PCB007 Magazine

PCB-Feb2014

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22 The PCB Magazine • February 2014 any burring at final rout is acceptable with the customer. This is where their understanding of electrical function comes in as well, plus years of practical experience. And when I say CAM depart- ment, I mean both people and tools, as the tools are only as good as the people behind it. I have said many times that a good CAM operator is worth his weight in gold. Likewise there are many levels of designers and lay- out folks,some with years of knowledge and good com- mon-sense layout practices. These are the guys and gals that ask the important ques- tions at the right time, which is to say at the design/layout stage and not after the PCB gets to the board fabricator. what does a fabricator do with data needed to produce parts? To start, let's go through what a board fabricator needs from an output CAD package briefly and talk about what ed- its are done at the manufacturing stage and why you need to know about them. Regarding output packages, what does a fab- ricator need to be able to fab your part? 1. Image data of all the layers involved in- cluding mask files, silkscreens and pastes ex- ported as either Gerber 274D, 274X, or ODB++. At least one of these files should have a part out- line on it, unless you are providing a drawing with a dimensioned hole or feature. 2. NC drill file or files (for blind or buried). Typically, Excellon 1 or Excellon 2 format. 3. An IPC-356 type netlist so the fabricator can run the design against the exported Gerber. 4. A drill drawing with hole sizes, dimen- sions and drill symbols chart. If the part is impedance controlled, the drawing should describe what lines are being controlled, where they reside and what thresh- old and tolerance they need to perform at. Now the fabricator has the job. Will my data undergo any modifications for manufacturing? Yes, so let's go through them. 1. Drill compensation for plated holes, slots or edge features Depending upon surface finish, fabricators will drill anywhere between .004" and .005" over your specified fin- ished hole size (FHS) to plate back down to your nominal size. Since this is something done by all manufactur- ers your output data should have already taken this into account. Fabricators like to see a minimum of .002" per side annular ring pad to hole after this drill compensation on signal layers and at least .005–.006" per side pad to copper pour after the drill compensation. Sometimes CAM is a com- promise between pad size, drilled hole size, copper weight and available space. Don't rope yourself into anything if you don't have to. If the hole is a legitimate via and you really don't care about its finished hole size make it +.003" minus the en- tire hole size. This tolerance tells the fabricator it is okay to drill smaller to deal with things like annular ring minimums and can mean the dif- ference between a quote from a fabricator and a "no bid." 2. Etch compensations for image data Depending upon the starting copper, all im- age data is compensated at the CAM stage to account for the known loss at the fabricators etcher. Typical rule of thumb is a half-mil of additional line or metal feature width for every half ounce of starting copper. Exception to this rule is quarter oz copper foil where no etch compensation needs to be Sometimes CAM is a compromise between pad size, drilled hole size, copper weight and available space. Don't rope yourself into anything if you don't have to. If the hole is a legitimate via and you really don't care about its finished hole size make it +.003" minus the entire hole size. " " UNDERSTANDING THE TyPICAL CAM PROCESS continues

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