by David Norton, TDK-Lambda Americas
Designers can avoid reliability problems by acquainting themselves with the way connector pin materials stand up to repeated use and corrosion.
Examine the input and output connectors on most open-frame power supplies and you’ll likely find parts from a supplier like Molex or JST. These connectors are inexpensive, readily available, reliable and easy to use. In addition, use of these readily available parts makes it easier for designers to second-source a power supply, if necessary, when some standardization exists.
Many power supply manufacturers will specify the name of the mating connector series in their product documentation. But they will often leave it up to the user to determine the actual part numbers. This usually provokes a call to power supply tech support for a recommendation.
An example illustrates why power supply makers adopt this practice. Consider the industry standard low-power 2×4-in. single output power supply. The Molex KK 09-50-3041 housing is widely specified as an output mating connector. Made of nylon, it has a friction lock and four circuits: two for the plus output and two for the minus output.
When looking for the mating pin, one has to be a little more careful. The suggested pin for the connector is available in two materials: brass and phosphor bronze.
Brass is a common material for contacts and pins. It is inexpensive, has good conductivity and is generally dependable in a benign, low-temperature environment like an office. Brass loses flexibility as it ages and, under repeated stress, is subject to crystallization, which significantly lowers it conductivity. However, most power supplies don’t undergo repeated connect/disconnect cycles, so loss of conductivity usually isn’t a problem.
Phosphor-bronze mating pins should be considered for more challenging environments. Phosphor bronze is an alloy of copper with 3.5 to 10% of tin and a phosphorus content of up to 1%. The phosphorus serves as a deoxidizing agent during melting. These alloys are tough, strong and have a low coefficient of friction. They are particularly helpful for applications at higher temperatures because heat can make brass contacts lose their spring properties. If there is some vibration, the lack of spring in a bronze pin can cause reliability problems. Phosphor-bronze contacts do not lose their spring. However, brass is more conductive than phosphor bronze, so check current rating capability.
Phosphor bronze is more expensive than brass, currently about $0.13 compared to $0.05 for brass (1,000-piece pricing from a distributor). For a 2×4-in. power supply, the additional few cents could add $0.56 to the bill-of-material cost. Designers should consider the environment and desired field life. As a note, on higher power 2 X 4-in., open-frame power supplies, there are alternatives to the single-point-of-contact KK style pins like those used with the Molex 09-50-1041 housing (SPOX series). These have multiple points of contact for lower resistance.
There are several other metals that can be found in power connector pins and plugs. Here are a few details about how other metals compare:
Beryllium copper is often used for its excellent conductivity and thermal properties. It has the best electrical conductivity of any spring alloy of comparable hardness. It beats other copper-based spring alloys in terms of resistance to fatigue and strength. So it is generally a candidate for applications experiencing numerous insertion and withdrawal cycles. But it costs more than any other basic contact material.
Nickel-silver alloys are actually copper alloys with nickel and often zinc. The typical makeup is 60% copper, 20% nickel and 20% zinc. These alloys resist oxidation but may be susceptible to stress corrosion, though not to the extent of brass.
Several materials can be used for plating connector pins. Gold is an excellent conductor and has a low contact resistance. Hard gold platings are applied in applications characterized by numerous insertion/withdrawal cycles. Gold can be impregnated with graphite to handle super-high insertion/withdrawal needs.
Of course, gold is expensive, so alternative plating materials have been developed. Silver is a general-purpose plating for power contacts. But it tarnishes when exposed to air. The resulting oxide layer can be problematic for low-level circuits, less so for power contacts. Like gold, silver has become expensive, so suppliers have developed alternatives.
Nickel is slow to react with air, so it resists corrosion and is relatively conductive. It often serves as an undercoat for contacts destined for high-temperature-use to prevent migration of plating materials. It also has good wear resistance.
Tin has good conductivity and is relatively inexpensive. But it has poor wipe resistance and best suits connectors that experience few mating cycles.
Finally, rhodium is sometimes found in connectors that need exceptional wear qualities. It is not as conductive as gold or silver, but its conductivity is generally acceptable when the material is deployed as a thin plating.
TDK-Lambda Americas
www.us.tdk-lambda.com
Filed Under: Connectors (electrical) • crimp technologies, Power supplies, Power Electronic Tips