PCBs are often produced in bulk, so any kind of fault that would hamper the effective operation of one PCB could affect dozens or even hundreds of them. At the same time, when you make any kind of purchase, you expect a certain level of quality. What is the IPC? IPC is a global trade association for the electronics industry. The IPC represents electronics design, printed circuit board manufacturing and electronics assembly. The organisation periodically releases acceptability standards in the form of qualification and performance specification standards for the design and manufacture of all major types of printed circuit boards, and other electronic components.

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LinkedIn The Hubble telescope, the Cassini-Huygens mission, and other exploratory spacecraft utilize high voltage dc power supplies for everything from vidicon camera tubes and mass spectrometers to radar and laser technologies. NASA has experienced performance problems with the 1. Designing a circuit that includes high voltages requires a different--and much more rigorous--approach than seen with other PCB designs. Along with that approach, design teams also must become familiar with terminology that covers insulation, board materials, clearance, creepage, and altitude.

Designers also should have an overall knowledge of regulations that can impact the circuit. In high voltage PCB design, trace spacing becomes even more important. If we rightfully consider the board as a series of conductive elements, the possibility of differences in potential creating high voltage flashover with narrow trace spacing becomes a certainty.

As a combination, the standards also set guidelines for PCB layouts that include two important parameters called clearance and creepage. Using the IEC definition, clearance equals the shortest distance between two conductive parts, or between a conductive part and the bounding surface of the equipment, measured through air.

A small clearance value between two conductors establishes the environment for a high-voltage flashover or arc. Clearance values vary according to the type of PCB material used for the circuit, the voltages, and operating environment conditions such as humidity and dust.

Those environmental factors--and others--decrease the breakdown voltage of air and increase the opportunities for a high voltage flashover and a short circuit. Since the bounding surface described in the IEC definition is the outer surface of an electrical enclosure, we can use 3D design tools and design rules to establish the clearance between enclosures and components for rigid and rigid-flex circuits.

We can also apply good PCB design principles by isolating high voltage circuits from low-voltage circuits. Fabricators often recommend placing the high voltage components on the top side of a multilayer board and the low voltage circuits on the bottom side of the PCB. Other methods involve placing the appropriate insulating materials between high voltage nodes and over any exposed high voltage leads.

Again referring to the IEC definition, creepage represents the shortest path between two conductive parts, or between a conductive part and the bounding surface of the equipment, measured along the surface of the insulation.

While clearance refers to the spacing through conductive elements through air, creepage considers the space between conductive elements over an insulating surface. The design rules that we establish for trace spacing, pad-to-pad spacing, and pad-to-trace spacing for PCB designs that incorporate high voltages address creepage.

The IPCA standard provides clearance and creepage tables that assist with setting design rules and with performing design rule checks and electrical rule checks for minimum requirements. Along with applying design rules, PCB layouts can also include slots or vertical insulation barriers between traces.

Because any metallic print pattern or printed circuit trace that has sharp edges can cause a high electric field across insulators and a flashover, the trace layout for a high voltage power supply must avoid sharp corners and acute angles. Along with those specifications, design teams should also consider tensile strength, hardness, surface breakdown strength, thermal expansion, chemical resistance, and stability against aging and oxidation.

While circuits require functional insulation to operate, other types of insulation prevent high voltage problems in PCBs. Insulating materials may include encapsulating resins applied to high voltage cavities, conformal coatings, or solid insulation that surrounds conductors. Regulatory standards require additional layers of insulation if the potential for human contact with the system exists. When selecting dielectrics and insulators for a PCB, use the comparative tracking index CTI to determine which material type works best for the specific application.

The CTI is the maximum voltage measured in volts at which a material withstands 50 drops of contaminated water without forming conductive paths because of electrical stress, contamination, or humidity.

Manufacturers uses the CTI to compare the performance of insulating materials under wet or contaminated conditions. Materials that have a high CTI value have a lower required minimum creepage distance and allow a shorter distance between two conductive parts. The shorter distance allows the use of high density circuits in a high voltage environment. UL standards divide the CTI levels for materials into the four groups shown in table one.


IPC-2221B: Generic Standard on Printed Board Design



PCB Design & Analysis


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