Many electronic connectors are gold plated. Gold plating increases conductivity, especially at low voltages, protects the surface from corrosion, and increases the connector cycle life.
But with gold at more than $1400 per ounce, using gold adds expense, especially if it can be avoided in your application. For example, you won’t find gold plating on your 120 or 220 V wall plug because the voltage is high enough that the electricity will flow right through normal surface contamination. But we seldom use such high voltage levels past the transformer in the electronics box.
So when can you get away with tin-plated interfaces?
If a contact is mated only a few times during its lifetime, tin may be a suitable solution. For this to work, however, several other conditions must also be considered:
- Tin interfaces will develop a nonconductive film on the contact surface over time with normal exposure to air. One way to overcome this problem is using connectors with high normal forces greater than 100 grams per contact. Normal force is the amount of force that a receptacle contact places on the mating pin or blade. With >100 grams force, the mating contacts plow through the insulating films to mate fresh tin to fresh tin.
- The higher the normal force, the higher the connector mating force will be, so you will find that tin connectors are most often used in connectors with a limited number of pins. One of the largest consumers of tin-plated contacts is the appliance industry where cost is very important, but a connector is only disconnected if the machine is being repaired. Even then, the replacement part is likely to have a new connector. Keep in mind that appliance manufacturers typically offer only a 1-3 year warranty. After that, you become a paying service customer.
- Connector engineers enhance normal force by designing connectors with contact geometry that concentrates force on a very small mating area. Imagine a pyramid mating with a flat surface, for example. That would be great for concentrating forces, but would be difficult to mate and would wear way too quickly. A lot of connector engineering involves designing the “spoon” on the receptacle contact so that it provides the optimum normal force, while keeping mating forces manageable.
- The best manufacturers of tin contacts will offer versions that are pre lubricated with a material that can help shield the tin from the oxygen in the air, reduce mating forces, and provide some lubricity to protect against damage from mating and micro-motion.
Some environments are totally unsuitable for tin contact systems:
- Vibration can cause mating contacts to move a tiny bit all the time. This micro-motion will rather quickly produce “Fretting corrosion.” Basically, as the contacts move relative to each other, fresh tin is exposed, which then oxidizes, and becomes insulating. Over time, the conductivity of the interface increases to unacceptable levels. This is one reason why when you have a connector failure, it will frequently be corrected by unmating and remating the connectors, clearing away some of the insulating debris. This vibration may be mechanical, or created by thermal cycling over time. One way to prevent fretting is to lock the mating connectors in place in such a way that the contacts cannot move relative to each other. This is normally accomplished by plastic housing features that positively lock mating connectors together.
- High temperatures must be avoided with tin-plated contacts. Several failure mechanisms, including intermetallic reactions between the copper and tin and mechanical creep of the tin plating, will create failures over time.
- If the contacts will be mated “hot,” under power, the resulting spark can quickly erode holes in the tin plating, revealing underlying copper, which can quickly corrode and cause connector failures.
Tin plating will always have a place in connector applications. If you just pay attention to the guidelines above, you can have a successful design at the right cost. For more info about using tin contacts, I encourage you to read a white paper from TE called “The Tin Commandments: Guidelines For The Use Of Tin On Connector Contacts.”