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December 2011

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Subject:
From:
Joyce Koo <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Joyce Koo <[log in to unmask]>
Date:
Wed, 21 Dec 2011 22:23:00 +0000
Content-Type:
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Pat, the reason I suggest kapton is due to it is on a signal line.   We were not told what kind signal line and performance requirements.  Kapton is a safer bet (still a bet) if low dielectric constant is required.  When I am in doubt, over kill usually saved day (just want to avoid nightmares). .  
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----- Original Message -----
From: Goodyear, Patrick [mailto:[log in to unmask]]
Sent: Wednesday, December 21, 2011 10:15 AM
To: [log in to unmask] <[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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