Connector Selection Design Guide

Connector selection is usually done late, from a catalogue, by whoever has the fewest other tasks. That is the wrong order. The connector sets the pin count, the board outline, the panel height and the assembly method, and a connector chosen at the end of a project can force a respin on its own. The work is a system decision and it starts with the interface the product has to meet.

Start with the Interface

The first question is what the connector has to mate with: a cable, another board, a panel cutout or a test fixture. The answer fixes the gender, the pin count and the mechanical envelope before any electrical question is asked.

The second question is how the product will be assembled and serviced. A connector that is mated once in the factory and a connector that a technician plugs in daily have very different requirements, and the difference is measured in mating cycles.

The third question is the environment: temperature, humidity, vibration, salt spray and cleaning. A connector that is rated for an office product will fail in a washdown enclosure, and the failure is usually corrosion of the contact rather than a broken housing.

Writing those three answers down before the search begins removes most of the catalogue. The rest of connector selection is checking numbers.

Electrical Ratings

Current rating is the first number to check, and it is rarely the number in the headline. The rating depends on the number of contacts that are energised, on the ambient temperature and on the allowed temperature rise of the contact.

Derating is normal: a connector rated for five amperes per contact at a single contact may be rated for two or three when every contact is loaded. The vendor derating curve is part of the specification and it should be read rather than assumed.

Voltage rating is set by the creepage and the clearance between contacts and by the material of the housing. A high voltage connector uses a wider spacing and a more tracking resistant plastic, and it is physically larger for the same pin count.

The last electrical item is the isolation between contacts, which is a question of the housing plastic and of the pollution degree of the environment.

Contact resistance is the third number. It rises with wear, with corrosion and with the contact force, so a new connector that measures a few milliohms may measure ten times that after the specified number of mating cycles.

Mechanical Life and Mating Cycles

The mating cycle rating is a wear figure, and it applies to the contact plating rather than to the housing. A connector rated for thirty cycles is a production only part, while one rated for thousands is intended for a serviceable interface.

The plating system sets the life. Gold over nickel is the standard for a high cycle count, while tin plating is cheap and wears through quickly. Tin against tin also frets, and the oxide that forms raises the contact resistance in a way that looks like an intermittent fault.

The contact force is the other half of the story. A higher force gives a more stable contact and a faster wear rate, so the vendor balances the two for the intended life. A connector that is operated beyond its rated cycles is a reliability risk with no visible symptom.

Where a high cycle count is needed, a connector with a sacrificial contact or a replaceable mating half is used, and the replacement interval belongs in the service manual.

Polarisation and Keying

Polarisation prevents a connector from being mated the wrong way round, and keying prevents one connector from being plugged into the wrong position. Both are mechanical features, and both are there to protect against a mistake that a human will eventually make.

A single polarising feature such as a chamfer or a rib is enough for a small connector, while a large interface may need several keys so that the harness cannot be cross connected. The key pattern has to be unique within the product family.

Keying also has to survive the mistake of forcing a connector. A well designed interface will not allow a wrong insertion even with a reasonable force, and the housing will break before the contacts are damaged.

Where two connectors of the same type sit next to each other, different key patterns or different colours are the usual way to keep them apart. The tolerance and reliability work should include the assembly error case, because a swapped connector is a common cause of a field return.

Board Interface and Footprint

The board side of the connector is a footprint, a mechanical outline and a soldering process. The footprint has to match the vendor drawing exactly, including the recommended pad sizes, because a pad that is too small gives a weak joint and one that is too large steals the clearance to the neighbour.

The retention of the connector on the board deserves as much attention as the contacts. A connector held only by its signal pins will be torn off by a cable pull, so the housing needs locating pegs, metal tabs or screws that carry the mechanical load.

The assembly process also matters: a through hole connector is soldered in a wave or a selective process, while a surface mount one goes through reflow. The choice affects the panel layout, the thermal profile and the order in which the parts are placed.

A high pin count connector on a thick board is a soldering challenge, and the release checklist should record the process and the inspection method before the footprint is frozen.

Qualification and Second Source

A connector that appears in one catalogue from one vendor is a supply risk, and the risk is largest for a part with tooling that no one else owns. A second source or a pin compatible alternative should be identified before the design is released.

Qualification includes the mechanical and the environmental tests that the product will see: vibration, thermal shock, humidity and the mating cycle test. A connector that passes the electrical tests and fails the vibration test is a design change rather than a batch problem.

The documentation should record the vendor part number, the plating option and the key pattern, because a suffix that looks insignificant in a catalogue can change the plating and the life of the part.

A connector choice is also a tooling commitment, so the decision should be reviewed with the purchasing team and with manufacturing. The quality characteristics of the product include the connector, even though it is not made on the board.

FAQ

How do I choose a connector for a high current path? From the derating curve, with every contact loaded and at the ambient temperature of the enclosure. The headline rating is measured with a single contact and is not the working value.

Is gold plating always better? For a high mating cycle count, yes. For a connector that is mated once, tin is cheaper and adequate, though tin fretting has to be considered in a vibrating product.

What is the most common connector mistake? Choosing the part before the interface and the assembly process are defined, which forces a respin of the footprint or a change of the enclosure.