By Larry Nembhard, Engineering Manager, Keystone Electronics Corp., Astoria, NY
Perhaps no process is more important to a reliable contact than the plating process. Whether it’s the power contact on your laptop, the contact for your phone jack, your stereo jacks in your head phones or the battery contact in your flash light, they must have the proper plating to function properly.
The struggle in today’s electronic design is to come up with reliable contacts that will function as well in 10 years as they do today. One of the most important features of contacts is the plating. Choosing the appropriate plating for each application is always a challenge for engineers. Some of the factors that affect the plating selection are galvanic reaction, conductivity, hardness, solderability, corrosion protection, and environmental concerns.
Galvanic reaction is also called dissimilar metal corrosion. It is a process by which material in contact with each other corrodes. This is managed by plating the dissimilar metals with compatible plating that protects the base metals. In harsh environments there should be no more than 0.15V difference in the anodic index. In a controlled environment, such as an office, the difference in the anodic index can be as much as 0.5V. The anodic index starts with gold at 0.0 to a high of 1.85 for beryllium. For this reason, the most popular plating for contacts is gold, silver (0.15), nickel (0.3) or tin (0.65).
Good conductivity is paramount in choosing plating for contact finishes. In demanding applications where cost is a concern, contacts must be functional but cost effective. Of the four popular platings silver is the best conductor but will oxidize easily without supplementary treatment. If the contact is being cleaned while in use, such as a wiping action, then silver is an excellent choice. Silver is soft which reduces the contact resistance, but will wear quickly.
Gold is also an excellent conductor but is very expensive. Gold is absolutely essential in low-current applications, not just for it superior electrical properties, but also for its corrosion resistance benefits and excellent solderability.
Nickel plating is maybe the most used plating for contacts, it is extremely hard, has excellent anti-corrosion properties, but needs a higher temperature and an active flux to solder properly. Tin plating has always been the most used plating on circuit boards and its components. Tin plating is usually alloyed with lead to extend its shelf life and to reduce whiskering. With the new environmental requirements, lead is no longer acceptable as an alloying element with tin.
Lower voltages and less amperage affect engineering on both sides of the connection, so contacts need to be plated with the proper plating for given applications. When designing a contact, consideration has to be given to the installation of these contacts into the boards as well as the function of these contacts after installation.
Tin plated contacts have excellent solderability and solders at low temperature, but it is a soft plating and will wear easily in use. It is also bothered by whiskering. Nickel plating solders at higher a temperature and may cause difficulties for other components on the board that cannot withstand higher temperature soldering. It is very hard and will not wear easily and makes excellent contacts, especially for batteries.
Silver plating is an excellent conductor, but oxidizes easily, solderability is excellent and also solders at a low temperature, but again it is soft and wears easily. Gold plating is an excellent conductor; solderability is excellent, solders at low a temperature and when plated with “hard” gold, does not wear easily.
The future of electronic components, like that of so many of the components inside, is clearly toward smaller, lighter and more powerful models. Innovation is the order of the day. Developing concurrently with these new designs to reduce costs and to extend the life of the product is the overall goal.
One innovative plating being used is Tin-Nickel plating. This is an alloy of tin and nickel (65% tin and 35% nickel). This plating exhibits extremely high resistance to corrosion and tarnishing; it is non-magnetic, has excellent solderability and has a hardness value in excess of 600 Vickers that will not wear easily. Where low current applications are necessary, gold flash over nickel plate is used. This offers the hardness of nickel plating and the advantages of gold plating without the cost.
Essentially, there is no single plating that is suitable for all applications, there are advantages and disadvantages to each. Your goal is to choose a plating that best suits your particular application, bearing in mind the mating cycles, solder profile, useful life of the product, the material of the mating contact, and the cost.