The term ‘high reliability’, often shortened in conversation to ‘Hi-Rel’, describes products that are designed to deliver excellent performance in demanding, mission-critical applications. For many, Hi-Rel is a euphemism for military and aerospace products. Indeed, there may be a perception that products approved to a MIL-Spec or BS number are naturally superior to other products that are not approved to these standards.
However, it may be time to stop thinking of high reliability and start talking about appropriate reliability.
One of the most important characteristics of any connector is the mating cycle, which is the act of mating and unmating a connector once. Most manufacturers publish a minimum mating cycle count in their specifications, as it provides a good indication of how long a connector will last.
The primary job of any connector is to join two electrical circuits together securely and efficiently. Every connector manufacturer strives to keep the contact resistance to a minimum. The act of repeatedly connecting and reconnecting will have a physical effect on the electrical contacts, which in turn will affect their electrical performance.
Testing a connector for its number of mating cycles is therefore about measuring its contact resistance. The manufacturer will conduct tests to discover how many times a connector can be mated and unmated before its electrical resistance rises above a pre-determined value. This number is affected by many factors, including contact design, the contact material and the thickness of any plating. It will also be affected by environmental issues, in particular the presence of dirt and other contaminants that might damage the mating surfaces of the contacts whilst in use.
A military specification connector might have a published performance of 500 or even 1000 mating cycles. But this is not the high point of the connector industry. The humble USB connector that has been with us for nearly three decades is designed to provide up to 5 000 mating cycles, and there are even versions available that will provide a service life of 20 000 mating cycles or more.
Another aspect to consider when looking at the reliability of a connector is how it performs with other components. Early in my career, I helped to make test cables for the TV industry. We had won an order for possibly the strangest cable assembly I have ever seen. Less than 1 m in length, one end was fitted with the cheapest 9-pin D-Sub connector that we could find. The other end boasted a military-grade, high-density D-Sub connector fitted with 1 000 mating-cycle contacts. We delivered 50 of these strange assemblies every month.
I later found out why we made so many. Every TV that left the production line was connected to a tester before being shipped to the customer, and our cable was the interface. The cheap connector was mated to the test machine and then ignored. It had completed the single mating cycle that it would be required to perform.
The other end of the cable was plugged into each TV as it passed the test station. It performed thousands of mating cycles per day. Once a test cable eventually failed it was thrown away, replaced by a new cable and the process would continue.
This anecdote has taught me several important lessons. First, it is important to choose connectors with the appropriate performance. It would have required less thought to manufacture the cable with high-reliability connectors on both ends, but this was unnecessary. The commercial connector had a required service life of exactly one mating cycle, and the difference in cost between it and its military equivalent was a factor of 100 or more. This would have made the cable unnecessarily expensive.
The second lesson is that connectors are capable of performance far beyond their expected limits. A connector will not simply cease to function once it has performed a certain number of cycles. The key challenge is to identify the products to deliver the appropriate performance you need for your application.
There will always be the need to choose Hi-Rel connectors for certain designs. Connector performance is not governed purely by mating cycles, and the environment in which the connector will be used must also be considered. We talk regularly of IP ratings, shock-, vibration- and electromagnetic interference (EMI)-resistance. The need to resist some or even all of these conditions will have an enormous effect on the design of any connector.
However, do not assume that a fancy, expensive, MIL part number is the only way to obtain this performance. Almost all manufacturers who make products conforming to a MIL standard also make the same connector under their own part number. These are usually made on the same production line, using the same materials and delivering the same performance. As an example, the ever-popular D38999 Series III circular connector is available from Amphenol as the TV, Souriau as the 8D and ITT Cannon as the KJA families.
The difference will be in the detail. A MIL-DTL part number might be subject to additional inspection or be supplied with a spare contact and removal tool. However, unless these are vital for your design, choosing the manufacturer’s own part number could significantly reduce your costs. The same is true for D-Sub connectors sold as M24308 and micro-D connectors as M83513. Find out what the manufacturer calls their equivalent product, and get ready to save money.
Another important thing to remember is that connectors do not have to be designed to a military specification to be capable of military-grade performance. Swiss manufacturer LEMO (www.lemo.com) has been making connectors for 80 years using its innovative push-pull locking system, which provides superior ease of use and reliability. Many of these connectors have been subjected to a range of military tests which prove their performance in certain applications, possibly the most aggressive of which are the gunfire vibration standards. Meeting these standards for shock and vibration makes them suitable for a wide range of uses, and LEMO connectors such as the M Series have proven time and again their suitability for use in tough conditions such as the motorsport industry.
There are other manufacturers of connectors that go above and beyond published standards to provide their customers with peace of mind. An excellent example is provided by US manufacturer Samtec (www.samtec.com), that has developed the Severe Environment Testing program. Samtec understands that the growth of high-speed electronics into almost every industry segment will see normal commercial connectors exposed to harsh conditions. The SET programme takes standard families of connectors such as the SEARAY and mPower, and subjects them to a battery of tests that goes far beyond the standards of ‘commercial’ products. In this way, designers are provided with cutting-edge board-to-board connectors that can withstand the rigours of demanding applications.
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