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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