Connectors are divided into four categories based on operating environments: commercial, industrial, military, and hermetic.

In commercial applications, outside temperatures and atmospheric conditions are the least critical. Connectors merely maintain electrical continuity, allowing low-cost materials to be used.

Industrial connectors are offshoots of commercial models for more rugged environments. Hazards include thermal shock, corrosion, vibration, physical jarring, and sand and dust.

Military connectors are more reliable and withstand extreme environmental conditions. They are made to military specifications and are available from a variety of sources.

Hermetically sealed connectors have performance requirements that are as rigid as military types. They are used in chemical-processing systems, industrial and commercial refrigeration units, and in underwater and aerospace electronic applications.

Designers can select from several different connector materials:

Brass
Brass has excellent conductivity but cannot withstand many insertion and withdrawal cycles. Brass loses flexibility as it ages and under repeated stress is subject to crystallization, which significantly lowers conductivity. It is suited for noncritical, low-contact-force applications, and is easily crimped, soldered, welded, and brazed.

Beryllium copper
Beryllium copper has excellent mechanical, electrical, and thermal properties and resists corrosion and wear. It is the best electrical conductor of any spring alloy of comparable hardness. Beryllium copper is stronger, more resistant to fatigue, and withstands more insertion and withdrawal cycles than any other copper-base spring alloy. But its cost is the highest of the basic contact materials.

Nickel-silver alloys
Nickel-silver alloys are oxidation resistant, and do not always require plating. Nickel-silver is susceptible to stress corrosion, although not to the extent of brass.

Gold
Gold is an excellent conductor and a highly stable plating material. It has the lowest contact resistance and provides the best protection from corrosion. Hard gold platings are recommended for frequent insertion/withdrawal cycles. For even greater cycling, gold can be impregnated with graphite with a minimum increase of contact resistance.

Gold-over-silver
Gold-over-silverunderplating is good for dry-circuit (millivolt, milliampere range) applications because it provides low contact resistance. But because its corrosion resistance is only moderate, the use of this combination is limited.

Gold-over-nickel
is a Gold-over-nickel widely used plating combination because it provides the surface characteristics of gold, while the hard underplating of nickel prevents migration of the base metal and minimizes the amount of gold required.

Silver
Silver is a general-purpose plating for power contacts. However, shelf life is poor and silver tarnishes when exposed to the atmosphere, increasing contact resistance. Although this oxide coating is undesirable in low-level circuits, it does not affect contacts carrying higher currents.

Nickel
Nickel has good corrosion resistance, fair conductivity, and is generally used as an undercoat for high-temperature environments to prevent migration of silver through gold. Nickel has good wear resistance, but it may crack during crimping if not properly plated onto the base metal.

Rhodium
Rhodium is used when exceptional wear characteristics are required. It has a lower conductivity than gold or silver, but on thin platings this increase in resistance is acceptable.

Tin
Tin has good conductivity and excellent solderability. It is a low-cost finish with poor wipe resistance best suited for connections requiring very few mating cycles. Tin is not a noble metal and will corrode.

Rhodium-over-nickel
Rhodium-over-nickel provides maximum wear resistance and is suitable for high-temperature operation. However, this combination has higher contact resistance than other platings.

Insulator materials used in connectors are normally plastics.

Plastics
Plastics provide the dielectric and mechanical characteristics required of the connector body. The dielectric selected is governed not only by the application, but also by the configuration and size of the connector body. These considerations include wall thickness, section-thickness variations, and types of inserts needed. The plastic is normally chosen for lowest material cost and shortest mold-cycle time while meeting the electrical, mechanical, thermal, and chemical requirements of the application. Thermoplastics and thermosets are the two main groups under which plastics are classified.