For those new to PCBs, let’s start with the basics. A Printed Circuit Board (PCB) is a structure made up of one or more layers of insulating material with electrical conductors designed to mount and connect electronic components. The insulator is usually made of fiber reinforced resins and fiberglass, but can also be made of ceramics, plastic, or other dielectric materials. The boards typically list their flame-resistant category, such as FR4 or FR2. During manufacturing, the unneeded portions of the conductors are etched away, leaving the desired tracks that connect the components.
The main generic standard for PCB design is IPC-2221B, which provides rules for manufacturability and quality, including material properties, surface plating, conductor thickness, component placement, dimensioning and tolerance rules, and more. This standard applies to all types of printed circuit boards, including single-sided, double-sided, and multilayer. For specific technologies, you may choose an appropriate sectional standard from the IPC-2220 series. For SMPS power conversion designs, additional parameters are recommended by IPC-9592B.
To design a PCB, you’ll need a design software. Popular options include Allegro by Cadence, PADS by Mentor, and Altium. There are also some free open-source programs available.
The PCB design process includes the following steps:
- Create footprints of all components and make a schematic using a schematic capture program.
- Set design rules, such as conductor size and spacing between nets.
- Place and route all components.
- Pour ground planes connected to the ground nets.
- Run connection and design rule checks.
- Generate files for board fabrication, typically Gerber files.
When setting design rules, the conductor width and thickness should be chosen based on the maximum allowable temperature rise at the rated current per IPC-2152 and an acceptable impedance of the specific traces. Additionally, the trace should not melt during brief surge currents or short circuit conditions. This requires a sufficient cross-sectional area of copper as a function of overload amp-seconds. The spacing between the PCB tracks is determined by the peak working voltage, coating, circuit location, and product application. The minimum trace width and spacing are limited by the manufacturing capabilities of the fabricator, with typical guidelines being 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 take precedence over IPC.
Once the design rules are set, you can place the components and do the routing. IPC and other standards do not provide guidance on how to properly route the board. Good PCB layout techniques require an understanding of trace impedance, signal coupling, and circuit operation. Auto placement software may be used for most control circuits, but power, ground, and high di/dt circuits should be routed by hand. Finally, the design information must be transferred into a form that can be sent to the fabricator, typically in the form of Gerber files and files in Excellon and other formats.