Cost vs. Quality
Two important considerations within any product development cycle; But did you know you do not have to sacrifice one for the other with a little up front planning?
You have already begun to lower costs when you realize cost control starts with the very first CAD command!
Drafters and designers are an exacting bunch, a very good trait to have for the job. The problem begins when we set-up tolerance blocks on flex circuits like we would use for a PCB. Flex materials are inherently unstable, changes in temperature and humidity can have a big impact on finished dimensions.
Our experience has shown the potential for a mil or better of stretch or shrink per inch. I see many drawings with outside trim dimensions (edge to edge) held to +/-.005”. While this may be acceptable for a PCB, it is far beyond the ability of common flex circuits and materials. The very nature of flex circuits is to yield and bend, super tight tolerances are rarely required.
So how does dimensional tolerance impact cost? When given a trim profile window of .010”, we as the manufacturer are “forced” into laser tooling. If we are lucky and the quantity justifies it, we can order a hard tool punch & die. Neither method of tooling would be considered inexpensive. Changing the trim tolerance to a .020” profile window can have a dramatic impact on cost reduction as it allows the use of “soft” tooling. Ideally larger circuit trim dimensions should be “reference”.
Keep in mind profile dimensions are not the only dimension that is often toleranced beyond the base materials inherent properties. While hole sizing can be easily held to +/- .003” “hole to hole” or “hole to edge” positioning cannot. Within a given hole pattern cluster tolerances can be held tight, but “cluster to cluster” tolerances are limited by the same stretch/shrink issues as our trim dimensions. Demanding “hole to edge” tolerances are indeed possible but remember tooling and production costs will rise.
Here is where the concept of “key characteristics” becomes our best hope for controlling cost. Examine your design for what features are truly important and call them out for special attention. If only one dimension is critical then by all means ask for tight control, but do not pass the requirement on to other features that do not require it.
I wish I could recall who said it but I am reminded of a comment I overheard at a recent design conference… “75% of your circuit cost is under the control of the designer”. From what I have seen after reviewing countless drawings I certainly believe it!
Best defense against rising program cost is to begin interaction with your circuit supplier at the earliest stages of design. Use them to help lower costs before you release to production.