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12 The PCB Design Magazine • January 2016 and circuits. The engineering physicist part of me adds basic principles analysis of all sorts of fun stuff including optics, algorithmic process- ing for ultrasound, or image processing. Like most electrical engineers, I also work on firm- ware and logic design. Shaughnessy: how is designing a medical pcB different than designing any other high-reliability board? MacCallum: Although the medical industry is fairly heavily regulated, designing a medical PCB is not that different than designing for lab equipment or a consumer device. There are still safety standards to meet. The medical standards are more stringent and they span a bit more. I've worked in metrology, general engineer- ing (remotely operated vehicles), and consulting engineering for consumer and dental devices. I've seen a bit about what is required for various product sectors, developed products and gone through regulatory hurdles for each. They all have regulatory requirements—especially in Eu- rope—but the requirements are generally har- monized around the world. We meet ISO 60601 for medical and ISO 61010 for laboratory equip- ment. These are standards we must comply to or we can't sell in the various markets. Shaughnessy: how important is simulation and analysis of medical pcB designs? MacCallum: I'm a big fan of simulation, and analysis is sort of my bread and butter. I also like walking into the lab and trying something, but I start at an intuitive level: How do I be- lieve a circuit is working? Then I back it up with simulation to see if it works the way I expect it to work. Often a circuit will see an evolution or even a total rip-up and re-do based upon simu- lation results. I apply a sort of quality filter in my head be- cause many times simulation will take you into the weeds. You never know, because simulation doesn't smoke. Simulation is only as good as the quality of the component models you use and often there are a lot of inherent assump- tions based in those models. I'm a big fan of FEA (finite element analysis) to compute impedances of traces, like differen- tial pairs, rather than using some of the ana- lytical solutions. FEA allows me to accurately model the right PCB stack and allows me the flexibility to adjust my space and trace settings depending on whether I've got a nearby plane below or maybe just a fill on each side. One strategy I use often is to make the fills to the side dominate and therefore I need less control over the dielectric and plane below. Whenever I'm developing circuits or apply- ing rules in circuit layout, I like to think of it at a basic principle level and do a thorough anal- ysis. I don't like apply a rule of thumb unless I've already hashed out in my mind or on paper the purpose of that rule and what problem it's really trying to solve. In many cases there's a big trade-off in all the requirements of any cir- cuit. Whether it is EMC, power, cost or noise, I want to understand all of the tradeoffs and how they may impact all the other circuits that are around, under or on the same board. It's impor- tant to understand how all those fit together, putting the right emphasis on the right require- ments and not just arbitrarily saying there must always be a certain space trace clearance, com- ponent spacing or whatever. Shaughnessy: the medical electronics field is so heavily regulated. What sort of "hoops" do you have to jump through, such as certifications and standards, when dealing with medical pcBs? MacCallum: There are a number of regulatory bodies we are concerned with aside from the typical UL, CSA, and FCC. We have to worry " There are still safety standards to meet. The medical standards are more stringent and they span a bit more. " MEDICAL PCB DESIgN: NoT JuST ANoTHER HIgH-REL BoARD feature interview