Hello Tom,

My that is an unusual request/requirement.  .004 is the min I like, it can go down to .003.  Below that... I wouldn't trust it.  Actually the "ring" configuration you speak of is unique enough that I believe it will be problematic.  I would not do it.

As to the statement of board warping due to soldermask on one side; I disagree.  Soldermask, especially LPI is pretty friendy stuff relative to warp.  Unless there is some unusual condition of board thickness or finish, one side solder mask will not warp a board.

Brad Saunders
Coretec Boston Office
781 858 0783