Having chosen your enclosure from our wide range of ABS products, used our PCB test pins and had your enclosure machined for all your i/o connectors, you need to think of the current carrying traces that might be important.

Did you know that there is a maximum thickness of copper that will carry an AC current, hence signal, and that if you make it thicker you don't get any more current? So, even at 20kHz, if your audio cable has a diameter of more than 0.9mm you are wasting copper (remember Litz wires... see later). The 'Skin Effect' happens in all wire or conductors. When the signal is DC, then the current density is the same throughout the cross-section. However, when an AC current goes down a wire then the skin effect occurs.

More worryingly for Power Transmission lines running at 50Hz, the max useful diameter the cable can be is only 19mm, any diameter larger than this is wasting copper. For example, the Boulder Dam power station in the USA used hollow tube to improve the power/weight ratio of their cables.

SKIN DEPTH for Copper
Frequency microns Inches
50 Hz 9300 0.367
10 kHz 660 0.026
100 kHz 206 0.008
1 MHz 66 0.0026
100 MHz 7 0.0003

Hence if you have signals in the GHz region the broadband cables used are most often copper clad steel. There is only a thin layer of copper on a steel wire, the steel being there for strength only. This means such a cable will only work at high frequencies. But if you try to use that cable for low frequencies, such as audio, or to carry DC to power up a satellite dish, you will wonder what's wrong with the cable! All the DC power will be going down the steel wire, which is seven times the resistance of copper. What you want for audio or DC power is an all-copper conductor, ideally with Litz wire type of construction to increase the current carrying capacity.

All of Vero Technologies' prototyping boards use a copper thickness of 1 oz copper (35 microns). With 2 sides this gives a current carrying thickness of 17 microns. This means that signals up to approx. 20MHz use the whole copper trace. Above that frequency you only use part of it! If you need to carry more current then Litz wires are the solution.

What parameters of the “Skin Depth” affect design?

1.      Can a change in material affect the Skin depth?

There are no wonderful superconducting materials at ambient temperature, but there are certainly materials to be avoided!At 60Hz

Material Resistivity (nOhms/m) Skin Depth Comment
Copper 17 8.3mm Copper wire USA Power Grid
Steel 100 0.3 mm Steel wire, USA Power Grid
Aluminium 28 10.7mm Aluminium wire, USA Power Grid

Hence the steel core in power lines is used purely for strength, with stranded aluminium wires (light, with a better skin depth than copper) being used for current carrying.

Copper at 1MHz to 100 MHz

Material Skin Depth in microns Comment
At 1MHz, Copper 66 Wire carrying logic
At 100 MHz, Copper 7 Copper track on board

The thickness of the base copper on a pcb is normally 1 oz copper, 35microns, or more likely ½ oz copper, which is 17 microns, where fine traces are required.

At 100MHz the max useable size of trace on a pcb is 13x13 microns!

At 1GHz 

Material Skin Depth

 

In Microns

 Comment
Copper 2.0 Coax cable core
Aluminium 2.6 Waveguide wall
Silver 2.0 Coaxial cable core


2.      What are the design implications of the skin effect?

As always it is at the extremes of the frequency range that designers have to be careful about their designs to make them practical, but cost effective. Power transmissions line therefore need steel to provide the strength, but lots of aluminium or copper wires to provide the current carrying.

At high frequencies, it is exactly the same! Many coax cables for satellite dishes need the strength of the steel core, but use a copper “skin” to carry the signal. Be careful, these cables cannot be used for audio frequencies because the “skin” of copper is not thick enough & the steel resistivity is 6x that of copper.

3.      What can you do if you need the frequency and the current? (for example in audio systems, motors, transformers, inductive cookers & inductive charging !)

This is where Litz wires come into their own. Basically, they consist of small diameter wires (you can calculate the diameters needed from the skin effect at your required frequency) covered in a non-conducting material and then interwoven into bundles.

The insulation material required depends on the application, but varies from a plastic type such as a polyvinyl to silk!

For high performance applications the current carrying capacity of the wires can be optimised by weaving the bundle into patterns. This ensures that the impedance experienced by each wire is the same,  so that each wire carries the same current.

Summary

For engineers, the skin effect has significant implications on the design of AC electronic and electrical products, infrastructures, electrical power transmission and distribution systems. Designers must balance power, resistance, size, cost and strength requirements in conductors to produce practical products and solutions for each application.

5.      Further reading

If you’d like to check what skin depth your signals are experiencing then Chemandy’s calculator is really useful:-

To calculate the skin depth for a frequency:-

http://chemandy.com/calculators/skin-effect-calculator.htm

Audio Frequencies:-

Audio cables for those of us with good hearing are equally complex. For an in-depth analysis of  audio cables design criteria, Jim, at St. Andrews University, provides a good paper:

https://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/skineffect/page1.html

6.      Reduce your time-to-market

Your time-to-market will be shortened dramatically if you use one of our standard ABS enclosures and have it modified with all the cut-outs needed for your i/o connectors. Vero will also print your logo and id /notation on the product to give you a usable enclosure in a very short lead-time.

Don’t forget to use our pcb test pins to make it easier to fault find problems in the field!

Please contact us if you have questions.

The Vero Technologies team