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30 DESIGN007 MAGAZINE I JANUARY 2022 correct by construction approach is required for complex designs. Once the rules are estab- lished, they will be followed by downstream tools and validated to conform by the various design rule checkers (DRCs). Along with applying design rules, PCB lay- outs can also include slots or vertical insulation barriers between traces. Any metallic print pattern or printed circuit trace that has sharp edges can cause a high electric field across insulators and a flashover. Whenever routing traces for high voltage PCBs, the following points are most impor- tant: 1. Keep the specified clearance between traces and pads that have a high voltage disparity. 2. Avoid any sharp turns and edges as these can act as areas of the concentrated electric field. 3. Avoid running very high voltage traces on the internal layers of the board. High voltage design requires insulation that has high dielectric strength, high resistivity to prevent arcing, and a low power factor that reduces heating effects and the possibility of thermal breakdown. Insulating materials may include encapsulating resins applied to high voltage cavities, conformal coatings, or solid insulation that surrounds conductors. e Comparative Tracking Index (CTI) is used to measure the electrical breakdown properties of an insulating material. It is the maximum voltage at which a material with- stands 50 drops of contaminated water without forming conductive paths because of electri- cal stress, contamination, or humidity. Mate- rials that have a high CTI value have a lower required minimum creepage distance and allow a shorter distance between two conduc- tive parts. e shorter distance allows the use of high-density circuits in a high voltage envi- ronment, which is commonplace these days. Along with requiring different types of insu- lating materials, high voltage PCBs also need board materials that protect from voltage breakdown and offer the physical properties that match application needs. Although FR-4 laminates have a high breakdown voltage, the weaker structure and porosity of FR-4 can allow the material to become prone to contam- ination and a gradual lessening of the dielectric value. Because of the FR-4 limitations, high voltage laminates that have a non-conductive base layer and prevent arcing are best for high voltage PCB design. High voltage laminates have higher levels of resin and glass than stan- dard board materials. A board material that is specifically designed to tolerate an overvoltage event, as well as the regular high voltage operating conditions is best. ere are a few material options to con- sider when dealing with high voltage: 1. FR-4 laminate: FR-4 has a very high dielectric breakdown. However, it is more porous than BT epoxy and polyimide, which makes it easier for the substrate to become contaminated. It also has a weak edge structure, and as the edge cracks, the dielectric value will decrease. Aging is a likely problem, especially for electronics near the edge. FR-4 also has no recovery or protection from carbonization that occurs during over-voltage events. 2. BT epoxy: A thermoset resin, BT epoxy (bismaleimide triazine resin) has strong sidewalls and is better for applications with planar coils and medium voltage circuits. For example, AGC (Nelco) N5000 BT epoxy laminate and prepreg provides superior electrical properties. 3. High V laminates: ere are several high voltage laminates. Isola's is one of the most well-known, that actually extinguishes arcs and leaves a non-conductive base layer. For example, IS550H was developed in conjunction with a consortium of industry experts for high power and high voltage applications and PEV and HEV automo-