Copper would be a better filler, but how to completely fill a hole with
copper without massive plating weight on the board surface and large
increase in board cost? It would still be relatively inflexible, though, in
place of one or two mils of barrel plating.
Peter
"joyce" <[log in to unmask]> 02/10/2002 08:02 PM
Please respond to joyce.koo
To: "TechNet E-Mail Forum." <[log in to unmask]>, DUNCAN Peter/Asst Prin Engr/ST
Aero/ST Group@ST Domain
cc:
Subject: RE: [TN] PCB design with mask over vias.
copper would be better filler ...;-)
jk
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]]On Behalf Of
>[log in to unmask]
>Sent: Monday, September 30, 2002 7:39 PM
>To: [log in to unmask]
>Subject: Re: [TN] PCB design with mask over vias.
>
>
>There is a theory in my Company that solder-filling holes, especially in
>the likes of FR4 with a relatively high LCTE in the Z axis, negates the
>ductility of the Cu plating inside the holes. Without the ductiity of the
>plating, the risk of shearing the barrel at the interconnections with
inner
>layers is greatly increased with a corresponding decrease in board
>reliability. I'm talking about high-rel boards here, and I subscribe to
>this theory. Any thoughts, anyone?
>
>Peter
>
>
>
>Dave Chapman <[log in to unmask]> 30/09/2002 06:02 PM
>Sent by: TechNet <[log in to unmask]>
>
>Please respond to "TechNet E-Mail Forum."; Please respond to Dave Chapman
>
> To: [log in to unmask]
> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
> Subject: Re: [TN] PCB design with mask over vias.
>
>
>
>
>
>
>
>
>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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