Selecting wire size should be the first task to perform when designing a new micro coaxial cable. Appropriate sizing is crucial to the success of cable design. Below is a list of key factors to account for wire size selection, followed by insights on each item.
- Space Constraints
- Compatibility with Connector
- Current Capability
- Allowed Voltage Drop and Ripple
- Allowed Signal Attenuation
- Cable Flexibility
- Cost and Lead Time
Space Constraints
Micro coaxial cables are usually used to go through very narrow spaces such as hinges of laptops. In such cases, the first priority is to make sure the bundled cable can safely go through limited space with enough margin. Assuming we have N wires bundled as a cylinder and each wire has a diameter of d mm, we could use a simple formula to make a quick estimation on the diameter (D) of the cylinder as below.
The d should be appropriately chosen to make sure D does not exceed allowed value.
Compatibility with Connector
It is recommended to determine wire size before determining connectors. However, that is not always possible (e.g. when trying to connect to an off-the-shelf module). If the connector is determined in prior, we need to make sure the selected wire is supported by connectors at both ends. It is apparent that the wire size should never go bigger than connector pitch. However, that is not the only rule. While many connectors can support any sizes smaller than pin pitch, there are also connectors that only support one or two specific sizes. The connector datasheet should always be checked to ensure compatibility.
Usually, connector makers list supported wire sizes with AWG numbers. However, it should be noted that AWG only defines the conductor sizes, but not outer diameters. For micro coaxial cable, outer diameter depends on cable properties such as characteristic impedance. The outer diameter should be checked against connector pitch even if the AWG number is in the support list.
- In practice, it is safer to keep nominal wire size at least 5% smaller than connector pitch, to avoid any tolerance issue.
Current Capability
If a wire is used as power line, it should be thick enough to carry required current. Since the current capability of micro coaxial cable is very limited, when large current is desired, it is recommended to use discrete wires (a.k.a. Teflon cable or PTFE cable) for power lines. Then the cable will be a hybrid of discrete wires (for power) and coaxial cables (for signals).
There are some special rules for such hybrid cable.
- The outer diameter of individual wires should be close enough with each other. As a rule of thumb, the difference of outer diameter between any two wires should be no more than 10%.
- All discrete wires should be collectively arranged at one side and coaxial cables at the other side. i.e. alternating discrete-coaxial-discrete arrangement is not allowed.
Currently our online design tool does not support mixed wire types. However, our manufacturing line does support it. Please upload your own drawing, or drop a message to our support team if you want to make micro coaxial cable with mixed types of wires.
Allowed Voltage Drop and Ripple
Current capability is the first factor to determine minimum wire size, but not the only one. For lengthy cables, the voltage drop must be considered. Because micro coaxial cables are so thin, the dc resistance plays a much larger role than other cables. It is important to make sure the voltage arrived at the terminal meets minimum voltage requirement. Moreover, for those that have strict requirement on voltage ripple, thicker wire is preferred because they have smaller resistance and parasitic inductance, both contribute to smaller ripple.
Allowed Signal Attenuation
For high speed signals, allowed attenuation is also an important factor to consider. Similar with power lines, thinner wires tend to be lossier, so thicker wire is preferred from signal attenuation perspective.
Cable Flexibility
Keeping the concern on voltage ripple and signal loss in mind, one may think thicker wire is always better as long as the size meets space constraints. That is not true because the stiffness of bundled cable grows quickly as the diameter of individual wires increases. It will be difficult to arrange a cable in a complex space if it is too stiff. So the wire size should be selected with stiffness in mind. If there is any doubt about that, the best practice is to take a sample of existing design, identify its difference with the cable under assessment, then make a reasonable estimation according to the stiffness of the sample in hand.
Cost and Lead Time
The priority of cost and lead time largely depends on the budget and scale of your project. It is worthy to note that 40 AWG and 42 AWG are the most widely used sizes in micro coaxial cable industry. Thanks to the large amount in the market place, they usually have more attractive price and lead time than others. They should be your first choice if you do not have bias on other properties.
Conclusion
Putting all above together, the wire size should be carefully selected to achieve best balance among flexibility, reliability, cost and electrical performance. There is no single answer to fit all cases. If you do not have enough confidence about the best choice, feel free to drop a message to our support team. We could make the best solution for you. In next post we will share more details on the definition of cable sizes. Keep tuned!