Four Rules of Setting Circuit Width and Spacing
Four Rules of Setting Circuit Width and Spacing
There are four basic rules to setting circuit width and spacing. These include the x/y rule, the 2/2 rule, the 90-degree trace angle rule, and the PCB stack-up rule. Knowing these rules will make your design a lot easier to work with. Using these guidelines will help you design your PCB with the proper circuit width and spacing.
x/y rule
When designing a circuit board, it’s important to consider the x/y rule of setting circuit widths and spacings. This is the rule that dictates the width between two circuits on the board. For example, an x/y rule of 12/12 means that a local circuit’s width and spacing should be smaller than its area. In contrast, an x/y rule of 10/10 means that a local circuit’s width should be larger than its surrounding area.
2/2 rule
The two-part rule of setting circuit width and spacing refers to the size of the space between the circuits. It’s also known as the area rule. In most cases, the width and spacing are set to the same value. However, this rule is ineffective if the spacing is too narrow. In such a case, the probability of shorts doubles.
The width and spacing of traces on a printed circuit board are critical to the design process. While most digital routing relies on default values, more complex circuit boards may have trace widths that need to be precisely calculated based on layer stackup. High-speed traces with sensitive impedance may require wider spacing to prevent signal integrity issues.
90-degree trace angle rule
Traditionally, the PCB design industry has avoided 90-degree corners. Modern PCB layout tools come with mitering capabilities, which will automatically replace corners of 90-degrees with two 45-degree angles. However, if you do need to create a layout with 90-degree corners, it’s best to avoid them, since they can lead to antenna-like loops that can add inductance. While mitering angles to 135 degrees may help in these cases, it’s not a very good solution.
The 90-degree trace angle rule when setting circuit spacing and width should be applied with care. This is because the corner creates a discontinuity that can result in reflections and radiating. The 90-degree corner is also the most prone to phase-shifted reflections. Hence, it’s best to avoid using corners with 90-degree angles unless you are planning to place them in extremely tight areas.
Another reason to avoid corners is that a sharp angle will take up more space. Sharp corners are also more fragile and will cause impedance discontinuities. These problems will reduce signal fidelity. Therefore, modern PCB layout software is more likely to recommend right-angle tracks and doesn’t require 45-degree angle routing.
PCB stack-up rule
The PCB stack-up rule of circuit width and spacing is an important guide when designing multilayer boards. Basically, it means that if you want to make sure that a signal is balanced and runs from one corner to the other, you need to set the circuit width and spacing accordingly. Often, the width and spacing are calculated by taking into consideration the impedance of the circuits.
A good stackup allows you to distribute energy evenly, eliminate electromagnetic interference, and support high-speed signals. Moreover, it also reduces EMI and ensures that your product is reliable. However, there are some challenges in managing a good stackup. To overcome these problems, you need to use the right materials and set the circuit width and spacing properly. A good PCB stackup software helps you with these tasks. It will also help you choose the proper materials for your multilayer designs.
As the number of layers increases, so do the requirements for PCB stackup. For instance, the simplest stackups typically consist of four-layer PCBs, while more complicated stackups require professional sequential lamination. Higher layer counts also allow designers to have more flexibility in circuit layout.
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