PCB DESIGN GUIDE

Let's begin with the basics for those new to PCBs. Printed circuit board (PCB) in general is any structure made of one or more layers of insulating material with electrical conductors designed to mount and connect electronic components. The insulator is typically made on the base of fiber reinforced resins and fiberglass, although it can also use ceramics, plastic, or some other dielectric materials. The boards normally list their flame resistant category (such as FR4, FR2). During manufacturing, the portions of conductors that are not needed are etched off, leaving desired tracks that connect the parts. Alternatively, the conductors can be added by an electroplating process.

Currently, the main generic standard for PCB design, regardless of materials, is IPC-2221B. It provides rules for manufacturability and quality, such as requirements for material properties, criteria for surface plating, conductor thickness, component placement, dimensioning and tolerance rules, and more. This standard applies to all printed circuit board types including single-sided, double-sided and multilayer. For a specific technology, you may then choose an appropriate sectional standard from the IPC-2220 series. For SMPS power conversion designs, additional parameters are recommended by IPC-9592B.

To do a board layout you obviously need a design software. Popular choices are Allegro by Cadence, PADS by Mentor, and Altium. There are also some free open source programs listed below.

PCB design includes the following main steps:

• Create footprints of all the components and make a schematic by using a schematic capture program;
• Set design rules (such as size of the conductors and spacing between nets);
• Place and route all the parts;
• Pour ground planes connected to the ground nets;
• Run connection check and design rules check;
• Generate files for the board fabrication- usually it is Gerber files.

When you enter design rules, the width and thickness of the conductors should be chosen based on maximum allowable temperature rise at the rated current per IPC-2152 and an acceptable impedance of the specific traces. There is another criterion that is often overlooked. The trace should not melt during brief surge currents or short circuit condition that can develop in the circuit. This requires sufficient cross-sectional area of copper as a function of overload amp-seconds. The spacing between the PCB tracks is determined by peak working voltage, the coating, location of the circuit, and the product application. The minimum possible widths of the traces and of the spacing between them are both limited by the manufacturing capabilities of your fabricator. Some vendors can do as low as 3 mils. I would avoid using the width and spacing less than 4 mils though. Typical guidelines for minimum values are 6/6 mils.
Depending on the application and product end use, other standards may also apply. For example, for mains-powered or battery-powered information technology and audio equipment, the creepage and clearance requirements of IEC/UL 62368-1 (which replaced IEC/UL 60950-1) take precedence over IPC.

Once you set the design rules you can place the parts and do the routing. IPC and other standards do not tell you how to properly route the board. Good PCB layout techniques require an understanding of the effects of non-zero trace impedance and the coupling of signals from one circuit to another through parasitic capacitances and radio transmission, as well as a basic understanding of circuit operation. Auto placement software may be used for most parts of control circuits, but power, ground and high di/dt circuits should be routed by hand. At the end, the design information has to be transferred into the form that can be sent to a fabricator. The set of the data that describes the board normally includes Gerber files as well as files in Excellon and other formats.

Below you will find more guidelines for designing PCB, links to software downloads, trace calculators, tools and other online resources.

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