NEPP - Connectors and Contacts

General
When choosing a connector, consideration must be given to the application space environment (operating temperature extremes, shock, vibration during launch, material properties at cold temperature and outgassing in vacuum), electrical requirements (working voltage, current, frequency, duty cycle, EMI/RFI requirements, durability) and physical constraints (weight, volume, unit seal). Voltage rating is determined by dielectric material and contact spacing; contact size determines current rating; these and the number of circuits all combine to affect the physical size of the each connector.

Larger connectors are generally easier to work with and are more robust, but are costly in terms of volume and weight. Small connectors, particularly surface mount types, conserve volume and weight, but require special handling and can be costly as they can be labor intensive to install. For cable harnesses, removable crimp terminated contacts are generally preferred over non removable solderable contacts, as solder requires more attention to pre-tinning, pre and post solder cleaning, and has more inspection and labor costs. Crimp removable contacts require tooling to be monitored for process certification and calibration. Smaller high tech connectors often require special installation tools.

In certain applications, where connectors will be mated with energized circuits (i.e. hot mating), it is important to select scoop proof designs, where it is not possible for connector shells to short energized contacts together during mating.

Design practices
Proper design practices must also be considered. For guidance, the NASA Workmanship Standard for Crimping, Interconnecting Cables, Harness and Wiring can be viewed at http://workmanship.nasa.gov/index.jsp ).

A recommended design practice is to include spare (unused) contacts to compensate for any last minute modifications that must be made to accommodate an additional circuit. If spare (unused) contacts exist, modifications can often be incorporated into existing designs without having to order new larger connectors and rework panels. Connectors with removable contacts that have unused contact positions should have seal plugs installed in vacant cavities. When multiple connectors of the same type must be used, selection of connectors with a polarization feature such as alternate keyway clocking or polarized guide pins shall be used so that it is impossible to cross-mate similar connectors.
Four types of contacts exist. Pin and socket contacts are by far the most commonly used. Stamped spade and fork contacts are not recommended for use in high reliability space environments.

Connector reliability.
Proper selection of connectors is important to avoid problems in their application. Most connector failures result from chemical effects of contamination or excessive temperature. Chemical effects are induced by growth of films at the contacts. Ions in contamination can migrate to the points of highest potential and accumulate, causing non-conductive film growth. These films can increase contact resistance, all the way from a few ohms to open circuit. Non-metallic material outgassing results in a source of contamination. Some of these outgassed materials can condense resulting in clouded optical surfaces and degraded performance, while other outgassed materials may be corrosive or conductive in nature. Non-metallic connector materials must be evaluated for acceptable levels of outgassing. Cadmium and Zinc should not be used as platings due to evaporation problems in a vacuum at high temperatures. Pure tin is prohibited as a plating due to tin whisker growth concerns.

Excessive current can cause excessive temperature, which can cause failure due to chemical decomposition of connector dielectric insert properties. Failure due to excessive voltage can induce failure by causing dielectric breakdown through insulating materials. Either failure mechanism can result in high leakage currents or short circuits.