Connector insertion and withdrawal force

What the numbers mean

Insertion force is the peak force needed to mate a connector. Withdrawal force is the force needed to separate it. Mating force usually means the insertion figure, and unmating force means the withdrawal figure. The terms are close enough that a drawing should use one pair and define it.

Neither number is a single value. The force changes through the travel: it rises as the contacts engage, falls as they settle, and rises again when a seal or a latch is compressed. The peak is what the user feels and what the test records. The average is useful for the mechanism, but the peak decides whether the housing cracks.

The force also changes with the number of mates. A new connector is usually stiffer than one that has been mated twenty times. A specification that quotes only the first mate is incomplete.

Where the force goes

The main term is contact friction. Each contact spring presses on the mating pin with a normal force, and the sliding surfaces resist with a friction force. Multiply by the number of contacts and add the effect of any wipe or lead-in geometry.

Seals add a second term. An interfacial seal, a cable gland or a radial O-ring is compressed during mating, and that compression is felt as force. A sealed connector can need noticeably more force than the same housing without seals.

Latches and locking rings add a third term. A push-pull latch, a bayonet ring or a screw coupling has its own force profile, and the peak often sits at the point where the latch rides over its detent. A connector with a secondary lock can have a separate, smaller force event after the main contacts engage.

Finally, misalignment adds force. If the two halves are not parallel, the contacts bind and the force rises. A floating panel mount or a lead-in chamfer reduces this term. A rigid, badly aligned installation can double the force that the contact design predicts.

Setting a force window

The upper limit is set by the housing, the panel and the user. The lower limit is set by contact stability. Somewhere between them is the window that the design must hold over life, temperature and the permitted number of mates.

For the upper limit, consider the hand force that a technician can apply in the installed position. A connector behind a panel, at arm's length, with no lever, is much harder to mate than the same connector on a bench. If the force is high, add a lever, a screw coupling or a cam, and specify the force at the lever rather than at the contacts.

For the lower limit, think about contact resistance and fretting. A contact that is barely touching will pass a continuity check and then drift. Vibration and thermal cycling cause small movements at the contact interface, and a low normal force allows oxide to build and resistance to rise. The withdrawal force is a useful proxy for normal force, which is why it is tested.

Specify the force at the first mate, after a defined number of mates, and at the temperature extremes if the application is severe. A single room-temperature figure does not cover a connector that runs hot or cold.

How to test it

Use a tensile or compression tester with a controlled speed and a free-floating alignment fixture. The fixture should let the connector self-align in the two lateral axes, so that the test measures the connector rather than the rigidity of the machine.

The two common references are EIA-364-13 for mating and unmating forces and IEC 60512-13-1, test 13a, for engaging and separating forces. They define the rate of travel, the preparation and the record. Follow the rate in the standard rather than choosing a convenient one, because force depends on speed.

Record the full force-displacement curve, not only the peak. The curve shows where the seal compresses, where the latch engages and whether the contacts are binding. A single peak number hides the shape of the problem.

Test the finished assembly with the correct cable, the correct backshell and the correct panel mounting. A bare connector on a bench is a different mechanical system from a harness hanging from a panel. If the connector is sealed, test it with the seal fitted and lubricated as specified.

Contact force, resistance and wear

The contact spring provides the normal force. For many small signal contacts that force is in the range of 1 to 3 N, and for power contacts it is higher. The exact value belongs to the contact design, not to a general rule.

More normal force lowers contact resistance and makes the joint more tolerant of vibration and oxide. It also increases insertion force and wear. A high-force contact in a connector that is mated daily will wear its plating sooner than a lower-force design. The right balance depends on the number of mates and the environment.

Plating choice changes the friction. Tin plating is soft and has a relatively high friction coefficient, so a tin contact needs more insertion force than a gold contact of the same geometry. Gold is smoother and more stable at low force, which is one reason it is used on signal contacts that see many mates.

Lubrication can reduce insertion force without changing the spring. A thin, compatible lubricant on the contact and the seal is common in sealed connectors. It must be specified, because the wrong lubricant can swell the seal or attract dust.

The user experience

A connector that is hard to mate will be forced, and forcing damages housings, bent pins and cracked solder joints. A connector that is too easy to mate may not latch, or may feel connected when it is not. The force profile is part of the user interface.

Give the user feedback at the end of travel. An audible or tactile click from a latch, a visual witness line or a secondary lock that only closes when the connector is fully home all reduce half-mated connections. The force at that point should be distinct but not so high that the user stops before the latch engages.

For high circuit counts, a lever or a screw coupling changes the effort and the feel. The specification should then be written at the actuation point, with the mechanical advantage stated. A contact force inside the connector can be correct while the lever effort is unacceptable, or the reverse.

Test with a representative group of users if the connector is hand-mated in the field. A force that is comfortable on a bench can be difficult overhead, in gloves, in the cold or in a restricted space.

Illustrative force bands by application

The table gives starting bands for discussion, not acceptance limits. The real limit depends on the contact system, the circuit count, the seal and the installation. All values are illustrative.

What to put on the drawing and the test report

State the maximum insertion force and the minimum withdrawal force, each with the test standard, the rate, the number of mates and the temperature. State whether the values apply to the first mate or to a conditioned sample.

Add the alignment condition. A force measured with a free-floating fixture is not the same as a force measured with a rigid fixture and a misaligned panel. If the installation cannot guarantee alignment, the drawing should say so and the test should reproduce it.

Ask for the force-displacement curve with the report. Note the peak, the travel at which it occurs and the force at the latch. If the connector has a seal, record the force with and without it so that the seal contribution is visible.

Finally, link the force to the electrical test. A sample that passes the force window but has a rising contact resistance is not a good connector. The force test and the resistance test belong in the same qualification plan.

• Maximum insertion force and minimum withdrawal force
• Test standard, rate and alignment fixture
• First mate and after a stated number of mates
• Temperature at test and any conditioning
• Seal fitted or removed, and lubricant
• Force-displacement curve with the report
• Contact resistance before and after the force test