Four numbers, and one of them is usually misread
Connectors are frequently selected on a single headline figure — almost always the current rating — and then cause problems that have nothing to do with current. The four attributes that actually decide fitness are pitch, current per contact, mating cycles, and plating. Two further attributes, keying and retention, decide whether the assembly survives its environment.
Pitch
Pitch is the centre-to-centre spacing between contacts, and it must match both the PCB footprint and the mating cable assembly. Common values run from the familiar through-hole spacing down through a series of finer surface-mount pitches, and the families are not interchangeable.
- Pitch determines how much board area a connector consumes, which is usually why fine pitch is chosen.
- Fine pitch brings consequences: tighter footprint tolerance, harder inspection, and rework that is far less forgiving by hand.
- Do not infer pitch from the overall connector width. Body width and contact count both vary independently of pitch, and a size measured off a drawing is a common source of wrong parts.
- A connector's pitch is part of its identity, so two connectors with the same contact count can be entirely incompatible.
Current rating, and why it is not the number on the front page
A connector's current rating is quoted per contact, and it carries two important conditions that are easily dropped:
- Derating for multiple contacts. The rating normally assumes only a limited number of contacts are carrying current at once. Energise all of them and adjacent contacts heat each other, so the permissible current per contact falls. Housing material, contact size and airflow all affect how much.
- Derating for ambient temperature. The contact's temperature rise adds to the surrounding temperature. At high ambient, or inside an enclosure with no airflow, the same connector carries meaningfully less current.
The practical discipline is to read the manufacturer's derating curves rather than the summary rating. A connector quoted at one figure in the headline may permit a small fraction of that across all contacts in a hot, sealed enclosure — which is exactly the situation in most real equipment.
Mating cycles
Connectors carry a durability rating expressed in mating cycles, and the spread across families is enormous — from a couple of dozen cycles for a board-to-board part that is meant to be mated once, to thousands for a connector intended for repeated connection.
This matters most where a connector is used for something other than its design intent: a service connector treated as a test point, or a development board that gets re-mated daily. Contact resistance degrades with cycling as plating wears, and the failure appears as intermittent connection long before the connector looks worn.
Plating, and the gold-versus-tin decision
- Gold is chosen for low-level signals and low mating force, where a stable contact resistance matters and where the contact force is too light to break through a tin oxide film.
- Tin is cheaper and suits higher-force contacts that wipe through their own oxide layer. It is not the right choice for very low-current, low-force applications.
- Gold thickness matters. A thin gold flash over a barrier layer performs nothing like a thick hard-gold contact, and thin gold over a porous layer can leave the underlying metal exposed.
- Mixing finishes on a mating pair deserves care. Dissimilar metals in contact under vibration or thermal cycling are associated with fretting corrosion and rising contact resistance, and tin-to-gold combinations are a well-known example.
Keying and retention
Two connectors with the same pitch and contact count will usually mate with each other, including in the wrong orientation. Keying and polarisation exist to prevent that, and they are worth insisting on wherever a mis-mate would damage the assembly.
Retention decides whether the connection stays made. Options run from simple friction through to latches, locking ramps and screw fixings. In a vibrating environment, friction alone is rarely enough, and strain relief on the cable — a separate feature from the connector's own retention — is what stops the load reaching the contacts.
Also worth specifying
- Voltage rating, which is often limited by pitch: fine pitch reduces the creepage distance available
- Contact resistance and the resistance change after cycling
- Operating temperature range and housing flammability rating
- Wiping length, and whether the contacts mate before or after the ground contacts
- The termination method: solder, crimp, insulation-displacement or press-fit, each with its own tooling
Where connector selection usually goes wrong
- Headline current rating applied to all contacts simultaneously
- No temperature derating for a sealed enclosure
- Mating cycle rating ignored because the connector is assumed to be plugged in only rarely
- Pitch measured from an overall dimension instead of the datasheet
- Gold and tin finishes mixed on a mating pair
- Connector retention relied on where the cable needs strain relief
- A replacement chosen on pitch and contact count alone, with keying, plating and current rating left unchecked — the same trap that catches buyers sourcing through the suffix, as our guide to reading a part number describes
Specifying a connector
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Send the full ordering code with quantity, the packaging format you need and your destination and we will confirm availability, pricing and lead time. Our notes on tubes, trays and reels explain how connectors are supplied for a line, and you can browse the full component inventory or start from Samtec, Molex or JST.