Connector Selection for PCB Assemblies
Why Connectors Decide Reliability
A connector is often the component most likely to fail in a product, because it carries the mechanical loads as well as the signals. It is mated and unmated, it is pulled and bent, it is exposed to the environment, and it is the interface between the product and the world. The choice of a connector therefore affects the reliability of the whole assembly, and it deserves more attention than the last item on the schematic. A connector selected for cost alone frequently becomes the most expensive part in the product.
The Electrical Ratings
The current rating of a connector is quoted per contact and depends on the temperature rise of the contact, not on the wire alone. It is usually given for a single contact with all others unloaded, so a connector with many contacts carrying current simultaneously has a lower effective rating, and that derating has to be applied. The voltage rating depends on the spacing and the insulation, and it must consider the altitude and the pollution degree of the application, since the breakdown voltage falls with a reduced air pressure and with contamination. The contact resistance matters for a low level signal and the insulation resistance for a high impedance one, and a connector specified for power may be poor for a sensitive analogue signal.
Mating Cycles and Wear
The number of times a connector will be mated and unmated is one of the most useful selection criteria and one of the most often ignored. A board to board connector that is mated once during assembly needs a very different design from a service connector that is plugged in daily. The plating determines the wear: gold on gold is the standard for a high cycle count, gold on tin is acceptable for a moderate one, and tin on tin is suitable only for a few cycles because the tin oxide is abrasive. Where the cycle count is high, a connector with a wiping action and a gold interface is the practical choice, and the cost difference is usually small compared with a field failure.
Mechanical and Environmental Factors
The connector has to survive the mechanical environment. A locking or latching mechanism prevents the connector from working loose under vibration, and a polarising key prevents it from being mated the wrong way round. The retention force of the contacts holds the wire and resists the pull that a cable harness will experience. The temperature range, the sealing requirement and the resistance to chemicals all follow from where the product will be used. Where a connector is expected to be mated by a user, its orientation and its tactile feedback matter as much as its specification, because a connector that can be forced in the wrong way will eventually be.
Board Level Considerations
The connector also has to suit the assembly process. A through hole connector may be easier to secure mechanically but requires a wave or selective soldering step and occupies space on both sides. A surface mount connector allows a single reflow but its solder joints carry the mechanical load, so it needs a larger footprint and often a reinforcement such as a through hole peg or a metal bracket. The pitch and the height matter for the routing and for the enclosure, and the connector’s height tolerance should match the mechanical design, because a connector that is taller than expected can prevent the enclosure from closing or strain the board when it is screwed down.
Second Sourcing and Availability
A connector is a part with a long life cycle, and the same physical connector is often available from several manufacturers in a compatible form. Where the product will be built for years, the availability of the connector over that life should be considered, along with the option of a pin compatible alternative. A connector that is unique to one supplier and is discontinued five years into a product’s life creates a redesign that is far more expensive than the small premium of a widely available part. For a high volume product, a second source qualified at the design stage is worth the effort.
Verification and Qualification
The selection should be confirmed by test rather than by catalogue. A mating and unmating test to the specified cycle count, a contact resistance measurement before and after, a vibration and shock test and a temperature cycling test are the minimum for a critical connector, and a salt spray or humidity test where the environment requires it. The tests should be run on the actual assembly rather than on the connector alone, because the solder joint and the board’s stiffness are part of the system. Where a connector has been used on a previous product with a good field history, that history is evidence, and it should be recorded so it can be used again.
FAQ
What is the most important connector rating? It depends on the application, but the current per contact with derating and the mating cycle count are usually the two that decide the design.
Why derate the current rating? The catalogue value assumes one contact loaded, so simultaneous current in several contacts produces more heat than the rating allows.
When is gold plating needed? For a high mating cycle count or a low level signal; tin is adequate for a low cycle count.
Why does a surface mount connector need a bracket? The solder joints carry the mechanical load, so they need reinforcement for a cable that is pulled.
How should a connector be qualified? By mating, resistance, vibration and thermal cycling tests on the actual assembly, not on the connector alone.
Conclusion
A connector carries the load, the environment and the user, so choose it for the electrical rating, the cycle count and the mechanical retention rather than for its price. Qualify it on the assembly. Interconnect design belongs to PCB design and layout, the assembly that mounts it sits in PCB assembly, and the qualification is part of PCBA testing. Connector choice for a new product is settled during prototype PCB assembly in 2026.
