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Global Connections: Cable Assemblies—Ideas for Improved Design
When you are tasked with a new project, even though other items seem to be more important, always start any cable designs early in the process. Talk to your connector supplier at the beginning to make sure there is enough stock available for your needs. Spend time talking with the connector manufacturer or representative to ensure the chosen connectors are suitable and have a proven reliability in field life. Many connectors look good but lack reliability or lack water intrusion specifications as required.
It is always a great idea to make the first cable assembly in your own R&D lab to test fit the size of each part. You can check electrical performance, measure over temperature power carrying of wires. Study the cable assembly design and complexity for overall manufacturability.
All assemblies can experience bottlenecks during the process. Some are unexpected but most can be anticipated and can be worked around. In cable assembly, the number one reason for delay is the unavailability of connectors. If your cable is using obscure parts, check availability and pre-buy what is needed to ensure they are in stock at the contract assembly company. Some connectors can be unreasonably delayed when stock supplies run out.
When designing your wiring harness or connector subassembly, always ensure you have the proper wire selected. Calculate the wire size carefully and always overestimate the wire size to ensure low wire temperature. Determining your electrical requirements is one of the most important parts of the cable assembly design process. In addition to operating voltage, insulation voltage breakdown numbers and operating current, you’ll need to specify the number of strands in the wire and the type of copper. Connector-related contact resistance can affect performance resulting in overheated contact points.
Additional electrical requirements may include signal speed, impedance, grounding, and the dielectric separation of power and signals, as well as twisted pair. There are many different types of cables, but it basically comes down to two basic types—flat and round. Both come in multi-conductor or twisted pair configurations, and both are available with or without shielding. Flat cables are used for mass termination with flat connectors. Round cables are used for long cable runs or when flexibility and compactness are required.
There are many different properties of the simple wire that can result in a failed sub connector or wiring harness later in use. Consider the following when specifying the wire conductor size:
- Check wire charts for temperatures with current to ensure the low temperature rise
- Look at the number of conductors in the wire, the fewer number, the lower the flexibility
- Research the wire coating and avoid some coatings, such as natural rubber, which can crack and fall apart with exposure to UV sunlight or salt water
- Look for the maximum and minimum temperatures of the wire coating, which can affect the flexibility, or crack at low temperature, resulting in easily broken wires
- Research the wire coatings chemical resistance charts if your design is to be used in a tough, chemically loaded environment
- Check outgassing numbers of the coating on the wire if used in aerospace applications
- Don’t over design the assembly which can add cost to your project
- Remember the “keep it simple” rule, but don’t under design cable assemblies either, which will result in cables that are too fragile or ones that will be prone to failure
- Add proper strain relief and stiffeners or include heat shrink tubing in critical areas where chaffing can occur
Always design with the appropriate safety and regulatory requirements for the cable assembly in mind. Ensure compliance with Restriction of Hazardous Substances (RoHS); compliance to UL or CSA may be required for your assembly certification.
If you intend to use overmolding, allow enough time to have the proper injection molds manufactured. Typical lead times are from 7–15 weeks. Once you prepay for the molds, they will be machined. Once finished, there are no changes that can be easily made. You need to get it right the first time. The molds are fixed as to the connectors which they use and the lead strain relief section as well as to the plastic injection machine they are used on.
The operating environment for a custom-made cable assembly is critical in the design. Medical cable assemblies are required to be easily cleaned and sterilized. Instrument cable assemblies such as ECG need to support higher data speeds and signal low voltages. Cable assemblies that are used in extreme environments will need to resist industrial chemicals, high temperature and UV sunlight.
Cable assembly designers have a choice of many different coupling methods. Threaded connectors provide a secure, mated connection for use in high vibration use, such as aircraft. Push-together connectors are easier to use but lack proper locking. Depending on your application, overmolding for bend relief is a significant improvement. Thermoplastic overmolds provide a more solid connector body, whereas softer, flexible material allow for some bendability. Silicone overmolds provide higher heat protection and maximum flexibility but lack strength.
Design your cable assembly to emphasize quality. Ensure your drawings have the correct and detailed information; leave nothing to guessing.
Cooler Cao is the engineering manager at Cabletree Industrial Co, Ltd. He has been in the cable assembly business for over 17 years holding a wide variety of positions including project management, engineering, and production. He has an abundance of experience in overmolding cable assembly and complex wire harness projects.