Basic Design Considerations
The following is to provide a brief overview of things to consider while designing a wire harness or cable assembly. Different industries and applications all have unique requirements and it is not possible to address all the specifics on this page. Please contact us for specific help with your individual requirements.
Mechanical:
Matching sizes
Two of the most common errors we see are, using too large of an insulation diameter (usually UL1007) in a connector that is on .100 centers, and terminals for multiple wire crimps not being matched for the correct total gauge or insulation diameter. Be sure your wire fits in the contacts and connectors.
Polarization or Keys
As a rule, it’s just good practice to remove the potential for assembly errors with keys and/or polarization. Typically, polarization and keys do not add any cost, so why not use them?
Locking Connectors
If your equipment or system has any of the following, locking connectors would be advised:
Vibration, Shock, Unsupported wires, connectors with .100 centers or less.
Temperature rating
The temperature rating of your wire and connectors needs to take into consideration the physical temperature of the system and any temperature rise due to current flow. Most connector systems are rated for a minimum of 105C. Most PVC wire will be rated at 80C or 105C. If you think you need the 105C and you are using 22 Awg wire, pay close attention to the wire diameters and the max diameters of the connectors. Many combinations will not work and other options will need to be considered.
Flame Rating
The two most common flame ratings for connectors are the UL 94V-0 and UL 94V-2. This is commonly confused with the temperature rating. The UL 94V tests refer to how quickly a wire or connector will extinguish itself after being removed from an open flame. The actual temperature rating difference between the flame ratings, in most cases, is less than 5-10 degrees however, the cost difference can be double. Be sure you know what you need and what you are paying for.
Abrasion
If your wire bundle goes around sharp metal edges or if it is in close contact with components that move or get hot, you may want to add some form of additional protection. If you have cables that move in tracks or are exposed to traffic, you my want to consider some of the more robust jacketing materials. Braided nylon sleeving is a good, low cost covering to use when additional protection is needed.
Tinning
To tin un-terminated wires or not….
If your wire is going to be soldered later, tinning is a good idea to keep the strands together and prevent bird caging. If the wires are going to be put into terminal blocks, tinning can cause delayed failures, especially with larger gauges. In these cases, if you still require strand management, you should consider using wire ferrules or request the supplier provide a partial strip (leaving the insulation slug on the end of the wire).
Contact Plating
Copper and its derivatives are great conductors but they oxidize easily. Plating is done to protect the base metal and reduce or control oxidation. Tin, Silver, Gold… don’t mix them! Tin oxide is non-conductive. When you mate tin to gold, fretting can cause tin oxide to build up at the contact interface and over time degrade the performance of the connection resulting in higher resistances and heat build up. Tin contact systems are designed with higher mating forces to provide sufficient wiping action to get a good connection. Tin is easy to plate and inexpensive. It is your best economic choice for general electronics applications. Gold is a great plating material (be sure it has a nickel under plate), but it is expensive. It should be used for harsh environments, very low signal and voltage levels or high mating cycle applications. Silver is the best conductor, it has a conductive oxide. Silver is your best choice for high voltage, high current or RF applications.
Electrical:
Voltage
The voltage that you can safely apply to any given conductor is dependant on the dielectric strength of the insulation around that conductor. In most industrial and electronic applications, PVC is the insulation of choice. If you have a high voltage application, you will need to consider using Teflon or silicone insulation. You should refer to the various UL and CSA ratings to allow for safe operating margins. For connectors and contacts, you should always stay within the manufactures recommended operating range.
Current
The amount of current that can be safely run through a given conductor is dependant upon many factors. You must take into consideration the wire gauge, the insulation type, the voltage drop across the conductor, is the conductor single or in a bundle, is it in free air or in an enclosure, and what is the ambient temperature of the environment the conductor will be exposed to. The more wires you bundle together, the more heat they will build up. Additional sheilding and over all jackets will act as thermal insulators, further reducing how much current a conductor or group of conductors can carry.
Shielding
When shielding is required for a multi conductor cable, consider how the shielding is best achieved for the level of isolation required for your application. For the best shield integrity, use a back shell system that crimps the shield to the shell. If the shield needs to be terminated through a pin in the connector, use a cable that has a drain wire. If you need a good shield connection through your back shell or have a cable that will be flexed frequently, don’t use a cable with just a foil shield. High flex environments will cause the foil to fracture and break up under the jacket. Over time, the shielding properties of the foil can be degraded or even lost.
Signal Levels
In applications where signal levels are down in the micro volt or micro amp range, special care should be taken to ensure the long term functionality of the connector system. At very low signal levels, even the smallest amount of oxidation can significantly attenuate and degrade the signal. You should choose a connector system with a high contact force, and good gold plating with a nickel under plate. Do not use tin, do not use gold without an under plate and never mix platings.