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Okay, so what is DFM, really? The term "design for manufacturability" has been used for many years now, but does everyone really understand this concept?
For instance, do you design for 10%? Do you design for a specific manufacturer’s capabilities, therefore making you less likely to seek alternative fabricators? How are your drawings worded?
In this article, I will be discussing the reality of DFM and what benefits you, the end-user, by embracing these practices.
Why Design For Manufacturability at All?
Good question. Even if you only buy your boards from a single source--if you have qualified the company already and feel you can expect certain press parameters and dielectric constants based on what they have provided you--it is STILL a good idea to at least design with some latitude. If your design is .1 mm lines and spaces there is not a whole lot of room to either expand or decrease the traces to achieve certain impedances. Clearly, when you have to ingress and egress out of tight-pitch components and your design takes you down to .003”/.003” there is NO ROOM at all for an etch compensation, so you are typically quoted by manufacturers as quarter-ounce foil start. This foil is so thin that we need not compensate for a loss at the etcher like the other copper weights.
Again, as I have mentioned before in my columns, the general rule of thumb is that for every half-ounce of starting copper, you give all the metal features an etch compensation of half a mil. Asking for 1 oz. starting copper, for instance, with 0.003”/0.003” will normally be a no-bid as fabricators would be hard-pressed to be able to run with .002” spaces at Image prior to etch. (Attempting to compensate the 0.003” traces for 1 oz. copper with 1 mil will result in 0.002” spaces at Image prior to etch.) So, 0.003”/0.003” is usually the limit.
Read the full article here.
Editor's Note: This article originally appeared in the May 2014 issue of The PCB Design Magazine.
