Tin and silver are the two most common used non-noble finishes. Tin finishes are used in a wide range of applications, from white goods to automotive. Silver is primarily used in power/high current applications.
Tin contact finishes are the most widely used non-noble contact finish. They are characterized by low durability (tin is a soft material) and high mating force (high friction coefficient and high contact force), and susceptibility to fretting corrosion.
These characteristics indicate that tin finishes are not suitable for connector applications requiring high mating cycles and high pin counts. These are intrinsic limitations though the range of mating cycles and pin counts can be extended by the use of contact lubricants.
Susceptibility to fretting corrosion suggests that tin finishes are questionable for use in demanding mechanical environments or those with large temperature variations. It must be noted, however, that tin finishes have been successfully used in under hood automotive applications; applications characterized by demanding mechanical and thermal requirements. This success has been realized by the use of high contact forces to ensure the mechanical stability necessary to minimize fretting corrosion degradation. This is a good example of a guideline rather than a rule with respect to concern for fretting corrosion. But, it is good practice to critically evaluate the mechan and thermal characteristics of an application when considering tin as a contact finish. Apart from fretting corrosion, tin may be a good choice for harsh chemical environments due to the self limiting passivating surface oxide. An additional temperature concern with tin finishes is the growth of copper-tin intermetallic compounds at the interface between the tin and the copper alloy contact spring. In worst case conditions the tin can be converted into intermetallic compounds with both mechanical and electrical effects. Mechanically the intermetallic compounds are brittle and electrically they are poor conductors. Tin finishes may not be appropriate in applications where the connector is exposed to temperatures in excess of 100 degrees Celsius.
The current carrying capacity and electrical resistance of tin finishes will be discussed simultaneously. High current applications, say tens of amperes, can be problematic due to the effects of Joule, or I2R, heating. Joule heating and its consequences for connector degradation will be discussed in detail in Chapter II/2.8.2 Power Applications. At this point it is sufficient to note that Joule heating can create a positive feedback loop because contact resistance increases with temperature; Joule heating increases resistance and increased resistance increases temperature and the loop continues. It is important, therefore, for connectors intended to carry high currents to have a low and stable contact resistance. Tin can readily realize low values of contact resistance. The stability of tin contact resistance, however, may be compromised by fretting corrosion driven increases in contact resistance as discussed earlier. Once again, practice has shown that tin interfaces can provide reliable high current performance as long as the mechanical stability of the contact interface is maintained, generally through the use of high contact forces. The success of tin finished connectors in the white goods industry is an example of this capability.
Tin finished connectors are intrinsically limited to low durability, low pin count applications due to the fact tin is a soft material (high friction) and high contact forces are needed to ensure the mechanical stability of the contact interface to reduce susceptibility to fretting corrosion. Another intrinsic limitation is the application temperature due to intermetallic compound effects on electrical and mechanical performance.
When concerns for fretting corrosion can be minimized or controlled, however, tin finished contacts can be used in a broad range of connector applications. The difficulty in minimizing the potential for fretting corrosion, however, should not be under emphasized.
The major limitation of silver as a contact finish is its susceptibility to surface tarnish films. The tarnish films are more an appearance than a functional problem because they are readily disrupted on mating and the desired silver-to-silver contact interface is easily established. The use of silver finishes in typical connector applications is minimal due to resistance failures in low force non-wiping relay applications, in which the tarnish films were not effectively disrupted, highlighting the potential for tarnish related failures. The major application area for silver finishes is in high current connectors where the high electrical conductivity of silver takes on enhanced importance due to Joule heating concerns.
Silver is intermediate in hardness between tin and gold and can be used at contact forces lower than those for tin because the requirement for mechanical stability of the contact interface is not as demanding. Thus, silver finished contacts will have higher durability and lower mating forces than tin finished contacts. The pin count does not come into play because power connectors do not generally require high pin counts.
The current capacity, resistance and stability of resistance of silver finishes are good due high conductivity and low susceptibility to fretting corrosion. High contact forces are typically used in power contacts, further adding to mechanical and electrical stability.
Environments containing sulfur promote tarnish formation. Sulfur plus chloride can result in more deleterious surface films. Silver plated contacts are often protected from tarnish by including special vapor phase inhibitor paper in the packaging.