Starr, 

The specification is IPC-D-275, Design Standard for Rigid Printed Boards and
Rigid Printed Board Assemblies. 

The Section is 3.8.4.3, and it calls for 1.3 um [0.00005 in] minimum, 2.5 um
[0.0001 in] maximum.

Regards,

Gary Ferrari
IPC Staff

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