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JANUARY 2022 I DESIGN007 MAGAZINE 29 if the HiPot tester did not reach maximum voltage, then locating the issue was done at arm's length. I used a matchbox to slightly open the lid, turned off the lights and watched the sparks fly from minute cracks in the ceramic capacitors. So, it was easily fixed by just replacing the offending caps. Unfortunately, I forgot to discharge the caps one day and found myself on the floor, on the other side of the room, still holding my spanner. Lesson learned. Appropriate trace spacing in a PCB design maintains signal integrity and crosstalk, and prevents the propagation of electromagnetic energy coupling between components. We typically use three times the trace-to-reference plane distance as a rule of thumb. However, this is for digital circuits with low supply volt- ages. In high voltage PCB design, trace spacing becomes even more important or you may end up with a charred board. To begin with, every designer needs a set of well-established design rules to base their constraints on. e IPC has provided the elec- tronics industry with guidelines for design and manufacture of printed circuit boards, com- piled over the years, with the support of both committee and industry members. e IPC has established design rules for trace spacing, pad-to-pad spacing, and pad-to-trace spacing for PCB designs that incorporate high voltages. e IPC-2221A Generic Standard on Printed Board Design provides clearance and creepage tables that assist with setting design rules and with performing design rule checks and electrical rule checks for minimum requirements. Table 1 is an extract from the IPC-2221A standard and provides the effective clearance required for both internal (stripline) layers and external (microstrip) layers. However, above 500V the clearance needs to be calculated. For 600V, for instance, the clearance is: 600V – 500V = 100V 0.25 mm + (100V x 0.0025 mm) = 0.5 mm clearance Design rules must keep up with the latest devices and fabrication processes—without losing sight of design for manufacturabil- ity (DFM). DFM is the practice of designing board products that can be produced in a cost- effective manner using existing manufacturing processes and equipment. If you follow the above IPC guidelines, you will be designing for both manufacturability and mass production. However, at times one must stretch the rules a little to meet the specific requirements of a design. is is fine, providing you can justify the reasons and tolerate the consequence of your decision. Entry-level EDA tools tend to rely on the skills of the PCB designer to detect possible issues as they arise during the design process. However, these days a more constraint-driven, Table 1: Electrical conductor spacing. *Voltage between conductors is DC or AC peak. (Source: IPC-2222A)

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