Hi folks - one reason that soldermask is not considered a dielectric
insulator is that the insulative properties from soldermask formulation to
formulation and also variation within a specific formulation can vary
greatly. No one has had enough pull or enough inertia to get the
characteristic insulation properties of soldermask documented for industry
use. Thank goodness for Kapton tape.
Dave Hillman
Rockwell Collins
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"Stadem, Richard D." <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
12/20/2011 12:49 PM
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Re: [TN] QFP center slug over signal vias
This is another example, and not the only one, where compliance to a
quality standard does not mean the product will be reliable.
Why be concerned whether it violates the J-STD-001 or IPC-2221? Does that
matter? The fact is that the design puts the product reliability at risk,
as a minimum.
Soldermask is not considered to be an insulator. But even if it was, would
that make you sleep well at night?
Just for the record, it IS a violation of the minimum electrical
clearance, because of the fact that the soldermask is not considered to be
an insulator, and if the soldermask thickness is less than the minimum
electrical clearance, the design will then violate that requirement.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
Sent: Tuesday, December 20, 2011 12:26 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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