Often neglected, connectors are essential parts of many electric and electronic devices, machines and systems. The overwhelming variety of shapes and versions can really make selecting a connector difficult and there is no universal solution in terms of material either – not even solid gold-plated contacts. This article should provide you with a brief overview of the key selection criteria to help you wade through the various options.
Electrical connectors are used to construct detachable connections to transmit power, signals or data between circuit boards, devices or cables. They are available in many – often standardised – sizes, shapes and configurations.
Form folgt Funktion
The form of a special connector is often prescribed by industry preferences or standards developed from experience. What’s more, the original purpose and arbitrary stipulations from the original manufacturer have significantly shaped the form of a specific connector.
Mechanical strength and corrosion resistance are important criteria for structuring and selecting material for a connector. Protection against polarity reversal or unintended separation is now an integral part of the geometry for most common types of connectors. In a specific application, many other factors determine the selection: in addition to the current, voltage and frequency of the signals to be transmitted, the number of mating cycles expected and the connection duration over the lifecycle of devices and connectors also play a role.
Connectors for industrial applications
This article primarily focuses on connectors for use in devices, machines and systems for industrial automation. Tobias Thelemann, Mechanical Components and Automation Product Manager at electronics distributor reichelt elektronik GmbH & Co. KG, says that ‘The digital transformation of production is not only leading to changes at the top of the automation pyramid’. ‘The ever-increasing number of sensors to record the necessary information need reliable connections to the industrial Internet of things.’
At field level in the mid-1980s, M12 connectors with screw locks developed into a universal standard that was designed for direct connection for sensors. However, a wide variety is still prevalent. With M8 technology, M12 connectors now have a ‘little brother’ and are being continuously enhanced. A push-pull quick lock should also make connection and separation easier.
Additionally, proven D-subminiature connectors remain essential and office standards such as RJ45 and USB have permanently established themselves and are now also available in industrial versions.
What’s more, the option of being able to mix data, signals and power within a single connector is becoming increasingly important. At system level in particular, it needs to be possible to connect replaceable machine modules appropriately in accordance with the principle of Industry 4.0.
Secure, durable connectors
The safety requirements for connectors are stipulated in the EN 61984:2009 standard. Although it is intended for connectors with rated voltages from 50 V to 1000 V and rated currents to 500 A, it can also be applied to calculate safety standards for voltages up to 50 V.
The structure and material of the respective enclosure are decisive to meeting the standard’s criteria. The dielectric strength, insulation resistance, creep resistance and the thermal and chemical stability of the insulation material, which surround the contacts and keep them in position, are also important. Reference to compliance with this standard therefore guarantees sufficient safety when choosing the right product.
Verarbeitung und Verwendung der Steckverbinder angepasst
Different versions for assembly also increase the variety of connectors. Various versions are not only needed for installation in enclosures and front panels but for assembly on cables with appropriate enclosures too.
The connection technology of individual connectors also varies to the same extent: on circuit boards, this connection is usually made by soldering on or through and by press fitting. Wires, stranded wires or cables are primarily connected via soldering or crimping – but also with insulation displacement connections. The form and material of the connecting elements used are the main factors to determine suitability for the respective connection process.
During processing and subsequent use, connectors may be exposed to significant loads as a result of temperature variations, vibrations or chemically contaminated atmospheres. These have an effect on the elasticity and the conductivity of the metal parts used for the connections and plug-in contacts.
Everything depends on the plug-in contact
The key element of each pluggable connection is the plug-in contact. It consists of the plug pin in the male part of the connector pair and the plug socket in the female part. The plug-in contact forms a non-positive connection in which the socket firmly encloses the pin using spring force. This also results in an electrical connection between the contact surfaces of these two elements.
There are a large number of various electrical, mechanical and lifecycle requirements for the contact material and they require different criteria to be considered, such as contact resistance. Material with high electrical conductivity between the pin and socket requires low resistance, which pure copper offers. At the same time, the socket contact should not lose its spring force when it is connected over longer periods or subject to frequent mating cycles. One solution for this would be spring steel.
Copper-based alloys such as bronze or nickel silver offer a good compromise between these requirements. Due to its tendency to form microcracks, brass is only suitable for forming to a limited extent. In general, beryllium copper is the best base material for contacts. However, it is only used in very high-quality connectors. As a special form for applications where there is no middle ground, there are also socket contacts made from pure copper, which plug into stainless steel oversprings and receive their spring force from these.
The right surface for every application
‘The base material is not suitable for the actual electrical contact surfaces,’ says Tobias Thelemann. ‘A surface finish is required to guarantee contact resistance that remains constant over the whole service life of a contact system, to prevent corrosion at the point of contact and to be able to minimise friction corrosion.’
To prevent corrosion, coating surfaces with tin is the cheapest option. However, relatively high contact forces are then required when trying to wear through the tin oxide on the surface. Additionally, tin-plated contacts are actually only suitable for connections with very low (a maximum of 25) numbers of mating cycles such as applying piggyback modules to circuit boards.
The surface is usually made from precious metals such as gold or silver due to the high conductivity. For cost reasons, the thickness of the precious metal layer is kept as thin as possible for this. Common layer thicknesses are 0.1 µm ‘gold flash’ for up to 100, 0.76 µm for up to 250 or 1.2 µm for up to 500 mating cycles.
All that glitters…
Silver is the first choice for high-current applications thanks to its high conductivity. However, it produces material in the form of silver sulphide (the black material on silver cutlery) over time. This is why silver should only be applied from a layer thickness of 2 µm. It is for this reason that some manufacturers are working on multilayers (tin-silver alloys) to obtain a cheaper alternative that simulates the gold-like behaviour of the contacts.
It is not possible to electroplate metal surfaces of less than 2 μm so that they are porefree. This is why an additional barrier layer needs to applied underneath in this case. This is the only way to prevent base atoms on the contact surface from diffusing through Brownian motion and creating corrosion, which would increase the contact resistance.
This is why, under the gold layer, high-quality plug-in contacts often have a nickel layer about 2 µm thick. This metal is resistant to air, water, dilute acids and the most common lyes and is therefore particularly suitable as a coating material.
Qualität der Steckverbinder mit Herkunft
Connectors with different contact surfaces should never be used. The different metals would form an electric current together that would inevitably cause surface corrosion. In contrast, different designs of connector pairs can be mixed within the same application since there is no risk of corrosion from this.
In industrial automation with its customised projects in particular, it is worth considering the costs and technical properties for every part. According to Tobias Thelemann, ‘It is a good idea to choose a partner that can supply a wide spectrum of connectors with industrial longevity, offer the ability to re-purchase over the long term and provide competent application advice.’ ‘This limits the time spent choosing the product and enables clients to benefit from economies of scale.’
Image: Adobe Stock
