The EMPF was recently involved with a project that required the design of many interconnecting cables. This included external (box to box) cables as well as internal (board to board) cables. The EMPF was tasked to design and document all cabling and build the internal cabling, while another source constructed the external cables to the EMPF specifications.

The design of the cabling required input from other areas of the project. Electrical and environmental conditions had to be documented. Electrical constraints are voltage and current of the conductors. Higher currents require larger gauge wire in the cabling. The length of the cable will also play a role in selecting the wire gauge because a longer wire will have a larger voltage drop for any given gauge. Therefore, longer cabling runs should have larger gauge wire to reduce the voltage drop through the wires.

The environmental conditions play a role in defining the insulation type of the wire as well as the cable. In our case, the insulation was defined by the organization that contracted the project. For internal wiring on an aircraft electronics box, the EMPF was required to use Teflon insulation. Some factors that need to be considered when not defined by customer requirements are temperature range, chemical resistance, and flame retarding characteristics.

The next decision was the physical connector configuration; the shell type and where to use pins and where to use sockets. The EMPF chose military D-38999 series connectors (Figure 4-1) for all external cables due to their reliability and environmental performance. These connectors have many configurations for wire count and wire gauge, as well as multiple keying options. Keying options allow a single connector type to be utilized for different cables without the risk of plugging a cable into the wrong receptacle. The standard convention for using pins or sockets in an interconnecting cable is based on the presence of voltage. Sockets should be used wherever the connector normally has voltage while pins should be used on the end where the connection will allow voltage into the cable.

Figure 4-1: MIL-D-38999 Series III Connectors. Unit required signals to four identical boxes; decision was to color code the box location in order to reduce quantity of spares required. The connectors shown are bulkhead receptacles with socket contacts.

When fabricating cable assemblies, there are a variety of specifications that may be required to define the proper methods for the assembly and routing of the cable assembly. The connector manufacturer has installation requirements and the Wire Harness Manufactures Association (WHMA) and the military have published guidelines for the fabrication and installation of cabling.

The IPC document IPC/WHMA-A-620 describes the fabrication and inspection requirements for cable assemblies in the electronics industry. Criteria are given for each of four levels of acceptance; Target (the ideal condition), Acceptable (the conditions that the industry has determined will not have reliability issues for the class of product), Process Indicator (conditions that are still acceptable, but are signs that something in the process should be changed to get back to target) and Defect (conditions that will cause field failures if not fixed during manufacture). The IPC/WHMA-A-620 covers everything from wire stripping and soldering to crimp connectors and cable bundling requirements.

Military requirements may vary slightly from the IPC/WHMA-A-620 based on type of service and expected environmental conditions. In the event that the project will be used in a military application, verify which military specifications are required.

Military connectors in external cabling assemblies are extensive. The basics of the connector system are the receptacle and the plug. Similar to house wiring, the receptacle is generally found mounted to a box or a bulkhead and the plug is mounted to the cable. For instances where an extension cable may be needed, one side of the cable will have a cable mounted receptacle. Both the receptacle and the plug consist of a shell, insulator, and contacts (Figure 4-2). They are assembled in order to create the completed connector assembly. As mentioned above, the contacts may be pins or sockets, depending on where the potential for voltage exists.

Figure 4-2: At top, bayonet box mount receptacle with socket contacts. Below, triple start box mount receptacle with pin contacts. Note the five keying slots on the inside edge of both receptacles.

All military connectors are keyed so that only their mates can interconnect. A typical connector will have a standard key orientation as well as up to five alternate positions. This allows the designer to create a unique cable mating scheme and prevent potential misconnections.

Military connectors also have multiple interlocking mechanisms. The most commonly used interlocking methods are threaded, bayonet, breech, and push-pull (Figure 4-2). Threaded systems work like a standard nut to bolt interface, where the locking nut is free to move about the fixed shell of the plug to secure the connectors. There are three types of threaded systems: Standard (like a common nut and bolt that engage within a 360° rotation), Double start (which have a double thread spiral and engage within a 180° rotation), and Triple start (which have a triple thread spiral and engage within a 120° rotation). Bayonet locking systems utilize three pins equally spaced on the outside of the receptacle and a locking ring that contains corresponding ramped grooves and a locking mechanism. Breech locking systems are similar to bayonet locks with the exception that they use solid lands on the receptacle and the plug locks require 90° to lock. Push-Pull couplings work similar to an air hose quick disconnect where sliding the lock ring allows the connection to be locked or removed.

When assembling wires and connectors into cable assemblies, it is important to use the correct tools. In IPC Class 3 as well as military applications, tools such as strippers and crimpers are required to force full cycle operation. Full-cycle crimpers have a locking ratcheting mechanism that does not allow the opening of the tool until it has been compressed to fully crimp the connector (Figure 4-3). This guarantees the technician makes a full crimp every time and ensures a quality cable is constructed.

Figure 4-3: An example of a full cycle tool. This crimper is for ring terminals. Note the ratcheting mechanism in the inset view. Once the crimp is started, the tool will not release until a complete crimp has been made.

Finally, care must be taken to route the completed cable assembly correctly during installation. Correct routing ensures that it will not rub on any components in the final assembly. Cable clamps, standoffs, and bundling ties must be installed to maintain the cable integrity through the life of the product.

Both internal and external interconnecting cabling are important in the proper function of all electronic devices. For more information on the design and manufacture of all types of cables, contact the EMPF Helpline at 610.362.1320.