SMT Processing and gasketing

SM-840, is a material qualification document only, with all the test requirements applicable only to a defined test coupon, not to production boards. So, it's useless for what you're trying to accomplish.

As assemblers use A-610 - Acceptability Of Electronic Assemblies, fabricators use A-600 - Acceptability of Printed Boards. A-600 is the language that you should use in conversing with your fabs. Luckily for you, we have verbage from an outdated version [A-600G] on the very topic, solder mask thickness, that you are in flux [so to speak] about. 3.3.11 Solder Resist Thickness: * Target condition, Class 1, 2, 3: Thickness as specified on procurement documentation. * Accpetable, Class 1, 2, 3: Specified: The solder resist thickness meets the thickness requirements on the procurement documentation [cannot be visually assessed]. * Nonconforming, Class 1, 2, 3: Defects do not meet the above critera.

Now, being from the middle of the back-end of the pack of the SMTneters from an astuteness stand-point, even we sense that's not the answer you were hoping to find. So, we checked IPC-6012 - Qualification & Performance For Rigid Printed Boards, just in case. Here's what we got for you ... 3.8.3 Solder Resist Thickness. Solder resist thickness is not measured unless specified in the procurement documents. If a thickness measurement is required, instrumental methods may be used or assessment may be made using a microsection of the parallel conductors on the E coupon.

So, we guess that we are going to be able to find you running the wuchacallit machine sectioning a bunch of boards from each supplier and using the two that you like as the basis for your 'customer requirement.' Actually, instumental methods [mentioned in 6012] will get you to the answer faster and cheaper. Just rent one of those eddy-current wuchacallit instruments. Wouldn't it be cool if you could sucker your fabs into doing this for you?

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