Hi Pat,
I haven't actually looked at the part but the insulation needs to be flat,
and thin, in order to provide adequate conditions in which to form the
perimeter solder connections. Doesn't glyptal form a pronounced meniscus?
If so, I think the bump could be a problem. Otherwise, it might be fine.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Goodyear, Patrick
Sent: Wednesday, December 21, 2011 10:16 AM
To: [log in to unmask]
Subject: Re: [TN] QFP center slug over signal vias
Ok I got into this discussion late as I have been off for a couple of days.
Is Glyptal red a good enough insulator? It works for transformers and
large motor windings. Instead of trying the tape you could use a small
applicator or stencil to dab a bit of Glyptal red onto the suspect problem
area, should provide a high level of insulation. I use it in ham radio
work when I need high voltage isolation.
Pat
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
Sent: Tuesday, December 20, 2011 11:52 AM
To: [log in to unmask]
Subject: Re: [TN] QFP center slug over signal vias
I agree completely. Sharing the pain always is a good learning experience.
-----Original Message-----
From: Joyce Koo [mailto:[log in to unmask]]
Sent: Tuesday, December 20, 2011 11:10 AM
To: Bavaro, Phillip @ MWG - TW; TechNet E-Mail Forum
Subject: RE: QFP center slug over signal vias
If you ask the designer to (1) tape the kapton tape on a piece of clean
glass (cleaned using IPA and DI, followed by IPA rinse and dry)(2) cut into
small pieces of the size using knife - under microscope if it is necessary,
(3) put the piece on to the PWB at desired location using tweezers with
proper ESD protection/with ionized gas blow on his hands.
I am sure you will never, ever see the same mistake again from the whole
design group (the words get around very very quickly ;-). My 1.92 cents.
Joyce Koo
Materials Researcher - Materials Interconnect Lab Research In Motion Limited
Office: (519) 888-7465 79945
Mobile: (226) 220-4760
-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Tuesday, December 20, 2011 1:39 PM
To: Joyce Koo; TechNet E-Mail Forum
Subject: RE: QFP center slug over signal vias
Yeah, that was my first solution but unfortunately it is too small to
put Kapton on reliably (.006" x .015"). I tried to get it die cut but
could not get a quote.
-----Original Message-----
From: Joyce Koo [mailto:[log in to unmask]]
Sent: Tuesday, December 20, 2011 10:32 AM
To: 'TechNet E-Mail Forum'; Bavaro, Phillip @ MWG - TW
Subject: RE: QFP center slug over signal vias
Based on my book, solder mask is not consider as valid dielectric for
signal. (Kapton tape would be).
Joyce Koo
Materials Researcher - Materials Interconnect Lab Research In Motion Limited
Office: (519) 888-7465 79945
Mobile: (226) 220-4760
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
Sent: Tuesday, December 20, 2011 1:26 PM
To: [log in to unmask]
Subject: [TN] QFP center slug over signal vias
We found an existing design which has a device (QFP with center slug) that
has too small of a center slug pad on the PWB. Within the corresponding
area where the center slug pad should have been, the designer put some
signal vias. Therefore the only thing preventing the signal via from
shorting out to the center slug (ground) is the solder mask. No failures to
date have been reported, even after a 20g vibe test.
Is this a violation of J-STD-001 or perhaps IPC-2221 -6.3.4 regarding
minimum electrical spacing?
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