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November 2014 • The PCB Magazine 25 PCBS FOR MEDICAL APPLICATIONS—A DESIGNER'S PERSPECTIvE continues Medical devices have some of strictest in- dustry EMC requirements and regulations. Ad- dressing these constraints is primarily achieved at the PCB level. This is yet another factor driv- ing PCB density and manufacturing tolerances. Design Considerations and Tips As PCBs in medical devices become more complex and dense, it is more important than ever to ensure that the designs are correct and will function as required. Although a typical medical product will see three or more signifi- cant PCB design cycles it is important that the value of each cycle is maximized. The more functionality is designed into a board, the more possibilities for errors and faults there is. General rules Electronic design automation (EDA) soft- ware now has sophisticated automatic design- rule checking, providing the rules are set up properly. These cannot prevent a bad design from getting into production, but they can pre- vent very common and frequent mistakes and oversights from getting out. As PCBs get more complicated, it is helpful to partition designs hierarchically. The top lev- el of a hierarchical design is much like a block diagram of the board. Putting all connectors on the top level schematic sheet if possible, makes that sheet very useful during troubleshooting and design reviews. Child sheets encapsulate lower level functionality and sub-circuits. These can be referenced multiple times if a circuit is repeated, rather than cut and pasted as separate sheets. This allows for easier duplication of lay- out and is less prone to mistakes creeping in. Once the schematic design is done, sub-cir- cuits are often routed first, without constrain- ing them to the board outline. This leads to a better overall layout compared to the tradi- tional approach of component placement fol- lowed by routing. Once the sub-circuit layouts are routed, these are then pushed and shoved together like puzzle pieces. It is helpful to mini- mize the number of layers of sub-circuits even figure 3: accelerated wear tester used to determine the durability or fatigue of prosthetic heart valves and other cardiac devices under pulsatile flow and physiological loading.