Short for "Universal Serial Bus," a USB cable is one that has become a nearly ubiquitous part of our lives, even if some people don't necessarily realize it. Technologically speaking, it involves an industry standard that created precise specifications for not only how these types of cables can be manufactured, but how they can connect one device to another, how power and data can be transmitted, and more.
What Types of Products Use USB?
Most consumer electronics still use some variation of the USB protocol. Smartphones, for example, typically come with a USB charging cable, be it traditional USB or micro/mini variants. One notable exception to this is the Apple iPhone, which still uses the company's own lightning cable design. Anytime you need to connect a computer to a peripheral device like a laptop or mouse, it will likely use a USB cable to do so.
The History of USB: Breaking Things Down
The USB 1.0 protocol was first released back in 1996. This standard specified necessary signal rates of up to 1.5 megabits per second at "low speed" and 12 megabits per second at full speed. Eventually, this was replaced by USB 2.0 in 2000. These cables had advanced to the point where their maximum (theoretical) data throughput grew to 1.2 megabytes per second.
USB 3.0 made its public debut in 2008. This was notable due to the addition of a "Super Speed" transfer mode enabling a theoretical data transfer rate of up to 5.0 gigabytes per second.
Of course, variations of this have been made over the proceeding years. USB 3.1 was released in 2013 with increased power capabilities and data speeds. USB 3.2 was released in 2017 and continued to make further improvements on the "Super Speed" mode, etc.
Advantages and Disadvantages of USB Cables
The major advantage of USB cables has to do with their versatility. As their popularity has grown, the economies of scale have helped reduce the overall cost of making USB cables and connectors. Because they are inexpensive, can be easily customized, and are able to carry high-speed data, they can be used with virtually any type of consumer electronic device as needed.
Along the same lines, USB devices are designed with the ability to carry more data faster than the other types of cables that preceded them. USB cables are typically built with one or more twisted pairs of data cables, allowing for higher data transfer speeds when compared to traditional multiconductor cables. Additionally, as the USB generations have progressed, their connectors have been downsized with more power and data being able to be transferred with decreasingly smaller connector footprints. Smaller connectors usually mean less cost and less real estate needed on your PCBA.
Perhaps the only major downside of these types of USB cables is that they don't have an unlimited lifespan, although they were never designed that way in the first place. They're inexpensive, they're effective, they're efficient. Yes, most people will replace their USB cables every few years, but that doesn't outshine the major benefits that they bring to the table.
The Most Common USB Connectors for Cables
One of the most common types of USB connectors for cables is called USB Type A. These are specifically designed to be a standard and low-cost interface. Usually, with 4 pins and a termination to “ground” or the connector “shell”, USB A connectors can transmit both power and data.
Another common interconnect type is the USB B, which is square in shape as opposed to the rectangular version that most people are familiar with.
Example of the most common USB cable connectors.
USB micro connectors are also common, especially in portable devices like smartphones, e-cigarettes, and many other types of consumer electronics. This is essentially a smaller version of the standard USB capable of transferring power and data. This interface is still very common today just as it was during the days of GPS units, MP3 players, and similar types of consumer electronics.
Common USB Cable Lengths and Wire Types
Generally speaking, the most common USB cable lengths are those manufactured at 3 feet, 6 feet, and 10 feet. This is because the types of devices that use these cables don't necessarily need to go beyond these standards. The USB charger that comes with your phone, for example, is very likely 3 feet in length.
Additionally, there is a maximum overall length allowed for early-generation USB protocols. The long cable has more wire and an increased electrical resistance creating a voltage drop over long distances. This can degrade signal integrity and reduce the total power transferred. Simply put, there are laws of physics that limit the maximum possible wire length that can be used for USB cables with standard copper conductor cables.
In terms of wire sizes, typically 26 AWG is used during construction however this size can vary depending on the application. USB wire typically follows UL2464 or another similar and readily available wire. It's important to note that some USB cables use multiconductor cables that are comprised of four discrete wires and an overall shield. Examples of this include USB Type A and USB Micro Type B. Two of those wires are used for power, while two are used to send and receive data.
Some USB cables use multi-conductor cables with up to 24 wires, with USB Type C being the prime example. This offers higher power and faster speeds when compared to previous generations as numerous differential pairs are needed.
How Overmold USB Cables are Built
Overall, USB cables are built to be inexpensive and to survive the rough handling that often comes with everyday use. Most USB cables are overmolded, so that they're as rugged and as cost-effective as possible.
Many factories maintain an array of "house tools," all of which can be used by any customer. This is particularly helpful when it comes to the connector families of USB Type A, Type B, and Type C cables. Mini and micro USB cables can benefit from this process, too. These house tools represent cost savings and a reduction in the total time it takes to make the first samples.
While saving money and scheduling on a new project is preferred, if a truly custom overmold tool is needed then this will have a small cost and timeline impact. Currently, production tooling is usually less than $2,000 per design and adds a few weeks to the total project. By considering a custom overmold tool, adding custom markings and artwork is possible. Additionally, the precise shape of the overmold can be completely customized to meet whatever design requirements exist.
Summary
Launching any new project for a custom cable assembly requires information regarding the wire specs, connector type, cut lengths, etc. Using a USB standard protocol and connector scheme helps standardize many of these variables, which can save time and money. Another benefit to using the USB standard for connectors is that this is not a single-sourced item, and there should always be stock of the raw materials. Early engagement with a full-service manufacturing partner like Epec can help review these critical cable assembly design questions before it’s too late.
