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22 The PCB Design Magazine • May 2014 feature 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 in - stance, with .003"/.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 .003" trac- es for 1 oz. copper with 1 mil will result in .002" spaces at Image prior to etch.) So, .003"/.003" is usually the limit. While we are on the topic of etch compensa- tions let's talk about drill compensations. I can- not over-emphasize this: So many times lately we have issues at CAM with insufficient annular rings. This is because many customers still do not realize that, in order to keep a plated hole at a nominal size, it has to be drilled larger (this is what we mean by drill compensation) to be able to plate back down to the nominal size and have continuity. When we talk to customers about drill compensations, they typically hear only the first part: That we need to drill approxi - mately .004"–.005" over the FINISHED nominal hole size to plate back down to NOMINAL. This OK, 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 alterna - tive 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 quali - fied the company already and feel you can ex- pect certain press parameters and dielectric con- stants 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, by Mark Thompson, CID ProToTron CirCuiTs What is DFM, Really?

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