How does one determine the track width for "slow" signals?

[matrixofdynamism]

For signals that are "high speed" signals, the track width is a function of the distance from the ground and required characteristic impedance.

However, for other tracks that carry signals that do not fall into the "high speed" category, how does one determine what track width to use? There are so many different packages with different pad spacing and pad size, so I am not sure if "one width fits all" is possible. Also, I am sure that there will be a rule of thumb regarding the track widths.

I am looking for something specific number that I can into the PCB design tool rules database. Also, when we talk of track width, the track spacing also is relevant.

Now, coming to the power supply. We could have power supply spread using power planes in the PCB. Is this how it must always be done? What if we use PCB tracks with decoupling capacitors to ground and not use any power planes at all?

 

[marce]

For power tracks current requirement, so width based on that and PCB copper weight/thickness.
Basic high speed/FPGA/BGA boards 0.1mm track/0.1mm gap, often 0.5/0.45mm via pad with 0.2mm hole..
Basic Analogue/Digital boards 0.2mm track/0.2mm gap, 0.6mm pad 0.3mm hole for vias.
Variations and mixing of these rules often used on mixed signal boards, where some smaller pitch components are used.

 

[barry]

I think you need to educate yourself about impedance, current, power, signal integrity, PCB layout, decoupling…You are looking for a simple answer to a complex question.

 

[KlausST]

Hi,

If there is no specific current requirement you may just use "easy to produce" track width and distance.

In my eyes nowadays PCB design needs to fulfill EMC/EMI rules. It's hard to achieve without a true GND plane.
And with GND plane I really mean a solid sheet of copper without traces.

A piece of trace acts like an antenna, receiving and transmitting. And it has some characteristic impedance.
And all this does not change much if you make it much thicker and much wider. Thus it simply can't replace a GND plane.

Klaus

 

[senilicus]

Simply use your fabricator's recommendations on width/spacings. Most of them have this available as a download.
Have a look at the attached file. (as an example)

 

[matrixofdynamism]

I am studying PCB design and this question came up. I have found answers elsewhere. From what I have learnt, a human hair thick PCB track can carry around 1 Amp without significant temperature rise. So it is down to the person how wide they want to make the tracks. Thinner tracks are better since we can do more routing on the same area of PCB. Always, there must be a distance between adjacent PCB tracks to minimise the coupling from EM field. This gap that is a function of the width of the tracks. So thinner tracks mean less gap between tracks and less area covered on the PCB.

The only important thing is, to not use the minimum value that is possible to manufacture by the PCB house. The closer we go to the minimum value they can manufacture, the cost increases and the yield decreases.

The videos from Robert Feranec and Altium Academy are quite helpful in this regard but obviously one can't ask questions from a pre-recorded video.

 

[barry]

I am studying PCB design and this question came up. I have found answers elsewhere. From what I have learnt, a human hair thick PCB track can carry around 1 Amp without significant temperature rise.

I do not want you designing PCBs for me.

For 1 amp, you need about 12 mils (0.3mm). A human hair is about 1 mil.

 

[marce]

Again, I would recommend using Saturn PCB toolkit, the current capacity of a trace depends on base copper weight, whether planes are present, etc. etc.
As to signal integrity/EMC that's a whole world of fun and games... To cover the topic in detail involves a lot of reading and study, one important factor being characteristic impedance of a signal, which involves understanding the geometry of a PCB, dielectric thickness and Er, position of return path planes...
I agree with Klaus, that the minimum requirement for all but the very simplest PCB's involves having at least one contiguous 0V (GND) plane...
To put this in perspective, I have been doing PCB design for 37+ years, I have a personal collection of related documentation that takes up nearly 6GB of disk space and has 2,700+ documents and I am still learning...

 

[matrixofdynamism]

@macre, do you use Altium designer? Have you done high speed design?

 

[marce]

I don't use Altium...
I do all aspect of PCB design, mainly analogue/digital (fun), high speed (boring), RF & Microwave (crazy) and have done for a while now...