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Dielectric Strength

The dielectric strength of a material is the maximum electric field the material can withstand without losing its insulating properties. Usually, this would be FR4 for most PCBs. So the first important thing to check is the dielectric strength of your PCB material. FR4 rated for 300 mils/mil; however, IPC standards will recommend a spacing of 3.9mils for 80V. It is often wise to leave room for it and therefore follow IPC standards.

Creepage Distance

Creepage distance is the shortest distance between two conductive parts along the surface of a solid insulating material. The main basis for the determination of creepage distance is the long-term voltage applied on the two conductors. Over time, there can be contamination on the board, so it is wise to have additional headroom. Creepage distance recommendations for most components and layouts are usually provided in their datasheets. It is one of the important parameters when it comes to High Voltage PCBs.

Clearance

Clearance is the shortest distance in air between two conductive parts. The dielectric strength of the air governs the clearance. Air usually breaks down at about 3KV per mm. However, under conditions of moisture and contamination, this value can go much lower. The IPC standards specify different clearance distances for design types. Most components are already compliant, and they are packaged in packages of high enough pin pitch. For example, a 600V rated Mosfet will usually come in a TO-220 or TO-247 package. More information about creepage and clearance can be found here:

Solder Mask

The solder mask also acts as an insulator to some extent. So for boards with fine pin pitch and high voltage between pins, it is important to go with a manufacturer who can apply a solder mask between PCB’s the fine pitch PCB.

How to do design practices for high voltage PCB?

Routing

Whenever routing traces for high voltage PCBs, the following points are most important:

1. Keeping clearance between traces that have a high voltage difference between them.

2. Avoiding 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.

Polygon Planes

For all high voltage PCBs, the polygon plane clearance should be increased to a safe value and certain sections of the board. For example, if there is a trace carrying 600V towards an edge connector on a board, then a lot of times, the design won’t have a polygon pour near that trace.

Also, internal planes on a multi-layer PCB should not have a very high voltage or a very small distance between them.

Internal Layers

It is possible to make multi-layers with the medium voltage on all layers. The major consideration is the proper filling of the spaces between the layers. The thickness of the separation between layers must be at least .005″ to maintain the filling medium’s integrity, prepreg. Any voids or pockets will degrade the dielectric value seriously.

The standard prepreg FR4 type multi-layer is not suited to medium or high voltage work. The material breaks down too fast and does not feature a homogeneous internal structure. The excess microvoids degrade the dielectric rating resulting in its poor performance.

EMI Considerations

High voltage PCBs are notorious for emitting disturbances over a wide spectrum. The most important thing to minimize these is to have a small loop area and extensive via stitching to the ground plane wherever possible. Additionally, the high voltage components can also be potted and then shielded by a metallic sheet.

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