If there is one part of an operating system that is regularly taken for granted, one would have to look at cable assemblies as the prime candidate. Usually the “unsung hero” of any electromechanical assembly, it quietly operates day after day without any issues. In fact, properly designed and built cables are rarely considered as the primary failure when the system stops operating correctly.
Despite the reputation of cables as a reliable component concerning most power and signal distribution systems, it’s important that close attention to detail is part of the technical requirements of the application. It is proper design and assembly expertise that gives cable assemblies their reputation for reliability. The value proposition comes from the company providing the design and assembly work.
The best cable assembly designs take into account a number of design criteria. These can be broadly classified as Electrical, Mechanical, and Useability criteria.
For point-to-point connections, often the cabling must carry both data signals and power for drive control, with a minimum power loss. A combined signal and power cable can cause interference (crosstalk) from the power line into the data lines. This can often be corrected through use of differential data signals and shielding. However, at some point the power lines may need to be carried separately to minimize interference.
Power and signal distribution becomes more complex when there are multiple sensors, actuators and power requirements. As a system becomes more complex, the solution may be to create wire harnesses that carry power and data signals through separate cables. This may include developing wire harness assemblies with specialized data cables depending on the type of signals that need to be carried: digital, analog, or some type of bus communication.
It is very common in most installations of electrical equipment, various types of sensors, actuators and power sources that cabling could be continually bombarded by potentially interfering electrical signals. It becomes critical to protect against any potential Electromechanical Interference (EMI), whether radiated or conducted. Some of the techniques include use of shielding, proper grounding, specialty cabling (such as coaxial cable) operating at lower power when possible or enclosing noise-susceptible components. Noise on data signals can sometimes be eliminated through use of twisted, shielded, differential pairs of data signals. These types of solutions require a higher level of technical skill to execute.
In addition to its primary duty to make sure that the right signals and power values get delivered where needed, cable assemblies need to work in a variety of demanding physical environments. Most commonly, cable jacketing decisions are specified on the basis of protection against the elements. We are talking mostly about temperature extremes, moisture intrusion and UV exposure. Other considerations are, resistance to abrasion, flexibility through thousands of bending cycles without breaking and strain relief where the cable enters the connector backshell to prevent wire breakage. In some applications, connectors must have pins which are sealed in the connector housing or have gold-plated contacts to prevent corrosion and reduce mating resistance.
Providing a keyed connector/mate system makes it easier to properly align and make/break connections. It also can prevent accidental bending of pins causing an expensive replacement. For cables with a pigtail terminations, which usually mate to a terminal strip, the individual connectors should have color coded jackets, along with a tagged or laser marked outer jacket that notes the function associated with each color. For certain applications, the jacketing material is chosen so that, in case of a fire, it will not produce toxic fumes. Finally, especially in commercial applications, RoHS 3 often applies. This requires the use of lead-free solders (or use of crimp pins) for making up the cable terminations at the connector and can restrict the plating material of the connector housings.
Though not completely comprehensive, this covers the main issues associated with understanding and having the experience, tooling, and manufacturing capability to correctly build cable assemblies. The same workmanship standards apply whether building a replacement cable assembly or a complex cable harness. One of the professional standards to look for in a supplier includes IPC J-STD-001; the worldwide authority for materials, methods, and verification of interconnects. Another standard is IPC/WHMA-A-620 (Classes 1, 2, and 3); where the progression from Class 1 to Class 3, represents increasing sophistication and tighter workmanship requirements for performance and testing. Class 1 is for general products and Class 3 operators are qualified to build and test military, aerospace, and medical equipment.
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