Scott, thanks for furthering the conversation.

I have a client (OEM) who wants us (Board House) to fabricate a copper plane to within +/- 0.002" of the PCB edge of FR-4 laminate.
I replied that the edge, per his print, allows a tolerance of +/- 0.005", making one tolerance incompatible with the other.

He replied that he'd simply change the print tolerance for the PCB length and width to  +/- 0.002".
When I replied that composite materials would neither punch nor CNC machine to that tolerance, he asked what the IPC comments were for board size tolerances.

That seemed an easy-enough task. After-all, circuit board size tolerance is about as basic or fundamental as it gets for design and fabrication considerations. But in the absence of actual values from the IPC (such as those defined for hole size dia.) anyone can ask for anything on a print.

Given that the IPC has geometric dimensioning and tolerance comments on virtually all other PCB features (machined, imaged or otherwise) I found the absence of same for board size tolerance surprising.

So I figured I overlooked it, and asked for help.

Thanks again -

Robert "Bob" Lazzara, CIT
Tel: (603) 318-3075

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Scott A. Bowles
Sent: Wednesday, August 5, 2015 10:07 AM
To: [log in to unmask]
Subject: Re: [TN] PCB length/width tolerance

Bob,

Maybe you can clarify what is driving your question.  You state that the ±.016" seems liberal (and I agree) but this is our subjective opinion. Are you asking from a design/OEM perspective or manufacturing/supplier perspective? Section 3.4 of IPC-6012 states the printed board shall meet the dimensional requirements specified in the procurement documentation. However, if there truly isn't any requirement specified then you default to the applicable design series specifications. I would think it is very rare for a drawing to have no requirements, to not even have the "Unless Otherwise Specified" title block tolerances on sheet 1, which typically default to something along the values of three place decimals ±.005", two-place decimals ±.010".

From a design perspective, IPC-2222A, paragraph 5.4.1 states the tolerances specified on the drawing shall accommodate the dimensions and tolerances of the mating part.

I suggest clarifying the requirement before manufacturing product.

Regards,
Scott


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