Phil,
I'm not sure why this is becoming an issue lately other than cost. I have
been told by more than one reliable source that
1mil thick is not a problem but some vendors try to save resources and make
it thinner. I've called out
SM thickness of 1mil minimum for many years with no problems. Lately, some
overseas vendors claim
they can only do 0.4mil thickness. I received a PCB with what they claim is
the standard (0.4mil) and I could almost see
through it and promptly rejected it. Bottom line is that mask needs to be
1mil minimum and if it is any thinner then it is up 
to the vendor to prove that it can handle 500VDC of insulation.
Hope this helps.
Ken

_____________________________________
Kenneth J. Wood
President
Saturn PCB Design, Inc.                [log in to unmask]
2737 Bishop Lane                       (407) 340-2668
Deltona, Fl 32725                       fax: (386) 789-2765
www.saturnpcb.com








-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Nutting
Sent: Tuesday, September 23, 2008 8:29 AM
To: [log in to unmask]
Subject: [TN] Solder mask thickness

Good afternoon,

OK, here is this afternoon's question.  Solder mask thickness, what is the
"industry standard"?  We currently specify 0.001" thickness and this may be
fine on a large area when tested via IP TM-650, 2.5.6.1.  Now use 2 ounce
copper traces in a normal board configuration.  Our board vendors are
stating that they cannot hold the 0.001" thickness at the edges of the etch,
it is more like 0.0003".  And if we spec 4 ounce or 6 ounce I'm sure it
doesn't get any better.

How are you specifying solder mask thickness on your heavy copper weight
circuit boards?

Do your board vendors use two coats of mask to obtain the thickness needed
to comply?

IPC-SM-840 section 3.4 says it must meet the requirements of section 3.8.1,
thickness of 0.001" at 500 Vdc.

Your guidance is requested.

Thanks,

Phil Nutting

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