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Design007-May2019

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52 DESIGN007 MAGAZINE I MAY 2019 The Bare (Board) Truth Feature Column by Mark Thompson, CID+, PROTOTRON CIRCUITS In this column, I explore design rules and the constraints they put on the PCB fabricator. As you will see (and may have experienced), some work, but others do not. 1. Applications Depending on the board application, you may need tighter tolerances. For example, with aerospace, automotive, and medical boards, the tolerances must be tighter. So, how do we get there? 2. Designing With Specific Tolerances Here are several examples of how a PCB fab- ricator can deal with various tolerances. Let's look at "press fit" applications for tool sizes. Typically, a given plated hole or slot is ±0.003" and a typical non-plated hole or slot is ±0.002". So, what does the fabricator do when a plated hole is called out as ±0.002"? The simple answer is to calculate how much plating there will be in the hole barrel, and then over-drill to accommo- date the ±0.002 toler- ance. Typically, this is done by labeling the ±0.002" hole as plus 0.004" minus zero in the CAM system. End-users on RF appli- cations, such as anten- nas and phased arrays, have precalculated the copper in the hole barrel for many years. Often, end users will stipulate the drilled hole size (not the finished hole size as most designers call out) here so that they can pre-calculate how much plating will be in the hole barrel based on the PCB fabricators capabilities. 3. Don't Be a Violator For instance, we may receive an IPC callout for a 0.002" annular ring minimum. But what happens when the end user does not allow for that, creating gap violations? Or worse, what if they don't understand that a PCB fabricator drills approximately 0.004–0.005" over the fin- ished hole size as expressed on the drill draw- ing to meet the nominal hole size? Making the pads on a signal layer or the anti-pads (relief pads) 0.004" over the finished hole size without account- ing for the additional plating to meet the IPC minimum of 0.002" per side annular ring does not work. In a perfect world, the machine tolerances and true position are both ±0.000", but that is just not the case in prac- tice. It used to be said that if you added up all the tol- erances needed to man- ufacture a given board, nothing would be pos- sible. As an example, let's add a ±0.003" true position tolerance to a ±0.003" machine Board Negotiations: Design Rules and Tolerances

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