Connector Ratings and Derating: Current, Temperature and Mating

A connector that is rated for five amps per contact does not deliver five amps in every application. The rating assumes a set of conditions, and where the conditions differ the rating must be derated.

What the Rating Assumes

A current rating is established with all contacts energised, at a specified ambient temperature, with a defined temperature rise limit and a defined wire size.

Changing any of those conditions changes the usable current. Energising fewer contacts reduces the heating and allows more current in each, which is why a connector may be rated higher when only some of its contacts carry current.

The wire size matters because the wire conducts heat away from the contact. A small wire removes less heat and the contact runs hotter for the same current. Our current capacity notes describe the same principle applied to a trace.

Derating Factors

The temperature derating accounts for the ambient. A connector rated at twenty five degrees must be derated at seventy, typically by a factor of one half or more.

The contact derating accounts for the number of energised contacts. It is specified by the manufacturer as a curve and it is often overlooked.

The altitude derating accounts for the reduced air density, which reduces both the cooling and the breakdown voltage. It matters for equipment that operates at high altitude. Our industrial assembly notes describe the environmental conditions that apply.

Contact Resistance and Its Growth

A contact resistance is specified for a new connector and it grows with the number of mating cycles and with contamination. The growth is caused by wear of the plating and by the accumulation of oxide and debris.

The mating cycle rating is the number of cycles over which the resistance stays within specification. A connector that is unmated and remated during service consumes that rating.

Where the contact resistance rises, the power dissipated in the contact rises, which raises the temperature and accelerates the growth. The failure is therefore progressive rather than a single event. Our quality notes describe the measurement of the contact resistance.

Voltage Rating and Creepage

The voltage rating of a connector is limited by the insulation and by the creepage distance between contacts. It is not the same as the current rating and it must be checked separately.

The rating is reduced at altitude and in polluted environments, for the same reasons as the board level creepage rules.

Where a connector carries both a high voltage and a sensitive signal, the physical separation between the contacts matters, and a connector with a mixed arrangement may need a shielded or separated version.

Mating and Mechanical Considerations

The mating force increases with the number of contacts, and it must be compatible with the mechanical support of the board. A connector with two hundred contacts requires a force that a connector soldered to a board cannot transmit without support.

The connector must be located so that the mating force is taken by the housing and the enclosure rather than by the solder joints. A connector that is only held by its solder joints will fail at the joints.

The number of mating cycles also depends on the application: a connector that is mated once in the factory has a different requirement from one that is used daily. Our component selection notes describe the parameters that are often omitted.

Verifying the Choice

The verification is a temperature measurement on the contacts at the maximum current and the maximum ambient, with the specified number of contacts energised.

The measurement is made with a thermocouple on the contact or by thermal imaging with the emissivity corrected. A temperature rise above the specification means the contact is outside its rating for the conditions.

The contact resistance should be measured before and after the temperature test, since a rise indicates that the contact has degraded rather than merely run hot. The comparison is more informative than either measurement alone.

Process Control and Verification

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

Verification and Records

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

Points to Confirm at First Article

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

Documented Process Control

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

FAQ

Can a rated current be used directly? Only where the ambient, the number of energised contacts, the wire size and the altitude all match the conditions of the rating.

Does a gold plating remove the need for derating? Gold resists oxidation and it does not change the thermal rating, which is set by the contact geometry and the material.

What does gopcb provide for connector selection? We provide a derating calculation for the actual conditions, contact temperature measurement at the maximum load and ambient, contact resistance measurement before and after the test, and support recommendations so that the mating force is not carried by the solder joints.