Terminal selection depends on the type of wire used, operating environment, and factors such as current levels and local electrical codes.
Temperature:
Crimping wires to terminals sets up the conditions for stress relaxation. This relaxation can be a product of creep or recrystallization. Creep is accompanied by dimensional changes and is caused by slow atomic diffusion of the material to accommodate crimp stresses. The rate of creep increases with temperature and is higher in aluminum than in copper. Aluminum, for example, creeps significantly at room temperature.
Corrosion:
The types of corrosion that occur in terminals are atmospheric and galvanic. Atmospheric corrosion depends largely upon the contaminant present in the environment. Most atmospheric corrosion, especially oxidation, increases with temperature.
Vacuum:
When metals are used in a vacuum at elevated temperatures, high vapor-pressure elements can vaporize from the material. Ordinarily, alloys selected for high-temperature applications do not contain such components. However, alloys such as brass contain zinc and are undesirable in a vacuum if they are load-bearing components at elevated temperatures. Under these conditions, the components vaporize rapidly and deposit in a cooler temperature zone.
Radiation:
Atomic radiation does not affect electrical performance of crimped terminals. However, the properties of most common plastic insulating materials used on wire and terminals degrade drastically under prolonged exposure and eventually become useless as an electric insulator.
Current:
Terminals should be able to carry at least as much current as the wire for which they are rated. Terminals have a larger heat-dissipating mass than wire, and usually operate at a lower temperature. In this instance, the effects are similar to heating a joint made of two metals having different rates of thermal expansion.
Voltage:
High operating voltage requires special insulation, spacing, and location of the terminals. Extremely low voltage requires special considerations as well. An electrical connection that performs satisfactorily at levels above 10 to 15 V may not be suited for signals in the mV range. With the exception of gold, metallic surfaces are usually covered with insulating oxide or sulfide films. In practice, these films are generally rubbed off when the connection is made, or they break down under circuit voltages. However, when these films remain, they can impede current flow at mV-signal levels. Generally, a connection that conducts well at 1 mV will conduct at lower voltages.
Insulation:
Preinsulated terminals come in many types and require less handling and only one crimp. They are also slightly more expensive than uninsulated ones.
