Hi Kelly -

May I direct your attention to an interesting little requirement out of
IPC-SM-840C?  A portion of para. 3.4.10 reads: "The board fabricator shall
confirm that the minimum thickness is sufficient to withstand  500 VDC per
solder mask vendor's specification.  The coating thickness shall be measured
by any micrometer or inidcator accurate to 0.0025 mm [0.0001 inch] or
microsection in accordance with TM 2.1.1 of IPC-TM-650."

"If a specific thickness or breakdown voltage is required it shall be
specified by the end user on the procurement document."


Thickness of the solder mask is considered critical only in an electrical
performance sense.  There is no relief for any areas which are to be coated
with permanent solder mask on the circuit board from this requirement.

Regards - Kelly

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