Hello,

You can not always keep the traces small.
For higher currents as in powersupplies and power amplifiers, you must keep track on the maximum current in a trace.
This PDF will give you the needd info:
Maximum_current_for_a_pcb_trace.pdf

Bertus

 

Time for a history lesson. What did we use before we got these cool computers with all this great PCB software? One thing we used was tape. Does anyone remember Bishop Graphics? They sold tape and stick on patterns of PCB components. This is how the artwork for a PCB was made. I made the following art work sometime in the late 1970s.

Its amazing, the tape is still stuck to the mylar after 36 years.

Imagine designing PCB artwork this way today.

 

Why not make fat traces and thin traces.

Old answer is, that the tape used to make the traces was only available in a few sizes.

But that was a long time ago.

But keep traces a small size.
capacitance and inductance of the track is always something to consider.

Use the same size tracks, and fill the spaces with plane.
saves copper etching.

You can't just "keep traces a small size." As you point out, capacitance and inductance of the track is something to consider (though not "always" something to consider). Inductance goes up as the trace width goes down, which is often not desirable. Also, trace resistance goes up as trace width goes down. This is not even considering the case of controlled-impedance traces. So there are factors that argue for making some tracks wider and others narrower.

 

I would like to add to the post @bertus made about current handling of traces.

A simple graph of current handling verses trace width and thickness is not enough. It basically says if you go above so much current for so much trace you are in danger of cooking that trace off the board. Traces are often burned off boards during catastrophic failures. No surprise here. What the bigger issue is, how does the traces resistance impact the circuits operation/performance.

Let me give you an example. You are building a 10 amp power supple. Your current sense resistor is 0.01 ohms. Bertus's graph says that you can use a .200" trace to carry 10 amps with 1 ounce copper. The trouble is, a 6 inch long, .200", 1 Oz. trace has about .013 ohms of resistance. That resistance is a little higher than your current sense resistor and can easily de-stabilize your regulator. In other words, if the traces that connect your current sense resistor have higher resistance than your sense resistor, you got problems.

Here is a real example. I am currently designing a 20(plus) amp solar battery charger. I kept having problems with stability and it outputting the correct current. For the input voltage and grounds I used 3/4" traces. Long story short, I had to reinforce the high current traces with 12 ga. wire. Here are some photos:

Component side

First high power prototype, This is where I found the PCB traces were inadequate. Soldering 12 ga wire on to the board cured the stability and accuracy problems.

The next high power prototype. Every jumper made a significant improvement.

So, trace size is not just about power handling, it is also about voltage drop.

The more you build and test PCBs the better you get.

Here is the trace table I use

 

. Does anyone remember Bishop Graphics? They sold tape and stick on patterns of PCB components. In 1970's.

Yes remember it well, made a few up this way.
Graduated to Orcad DOS, now a Kicad fan.
Max.

 

Attention all you experienced PCB designer/builders.
I would like to invite all of you to post a picture of a PCB you have built and talk about what concerns you had as you were laying it out. Also please share any insights you have about laying out a PCB in general.

The priorities are different for each kind of PCB. Audio amplifiers, power supplies, RF, digital, analog, etc., all require a slightly different mind set when being laid out. Each of you have a different way of looking at laying out a PCB and there is someone out there that needs to hear what you have to say. So, lets hear how you do it. Remember, you are talking to a beginner, so, give them a little push to help them getting started.

 

How do call it a "black art" when you gave a clear example that the golden rules of PCB layout and circuit design actually work as described?

Here is the starting point for all engineering - understand the science, use the science and get satisfying results.

There is a lot more to it than my simple reply.

 

There is a lot more to it than my simple reply.

I agree there is more to PCB Layout than your statement. In fact, I think there is so much known about PCB layout that I cannot agree in any way that it is Black Art.