3. How to choose a connector when designing electronic equipment?
Objective and subjective factors
What do I mean by electronic equipment?
I am writing to the designer of any device/equipment/system that involves transmission of electrical signals during its operation – be it mechanical, chemical, medical or even space-research.
The criteria for selection can basically be divided into two groups – there are completely objective and highly subjective factors. Let us now look at these with some comments of my own:
The OBJECTIVE (mainly technical) factors must be decided first,
- Standards required by the application conditions, or, if no standards are required, what environmental requirements need to be considered – for example, in case of components of charging equipment placed in the same room with the conventional lead-acid battery. Some of the areas where strict requirements apply not only to the complete equipment, but also to the materials used for the components or accessories installed, and even to the tools used in the manufacturing process, are: automotive, rail vehicles, aircraft, ships, medical equipment, deep-sea applications, aerospace, mining, oil and gas wells, geodesy, chemical industry, pharmaceuticals…
- Immunity
– Traditionally, referred to as protection against dust and moisture (IPXX), but the term impact resistance (IKXX), used for enclosures and switches, is also used for connectors.
– Even if not mentioned in the catalog, it is always necessary to clarify whether IPXX protection applies for a connector while connected or also while not being connected. Many connectors are available with a protective cap that provides some protection only while in place. For most of them, a chain or something similar is available to secure the cap close to its place, but this only protects against loss, it does not put the cap in place. There are spring-loaded, automatically locking caps, but these offer more modest protection.
– The need for possible EMI/RFI shielding should also be considered.
– Whether the application raises shock or vibration resistance issues should also be considered. This may not be critical for a desktop computer, but it may be critical for an industrial computer mounted on a production machine or in automotive electronics – indeed, companies offering connectors for space applications and satellites have specifications for up to 50g shock and 20g vibration resistance.
In 2005, work began on using concepts from an earlier IEC standard to describe the various aspects of protection in a common (joint) specification – this is the MICE concept.
The Mechanical, Ingress, Climate and EMC requirements are grouped into three categories: Office(1), Light industrial(2) and Heavy industrial(3). The environmental resistance of the connectors can be described by a M1I3C2E2. So far I have only encountered the use of the MICE concept when specifying Ethernet connectors. - Ambient temperature.
Some interesting examples: space (-180…+380 C), Gas Turbine (-55…+1200 C), car passenger compartment (-40…+85 C), but roof +125C, engine compartment +150-175C! - Operating voltage/breakdown voltage/leakage current path.
- Current.
– Manufacturers usually specify a load per contact, but it makes a difference whether this current can flow through all contacts of the connector at the same time, or, for example, only on two of the four contacts of a four-contact connector at the same time.
– It should be noted that the current carrying capacity of connectors is temperature dependent.
– The majority of manufacturers publish this (derate-) diagram in their catalogs, while the current flowing through the connector also generates heat on the transient resistance of the connector.
– For wired connectors, the applicable wire gauge is usually a factory specification, but often appears modest when measured against the maximum current specified. It is always necessary to consider whether the current density resulting from the actual load current and the applicable wire/cable construction/cross-section, as well as the voltage drop over the given wire length, is acceptable.
– At PCB connectors the pad design and conductor foil cross-section should be considered as described above.
– Remember that the connector is not a switch. In power inputs, especially in DC applications, inductive or capacitive loads, a load-free connection should be ensured by using a switch – chosen according to the load.
– When designing electronic equipment, it may be necessary to ensure that modules or cards can be connected or disconnected while being active – the use of leading contacts is helpful here. This is usually solved by connector manufacturers by using longer plug contacts. - Frequency/signal transmission rate.
- Transmission mode optical/copper.
In recent decades, more and more connector manufacturers have included optical signal transmission connectors in their portfolios, at PCB, device and cable-to-cable level – now I put into focus the devices for connecting copper conductors. - The connection between the connector and the copper conductor
– The legs of the first generation of PCB connectors were, like the other components, fitted into the hole of the PCB and soldered on the other (solder-) side. This is the through-hole (TH) technology, which is still in use today after many developments in soldering technology.
– Press-fit (Press-in, Pres-xx…) technology is in fact TH, but the connector legs are not soldered, the specially formed legs are pressed into plating PCB holes. Both the feet and the holes (required by the connector manufacturers) must meet very strict dimensional specifications. The catalogs/leaflets usually contain the technological specification for the press-fitting, some also recommend tools and even a back-panel manufacturer can be found. Later, perhaps in the 1980s, surface mount technology (SMT) caught up with connectors.
A Device described as SMD can be mounted in SMT way. In order to ensure a safe retention of the connectors on the PCB, mixed seating connectors were born: some of the signal wires or fixing points are TH-, the rest are SMT bonded.
The use of purely SMD connectors should be avoided in applications where the cabled counter-end is capable of movement that could sooner or later tear the seated connector off the PCB (e.g. powering LED desk lamps with uUSB or similar connectors). A good example to prevent this problem is the fixing of the power connector of a better mobile phone.
So-called one-pice connectors create a connection between two PCBs by being TH- or SMD mounted in one of them, the other one is connected by springing contacts.
The cylindrical version (pogo-pin) has a coil spring inside, which supports the pointed, cone, or crown-shaped contact to the other PCB. An interesting use for these is the use of needle beds to control planted or unplanted PCBs.
A newer version of one-pin connectors is not soldered into either PCB – springing contacts in both directions provide the connection.
The positioning and clamping of the one-piece connected PCBs must be done by the user, using a clamping device and screws sized to take into account the quantity, arrangement and spring force of the contacts – for help, see the connector manufacturers’ websites and catalogs. - Wiring of connectors mostly are
– screw-type
– soldering
– crimp
– IDC
– Splice
– Amplivar
– spring-clamp
– wire-wrap technology.To characterize the size of wires, instead of diameter (mm)/cross-section (mm2), you may find the AWG (American Wire Gauge) value, especially for products of American origin or destined for the US. The AWG standard was developed in America in connection with wire manufacturing technology in the early 20th century – the final version has been in use since 1957. There may also be another name for AWG, the B&S wire gauge.
Back in the last century, there was also the British standard BWG and SWG – neither of which is still in existence. Little used, but it does occur, is CMA (Circular Mil Area), used to describe the useful cross-section of flexible wires.
Conversion tables and diagrams (e.g. connector and cable manufacturers’ catalogs, websites) are available to help with the use of the above.
Screw termination is offered by manufacturers in two versions: with or without wire protection – the user has to choose between these depending on whether they use flexible (bunched) or solid (single strand) wire. When connecting a flexible wire to a socket without wire protection, the use of a ferrule is mandatory. Only flexible (fine (DIN EN 60 228-class 5 or superfine (-class 6)) should be used for axial connection (mostly screw).
When selecting crimpable contacts, it is essential to take into account the cross-section of the wire to be used and then to select the crimp tool (mandatory) for these. Even taking all this into account, crimping will only be of a satisfactory quality if a crimping tool is selected which can only be opened after full compression. Most crimping tools will connect the stripped conductive (copper) part of the wire to the contact, but some will also grip the insulation of the wire to the part of the contact which has been trained for this purpose. Crimping tools for turned and stamped contacts are of different construction!
The quality of crimping can be measured by the pull-out force – the extent of this (depending on the wire cross-section) is specified in a standard (DIN EN 60352-2) and should be checked as often as possible.
IDC means insulation displacement, where the receiving part of the contact is a springing V-shaped cut-out plate. The inner side of the cut-out is sharp, so that it cuts through the insulation of the conductor pressed into it and, due to its springy property, holds the conductor in the cut-out. Strict adherence to the applicable wire gauge, as determined by the size of the cut-out, is required. Connector manufacturers usually offer a tool (hand or bench press) for the use of the connectors.
IDC connectors are widely used for processing ribbon cable ends, but there are also those suitable for receiving solo wire. IDC solutions have also been developed for the termination of wires: the contact is soldered or pressed into the PCB, the wire receiving part is positioned above the PCB.
A splice connection uses a clamp to join two wires together, or a wire and the correspondingly trained leg of the connector. The technology is similar to crimping. In mass production, its heyday was in the manufacture of compact fluorescent lamps (used to connect components together in the header of the fluorescent lamp).
Amplivar bonding is functionally just a ferrule. It is used to bond enameled wires: the inner surface of the ferrule/clamp is designed to cut through the enamel insulation when crimped, similar to splice technology. It is used primarily for the connection of transformers, motors and electromagnets (magnet wire).
In spring wire bonding, there are solutions that require tools for both connection and disconnection, but there are also solutions that require tools only for disconnection.
For wire-wrap technology (a common method of back-wiring racks in the 1970s before the advent of multilayer back-panel technology – still used today), connectors must have specially formed legs and only solid (single) wire of a specified diameter by the tool to fit the connector foot can be used.
Connector manufacturers supply the screw, spring and wire-wrap contacts in all cases incorporated in the connector body, crimp contacts separately, and solder contacts either incorporated or separately. Contacts supplied separately often have to be selected and ordered separately – this is usually pointed out in the catalogs. - Contact forming (according to their production technology).
Most contacts are turned, stamped from sheet metal or a hybrid of these. In high-density PCB connectors, mainly for high-speed signal transmission, contacts manufactured by other technologies have also appeared. - Surface finishing.
The choice of material and surface for the connector body and housing should be primarily determined by the application environment, as discussed above – the surface coating of the contacts should be decided by taking into account, in addition to the above, the current flowing through the connector, the voltage and the expected lifetime (number of connections without significant loss of contact resistance). Some connector manufacturers provide recommendations in their catalogs/websites. - Location fixing.
– PCB connectors will often remain in place even when soldered to signal lines only, but those likely to be subjected to non-perpendicular stresses on the PCBs (especially those with SMT seating) should be held in place by additional TH foot, strap, screw, etc.
The pieces to be mounted on the panel (taking into account the mandatory protection):
– can be mounted from the outside or from the inside
– can be of screw, flange or snap-in type. - Fixing of coupling pairs to each other.
– The fixing of printed circuit board connectors to each other is not necessary if the PCBs are fixed to each other, by means of spacers or other supports.
– In the case of cable to PCB, the use of connector pairs where the connector bodies are not designed to include a locking function shall be avoided.
– Cable to cable or cable to panel, there are many options to choose from: threaded (single or multiple threads), bayonet or reverse bayonet, push-pull, breakaway, stirrup, screw…
Wherever possible, look for a locking solution with the connection in place, which means that extra activity – otherwise forgettable when plugging – is only required when dismissing the connection.
– A magnetic connector pair is found on Apple laptop power connectors. Ring-shaped magnetic connectors have appeared from other manufacturers, where the contacts on one side are pogo (spring) pins arranged one by one along a circular ring of different diameters, the contacts on the other side are gold-plated flat rings of the same diameter as above. In this way, the two sides of the coupling, while maintaining uniaxially, meet clearly at any angular position. The use of such connectors is particularly advantageous where blind mate is required. - Accessories.
– When choosing a housing/backshell/hood (the term backshell is common for round connectors, the hood for D-subs), in addition to the aforementioned question of fixing, the protection requirements, the cable routing, the diameter of the cable/wiring harness and the need for cable gripping and/or cable breakage protection must of course be taken into account. The use of grommets is also common for cable bushings for round connectors – there are also multi-hole rubber grommets for these to bush several wires or cables. Multi-hole housings are available from some connector manufacturers.
– Screws for fixing to PCB or panel, right-angle brackets, etc …
– Protective caps – be careful to choose the right one for the IPXX rating required (there are also versions with a chain to prevent loss or with automatic spring-locking when the other connector is not connected).
– Color coding (to distinguish between several identical-looking connectors mounted on the same device).
Among the technical factors, the possibility of using Hibrid (either PCB or wired) connectors should also be mentioned:
– Various contacts within one connector body (mixed layout).
– Various connector modules within one connector housing (modular, industrial and PCB).
After the technical specification of the connector, it is also an objective criterion to specify the quantity and the time when the connector is needed.
After this, the choice of manufacturer and distributor can be made – these are subjective factors.