Be aware of connector mating-cycle limits

It’s easy to overlook vendor-specified mating cycle numbers for connectors and thus encounter problems over time.

No, this title isn’t some kind of joke about how your connectors, if left alone at night, will magically spawn new mini-versions of themselves or miraculously grow new contact positions. Instead, it’s a serious issue that is easily overlooked and ignored until it hits the designer with frustrating consequences during debug, longer-term evaluation, or even in the field.

Connectors are meant to be plugged (mated) and unplugged (unmated) – obviously, that’s their role. But how many such mating cycles can the connector pair tolerate before performance degrades with high resistance or intermittent contacts, leading to hard-to-trace system problems? Designers may not give much thought to a connector’s requirements for mating cycles, which can range from low double digits to thousands of cycles depending on the product, its users, and the application – but they should.

Vendors specify their connector performance s for a defined number of such cycles, which is a function of overall connector design, as well as contact design, material, and plating and when used at defined voltage and current values. Yet it is easy to exceed that number in the real world of end-user applications.

Connectors: too often taken for granted

The physical contact may look simple, but it is not, as it has to implement multiple electrical and mechanical considerations in a cramped space while meeting many industry standards for performance, safety, flammability, and more. Among the top-tier connector performance metrics are contact resistance and mating/unmating forces. The first number should be low and stay that way, while the second is a paired function of the connector’s intention and application.

For non-power-handling signal-contact designs, the contact is almost always plated with a few micrometers (µm) of gold or less-expensive tin to reduce electrical resistance, minimize corrosion, and resist wear at the mating surfaces. That is asking a lot of a physically small contact with an even smaller contact area. Many connectors are designed to give adequate performance even s their plating wears down due to repeated cycles and underlying base material is exposed.

How many mating cycles are needed in a connector? Obviously, the answer depends on the application. In some cases, it is in the low double-digits, but it can also be in the hundreds and thousands; think of that charger connector on your phone. The same connector used in different situations can have dramatically different mating-cycle numbers.

Examples show connectivity diversity

Figure 1. This card-edge connector has contacts on a 0.50 mm pitch, meeting the PCI-SIG M.2 specification, and is specified at 60 mating cycles. (Image: JAE-Japan Aviation Electronics Industry, Ltd.)

A look at some connectors shows their wide range of cycles and contact resistances; it also shows you can’t judge by appearance or assumption:

The JAE Electronics SM3ZS067U410AMR1000 of Figure 1 is a high-reliability 67-position, female card-edge connector which is compatible with the PCI-SIG M.2 specification with 0.020″/0.50-millimeter (mm) contact pitch. It is rated for 60 mating cycles with 55-milliohms (mΩ) maximum contact resistance.

In contrast, the Hirose Electric Co Ltd UX60A-MB-5ST seen in Figure 2 is a surface-mount, right-angle, mini-B USB 2.0 receptacle connector (5-position) specified for 5000 cycles and 70 mΩ resistance.

Figure 2. This consumer-focused USB receptacle from Hirose is rated for 5000 cycles while maintaining a maximum of 70 milliohms of contact resistance. (Image: HIROSE Electric Go., LTD)

Figure 3. This direct-to-PC board insulation-displacement connector from Würth Elektronik has two mating circuit-board drill patterns: one for prototypes and one for the final product. (Image: Würth Elektronik)

Flat ribbon-cable connectors (also called an insulation displacement connector or IDC) of Figure 3, members of the Würth Elektronik 490107671012 “SKEDD” family, are normally used within the product enclosure and do not require as many mating cycles as a user-facing connector.

This 10-position connector is unique in that it fits directly into plated holes in the pc board rather than using a complementary mating part, and is rated for 10 cycles at 20 mΩ using the vendor-specified board hole pattern, diameter, and plating for production runs. Würth also defines a slightly different, more rugged set of numbers for prototyping, which boosts that rating to 25 cycles.

Finally, the Harting 09332062648 seen in Figure 4 is a six-contact ground-position connector for wires with diameters from 0.14 to 2.5 mm² (AWG 26 to AWG 14), which can handle up to 500 V at 16 A. It is intended for frequent connect/disconnect cycles, and needs extra-low resistance to minimize IR drop at the higher currents, and so is rated at over 10,000 cycles with just 3 mΩ maximum contact resistance.

Figure 4. This power-connector assembly from Harting is specified to maintain contact resistance below 3 milliohms up to at least 10,000 mating cycles. (Image: HARTING Technology Group via Digi-Key)

This array of disparate connectors demonstrates how vendors tailor their connector mating-cycle and maximum-resistance ratings to the target application, and these numbers may not be obvious solely from their physical size or appearance.

The right tactics can minimize the problem

When specifying connectors for the bill of materials, there are some mating-cycle issues to keep in mind in addition to basic connector selection:

Study data sheets with special attention to how and under what conditions the vendor specifies the number of mating cycles, as there is no industry standard (except in special cases). Is it defined with respect to a defined increase in contact resistance? Insertion force? Other?
If using the PC-board edge with fingers as one half of the connector pair, work with the board fabricator to determine what sort of extra or special plating is available, since the standard one- or two-ounce copper may not work well for long.
Consider using a more rugged two-piece connector rather than PCB edge fingers, if possible.
Check if the connector vendor offers thicker connector-contact plating as a custom or even standard option, as many do (also consider if it makes sense for the final bill of materials).
Recognize that mating-cycle longevity needs are often very different for power connectors than they are for signal connectors; the cycle-count limits are especially challenging for tiny gigahertz-class connectors. In general, especially for mass-market consumer applications, it is hard to determine how many mating cycles are a reasonable number as a goal. An AC plug may be used rarely (kitchen toaster oven) or every day (hair dryer). The same dilemma applies to a USB connector.
Finally, if the application supports hot plugging, there will be new issues of pitting and fretting of the contact surfaces during insertion and removal cycles.

Summary

Connectors are reliable when used within their defined specification ranges. However, it’s easy to overlook their limits, especially in the debug and evaluation stages, and exceed their ratings for mating cycles and other parameters. The result can be frustrating, intermittent, and inexplicable circuit misbehavior.

Add this to your “keep in mind” list: the humble connector is yet another component that will need some attention and consideration. Anyone who casually remarks “what’s the big deal?” about the topic is woefully naïve or has been very lucky.