask. The cross
section also showed that the LPI solder mask did not plug and did have pin
holes in the mask with bare copper in the air pockets that occur during LPI
screen deposition.
1. We feel the customer is responsible to specify the PCB and PCBA
performance criteria. Comments?
2. Is there an IPC document that covers solder mask covered vias and
guidelines associated with the practice? If not, why not?
3. Is it the PCB shop or the CM who should question, and if not
specified...should either question the practice?
4. What are the issues related to designing with solder mask film design
covered vias? In this case, the vias were .010-.014 diameter and space
constraints were not an issue. There were other holes on the PCB that were
larger diameter and were left open to coat with solder from the HASL
operation...and not coated with conformal coat.
5. What would you tell a customer to correct the issue?

There are some obvious issues and answers and I have had 12 years of
experience with PCB's. However, I really would like to have some comments
from my peers.
Best regards, Willie

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