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How to Design a Circuit Board

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • Circuit board design begins by incorporating design documents into the process workflow.

  • The designer has to build land patterns and design rules according to the manufacturer’s specifications and the demands of the design.

  • The layout process, as the final pre-manufacturing stage, represents the bulk of the designer’s obligation.

Isometric view of processor and traces

 

With some guidance and experience, laying out dense board designs can become second nature

The first time a designer lays out a circuit board is usually an experience fraught with mistakes, frustration, and necessary perseverance. This is not the fault of the user or even the interface - PCB design is a complex topic that touches many engineering disciplines, material science,  chemistry, and more. Even though the stakes are much lower, especially if the board is simply a test of a beginner’s knowledge rather than an actual board set for manufacturing, designers will want to establish healthy best practices early. Learning the how’s and why’s, as opposed to the do’s and don’ts, will further reinforce a designer’s knowledge and confidence. This brief introduction on how to design a circuit board will instruct the reader as to the basic steps of the design process and the lessons they should absorb along the way.

How to Design a Circuit Board: An Introduction

A common mistake of amateur layout designers is racing to get to the board level of design. It’s not hard to understand why: the board is the fun part of the design, where users play a 3D puzzle game trying to find the best-engineered solution with the schematic and constraints provided. It may be tempting to use an autorouter (an automated software algorithm that attempts to complete the placement and routing of the board) to save time. While an autorouter is a useful tool in many designs, it’s more often a last-ditch feature to get an exceptionally dense design over the hump, not something that is meant to be deployed at the first sign of trouble. As all good things come in time, designers must first invest in the underlying data structures that provide the building blocks of design before diving into the meat of the project.

The process of learning how to design a circuit board will likely begin with some controlling documents, usually a bill of materials (BOM), a schematic, and some additional design information:

  • BOM - The BOM contains all the information related to the components that will eventually be placed on the board surface(s). This includes quantity counts, manufacturer’s part numbers (MPNs), parameter values, and reference numbers. 

  • Schematic - If the board is the heart of the design, the schematic document is akin to the brain and nervous system. The schematic contains all the information on how components will connect and their shared networks (synonymous with nodes in the circuit/network analysis naming schema).

  • Additional documents - This could be many things, including a word processing or other similar text and image-rich file format that contains all of the requested changes from the initial work order and any other information the engineering team wants to call the designer’s attention to (power circuitry layouts, stackup and via design, or design rules).

Preparing Land Patterns and Building Design Rules

From this point, the layout designer begins by researching the MPNs on a part lookup catalog to find datasheets for all the components in the BOM (provided this was not included alongside the design documents). The purpose of the datasheet is two-fold: 

  1. It gives the part dimensions or pad layout for the designer to build an accurate land pattern for placement. 
  2. It provides additional information about how best to layout the circuitry containing the component in question for best performance.

Land patterns are the space the component takes when placed on the board and contain any pad and drilled hole information. Improperly sized or misaligned land pattern features can lead to a whole host of issues during assembly. As for the second point, many electronics need to have a carefully designed layout and copper feature design settings to maximize efficiency. This is often seen in power circuitry and is less of an issue in circuit blocks consisting of only passive components.

After the land patterns are correctly associated with their respective schematic symbols, one final step has to be taken before the layout can proceed: rule design. The idea of setting up design rules through the constraint manager is that then the designer doesn’t have to mentally juggle what is and isn’t valid design on the fly. It i much easier to leave something as tedious as that to the computer. By setting up rules, the rule check function can inform the designer when they’re being violated with some on-screen indication. This prevents the designer from investing significant time into a design that has incorrectly synthesized the design constraints, a direct reflection of the sophistication of the materials and machinery used to make the board.

Design Culminates With the Layout Process

The layout designer has now arrived at their eponymous moment. A netlist is exported for the schematic software and populates the board with all of the component land patterns, net names, reference designators, and any other information the designer includes. The layout should first begin by grouping - the designer, using the schematic as a reference, arranges circuit blocks on the board. Enough spacing should be provided for via fanout, but the exact distances are not as important as the placement and rotation of components. If there are any critical signals like clocks, data lines, differential pairs, etc., the distance these traces run should be prioritized. Otherwise, the general rule of thumb is short stubby traces for power and return, and the shortest and most direct traces possible for any remaining signals.

With an approved placement, most designers are ready to route the board and put the design to bed, but there is one final preceding step. The plane design of power and ground needs to be shaped on the relevant layers. All power and ground vias should fall within the perimeter of these copper shapes, but it may be necessary to split the same net planes across multiple layers or even run wide traces to reach the same net vias that are far away from the major areas of distribution. In any case, care needs to be taken when routing on layers coupled with power planes: for signal integrity purposes, crossing a split plane should be avoided in almost every circumstance.

The process of how to design a circuit board represents the core obligation of a layout designer; to aid in this as well as any supporting tasks, Cadence’s suite of PCB design and analysis software provides users with an unparalleled level of support and functionality.

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