Issue link: https://iconnect007.uberflip.com/i/981887
MAY 2018 I DESIGN007 MAGAZINE 63 the purpose of the build, or conversely, they may want to check all functionality via a net- work analyzer. In that case, you cannot sub materials. Beaulieu: Those are great for folks who are just prototyping, but are there other ways to save cost? Thompson: Yes, sometimes it comes down to technology. I see a lot of jobs set up as blind vias, for instance, that do not need to be blind vias. This happens on boards with tons of additional routing space and adequate distance between points internally for standard through-hole interconnects. Why use blind vias unless you absolutely have to? Beaulieu: Last question. What are some things that design- ers may not know about board shops that they really should know? Thompson: Ha! There are probably thousands of such things. I would say that designers not understanding manufacturing edits or not pro- viding an IPC netlist are the things that can bite a customer the most. Some things that are frequently not considered as far as necessary manufacturing edits are etch compensations and drill compensations. For every half ounce of starting copper, mean- ing prior to plating, we add a half a mil etch compensation to account for the known loss at our etcher. So, starting on half ounce with .005" trace and space, the image data, whether it is artwork or direct image electronic data, would be at .0055" traces with .0045" spaces prior to etch after image. These compensations cannot take the fab house below their process minimums. So, the same .005" traces with .005" spaces would not be possible if the cus- tomer asked for three oz. copper, as the image data would be below the shop's minimum space after etch compensation. Additionally, Class 3 6012 /AS9102 parts that have epoxy fill or blind vias will require additional wrap plating to encapsulate the pad. This too needs additional space metal-to-metal and should be accounted for in the design. Similarly, for drills we do a compensation so that after through-hole plating the drills end up as the nominal size specified on the drawing. This means that if you call out a component hole as .008" +/-.003" and only have .010" signal pads after drill compensation, we would drill those same .008" holes at approximately .0138" so a .010" pad would be insufficient. In fact, to even get close to meet- ing IPC the pad size should be closer to .020". If you have designed for .010" pads and a manufacturer tells you that you need to double that number for an .008" drill, for you as the designer, that means a whole new layout. Again, if the holes are being used for plug-in components, the fab shop cannot reduce the hole size to deal with the pad size. However, if meeting IPC/AS9102 is not necessary, sometimes the board shop will say that they can live with a smaller pad size as a compromise, so that it may not require an entirely new layout. Beaulieu: Any last thoughts, Mark? Thompson: Lastly, remember that the best way to make sure the part functions electrically as designed is to provide an IPC netlist of your design criteria to be run against your provided image data to make sure they are electrically the same. Word to the wise about netlists: Avoid assigning net points to things like targets, moi - rés, non-plated holes or castellated holes. Beaulieu: Thanks, Mark. Great seeing you again. Thompson: Thank you, Dan. It's been a plea- sure. DESIGN007 Dan Beaulieu