What is line width?
Let’s start with the basics. What exactly is trace width? Why is it important to specify a specific trace width? The purpose of PCB wiring is to connect any kind of electrical signal (analog, digital or power) from one node to another.
A node can be a pin of a component, a branch of a larger trace or plane, or an empty pad or test point for probing. Trace widths are usually measured in mils or thousands of inches. Standard wiring widths for ordinary signals (no special requirements) may be several inches in length in the 7-12 mils range, but many factors should be considered when defining the wiring width and length.
The application typically drives the wiring width and wiring type in PCB design and, at some point, usually balances PCB manufacturing cost, board density/size, and performance. If the board has specific design requirements, such as speed optimization, noise or coupling suppression, or high current/voltage, the width and type of trace may be more important than optimizing the manufacturing cost of a bare PCB or the overall board size.
Specification relating to wiring in PCB manufacturing
Typically, the following specifications related to wiring begin to increase the cost of manufacturing bare PCB.
High-density designs that combine PCB space taking, such as very finely spaced BGA or high signal count parallel buses, may require a line width of 2.5 mil, as well as special types of through-holes with diameters of up to 6 mil, such as laser drilled microthrough-holes. Conversely, some high-power designs may require very large wiring or planes, consuming entire layers and pouring ounces that are thicker than standard. In space-constrained applications, very thin plates containing several layers and a limited copper casting thickness of half an ounce (0.7 mil thickness) may be required.
In other cases, designs for high-speed communication from one peripheral to another may require wiring with controlled impedance and specific widths and spacing between each other to minimize reflection and inductive coupling. Or the design may require a certain length to match other relevant signals in the bus. High voltage applications require certain safety features, such as minimizing the distance between two exposed differential signals to prevent arcing. Regardless of characteristics or features, tracing definitions is important, so let’s explore various applications.
Various wiring widths and thicknesses
PCBS typically contain a variety of line widths, as they depend on signal requirements. The finer traces shown are for general-purpose TTL (transistor-transistor logic) level signals and have no special requirements for high current or noise protection.
These will be the most common wiring types on the board.
Thicker wiring has been optimized for current carrying capacity and can be used for peripherals or power-related functions that require higher power, such as fans, motors, and regular power transfers to lower-level components. The upper left part of the figure even shows a differential signal (USB high-speed) that defines a specific spacing and width to meet the impedance requirements of 90 ω. Figure 2 shows a slightly denser circuit board that has six layers and requires a BGA (ball grid array) assembly that requires finer wiring.
How to calculate PCB line width?
Let’s step through the process of calculating a certain trace width for a power signal that transfers current from a power component to a peripheral device. In this example, we will calculate the minimum line width of the power path for a DC motor. The power path starts at the fuse, crosses the H-bridge (the component used to manage power transmission across the DC motor windings), and terminates at the connector of the motor. The average continuous maximum current required by a DC motor is about 2 amperes.
Now, PCB wiring acts as a resistor, and the longer and narrower the wiring, the more resistance is added. If wiring is not defined correctly, the high current may damage wiring and/or cause a significant voltage drop to the motor (resulting in reduced speed). If we assume some general conditions, such as 1 ounce of copper pouring and room temperature during normal operation, we need to calculate the minimum line width and the expected pressure drop at that width.
PCB cable spacing and length
For digital designs with high-speed communications, specific spacing and adjusted lengths may be required to minimize crosstalk, coupling, and reflection. For this purpose, some common applications are USB-based serial differential signals and RAM-based parallel differential signals. Typically, USB 2.0 will require differential routing at 480Mbit/s (USB high speed class) or higher. This is partly because high-speed USB typically operates at much lower voltages and differences, bringing the overall signal level closer to background noise.
There are three important things to consider when routing high-speed USB cables: wire width, lead spacing, and cable length.
All of these are important, but the most critical of the three is to make sure the lengths of the two lines match as much as possible. As a general rule of thumb, if the lengths of the cables differ from each other by no more than 50 mils, this significantly increases the risk of reflection, which may result in poor communication. 90 ohm matching impedance is a general specification for differential pair wiring. To achieve this goal, routing should be optimized in width and spacing.
Figure 5 shows an example of a differential pair for wiring high-speed USB interfaces that contains 12 mil wide wiring in 15 mil intervals.
Interfaces for memory-based components that contain parallel interfaces will be more constrained in terms of wire length. Most high-end PCB design software will have length adjustment capabilities that optimize the line length to match all relevant signals in the parallel bus. Figure 6 shows an example of a DDR3 layout with length adjustment wiring.
Traces and planes of ground filling
Some applications with noise-sensitive components, such as wireless chips or antennas, may require a little extra protection. Designing wiring and planes with embedded ground holes can greatly help minimize coupling of nearby wiring or plane picking and off-board signals that crawl into the edges of the board.
Figure 7 shows an example of a Bluetooth module placed near the edge of the plate, with its antenna outside a thick line containing embedded through-holes connected to the ground formation. This helps isolate the antenna from other onboard circuits and planes.
This alternative method of routing through the ground can be used to protect the board circuit from external off-board wireless signals. Figure 8 shows a noise-sensitive PCB with a grounded through-hole embedded plane along the periphery of the board.
Best practices for PCB wiring
Many factors determine the wiring characteristics of the PCB field, so be sure to follow best practices when wiring your next PCB, and you’ll find a balance between PCB fab cost, circuit density, and overall performance.