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24 The PCB Design Magazine • May 2014 does not allow for any annular ring at all and most fabricators would like to see a minimum of .002" per side annular ring pad to drill compen- sated hole size. Normally, what we fabricators recommend is .010" over the finished hole size for all pads and lands with holes, and approximately .015" over for anti-pads (internal plane clearances, from drilled hole to adjacent cop- per plane). This allows for a drill compensation of .004"–.005" plus .005" for a minimum annular ring of .002" after our plating manipulation for drills. Speaking of Plating… Let's talk about some of the many assumptions about plating and surface finishes. So many times we see jobs that are externally imped - ance-controlled, but the cal- culations from the customer do not match the reality of board fab- rication. Remember, the holes and sur- faces need to be plated to achieve the through- hole continuity to make the part function. Many customers specify .0014" or 1 oz. fin- ish and use that number for the purpose of cal- culations. First off, in order to meet IPC with a minimum of 8/10 of a mil of plated copper in the hole, normally this means fabricators will plate up in whole-ounce increments, so we have to start on a base copper whether it be copper foil or copper clad core material. So, starting on half-ounce copper and plating up an ounce re - sults in 1.5 oz. or 2.1 mils finish. What does this mean to you at the design stage? It means if you are calculating your im- pedances with a lighter finished copper weight and not considering the additional plate-up for continuity, your trace sizes will end up having to be thinner than designed. If you are already at .0039"/0039" (.1mm/.1mm), this half a mil can mean the difference between a shop asking for +/-15% for the impedances vs. +/-10% or worse, getting no bid at all. Design for manufacturing has so many dif - ferent implications. At the design level, for in- stance, it implies you have already done a design review, so things like differential pair spacing or matched lengths are scrutinized and drawings are updated. But sometimes things change. An engineer may ask for additional layers for electrical reasons. Or changes to the various impedance needs may result in new revisions with all kinds of unforeseen issues. The engi - neer, for instance, may choose to remove some non-essential im- pedances or add some new ones; many times, drawings are not updated and you find your fabricator saying, "But there are no .005" traces on layer 3." Sometimes, as is the case with additional layers, the reference plane scenarios change, making the imped - ances impossible. Take the time to review the drawing for any notes pertaining to things that may have changed between revisions. OK, so we have talked about etch compensations, drill compensa - tions, impedances and drawing notes. Now, let's talk about DFM as it relates to profitability. Yes, that is another reason to design for manufactur- ability. Not taking the time to do a thorough design review prior to release to a manufacturer may result in a phone call from the fabricator and lost time. If you are on a particularly tight time schedule, this impact can reach thousands or even hundreds of thousands of dollars! But I know what you're thinking; after spending years in CAM, I can almost read your mind! You're probably thinking, "With all the fabricators in the world that perform an analysis to air out any issues prior to quoting my job, why should I take the time?" Because NOT tak - ing the time limits the type of fabricators who can do your job. (Not all fabricators can perform a full review to catch all possible CAM anom- alies prior to quoting your job.) Some fabrica- tors do a cursory review to make sure the basics are covered: The impedances are possible, and the drill files match the drawings. Additionally, should the CAM department catch something in pre-analysis, it will still need to be fixed. "With all the fabricators in the world that perform an analysis to air out any issues prior to quoting my job, why should I take the time?" Because NOT taking the time limits the type of fabricators who can do your job. " " feature WHAT IS DFM, REALLY? continues

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