Know Your Terminals Part 1Follow article
While the shell of a connector provides mechanical robustness, protection from the elements, and shielding against electromagnetic interference (EMI), it is the contact or terminal* that defines the performance of the finished product.
In this series of articles, we will take a look at terminal designs, how their construction affects their capabilities and why one technology might be chosen over another. In this first part, we will look at stamping and forming.
Connectors are designed to work in tough conditions. While the connector shell normally protects the contacts from contamination, they are still subjected to shocks and vibration. Ideally, even under these demanding conditions, the terminals must resist any vibration and stay in contact with each other. To achieve this, every terminal design uses spring pressure to ensure positive contact, even when being knocked around.
The simplest terminals are constructed using a technique known as stamping and forming. The process begins with a continuous strip of metal that passes through a number of dies. These remove unwanted parts and shape the metal into a 3-dimensional form as required. The image above shows just such a strip that has been removed from the presses partway through manufacturing. Beginning on the left, the incremental nature of the manufacturing process should be clear to see as each terminal begins to take shape. Once the terminals are fully formed, they remain joined to the strip of metal, which makes handling and plating much easier.
Plating is also carried out while the contacts are part of the strip. Plating is a vital step in ensuring the performance of any contact, as it improves electrical conductivity and provides protection for the metal beneath. The image below shows a strip of contacts moving through one of the baths used in a plating facility. A plating line is long, consisting of multiple baths, and the contacts enter one after the other to deposit the required plating onto the surface.
The end result of this process is a long strip or bandolier holding thousands of contacts that have been formed from a flat sheet of metal like high-tech origami. The continuous nature of the manufacturing process means that it is well suited for mass production. With the exception of power connectors, almost all of the connectors found in the home will use stamped and formed contacts, from the HDMI connector in a flat-screen television to the USB type C connector at the base of a smartphone.
Sometimes it is obvious that a terminal is constructed using this method – the wiping contacts found in USB connectorsare clearly made by shaping a flat sheet. Identification becomes harder when looking at cylindrical terminals. However, creating a 3-dimensional shape from a 2-dimensional sheet leaves a seam where the two sides join. In a complex contact such as the Souriau Trim Trio terminal shown below, the tell-tale is the line that can be seen in the lower right of the picture. Even if the rest of the contact is hidden inside the connector housing, this seamline gives it away.
Knowing the construction is important, as it defines the performance of the terminal itself. The first aspect to understand is the current rating. Stamped and formed terminals are hollow, and this means that there is less material. Carrying high power is largely a function of the cross-sectional area of the terminal. The fresh air in the centre of the terminal means that the area available to transmit power is a function of the thickness of the strip from which it was formed rather than the diameter of the terminal as a whole.
Power and Reliability
Power capability is also a function of the area of physical contact between the two halves of a connector system. Modern connectors such as the Mega-Fit from Molex feature multiple points of contact to increase this surface area. I remember being really impressed with the design of the Mega-Fit when it was released, as this enabled it to carry a current of up to 23 Amps, even with stamped and formed terminals.
The other key characteristic of the terminal design is reliability. Many traditional connectors, including the highly popular D-subminiature, use a pin and receptacle arrangement. A fixed male pin slides inside a receptacle. The inner diameter of the receptacle is slightly smaller than the pin diameter, and the spring-loaded design creates a positive spring pressure. The short video below demonstrates the process perfectly. As the pin enters the receptacle, it is possible to watch the spring parts of the receptacle displace. The strength of these spring parts, and their ability to perform this operation time after time, defines the reliability of the terminal.
More recently, connectors featuring stamped and formed terminals have moved towards a wiping or sliding design. In this, the two terminals slide against each other to provide positive pressure. This design will be familiar to anyone who has looked inside a USB connector. Wiping terminals have the advantage of being easier to manufacture, as the metal does not need so much work to form the final design. They also provide higher reliability as there is less stress placed on the terminal during each mating cycle, demonstrated by the animation below. Considering the thousands of times that a USB connector might be used in its lifetime, this reliability is vitally important.
Stamped and formed terminals are a huge part of the modern connector industry. So many of the connectors that we use daily employ stamped and formed designs. They are ideal for high-volume production and are therefore cost-effective. What is more, recent advances have improved both the current carrying capability and the mechanical reliability of these terminals.
However, for applications in harsh environments or where precision is needed, stamping and forming is not the correct solution. Under these circumstances, the interconnect industry turns to a different manufacturing process that can deliver superior performance when it really counts.
Tune in next time when we’ll investigate screw machined terminals. We’ll consider concepts such as open and closed entries, and we'll look at exotic designs including hyperboloid and lamella baskets.
* Please note: While the names "contact" and "terminal" are often interchangeable, in these articles we will talk about terminals and terminal design in order to distinguish them from the act of making contact.