Controlling the track width

In this chapter we will improve the electrical characteristics of the PCB by increasing the width of the Ground and Vcc tracks. As far the features of this PCB are concerned, I think what we have now is enough. We could stop here and send it to the manufacturer but in general there are two more things that you can do in any design. First, you can take into account the current requirements of particular pads, especially those that convey power like in this case, VCC and ground. Also you can take into account the heat dissipation requirements.

For this board the main component is the NRF2401L+ module. In its documentation we can learn that at peak receive in transmitter operation, it will draw up to 14 milliamps of current. We should confirm that the tracks that convey power can accommodate this amount of current going through them. From my own experience, I know that this module can get a little hot when it is operating at peak. It doesn’t get as hot as a CPU or RAM does but I would still like to take that into account and build my breakout board in a way that it manages the observed heat profile.

What I will show you next is not scientifically robust, but for our practical purposes it will be good enough. Let’s start with the track width. You can control the width of each track manually or automatically. The automatic method works with nets, which is something I haven’t discussed yet, but I will in the next project. The manual method involves creating custom track’s widths and then applying one of those to a new or existing track.

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To create custom track widths, select Design Rules.

Let’s go ahead and add a few custom track widths that should cover 99% of your requirements. To do that, let’s go to design rules and check on the first option design rules.

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Once in the Design Rules Editor, click on the Global Design Rules tab.

Switch to the global design rules tab. Notice, in the middle area of the window, two empty grids. On the left side you can see the Custom Via Sizes grid, and on the right side is the Custom Track Width. We will populate these grids with our own values. Let’s work on the widths first. I looked up the manufacturing specifications for OSH Park and I found the typical widths that they support. I will use these numbers here. These numbers tend to be used across the industry, so chances are they will be right for many other fabricators.

Please look at the screenshot below and copy the custom widths to your Custom Track Widths grid:

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The custom track width sizes that work with OSHPark (and many other fabricators).

Custom tracks 3, 4 and 5 are very large and unlikely I will ever use them, but we can include them for the sake of completeness.

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The custom via sizes that work with OSHPark (and many other fabricators).

Even though we are not going to be using any vias in this project, we will in the next project. We might as well put them in here to have them ready to use later. Copy the values from the screenshot above to your Custom Via Sizes grid.

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The minimum allowed values must be at least equal to the smallest values in the two grids.

At the top right side of the window you will see the minimum allowed values group. None of the values in the two grids of this window can be smaller than the corresponding values in this group. I have entered the correct values to satisfy the minimum value rules in the Minimum Allowed Values group, so feel free to copy these values across to your Design Rules Editor.

When you are finished, click on OK to commit the changes.

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The custom track widths now appear in the width drop down menu.

You can now see that the values that we have just entered are available from the Track Width drop down menu. Let’s change the width of one of the existing tracks. Let’s change the width of the Vcc track to something bigger. First, choose the new dimension, for example, 0.508 millimeters.

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First, choose the desired track width.

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Second, right click on the track you want to change, and select “Change Track Width”

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Notice that the track is now thicker.

Then, go to the track that you want to adjust the width for, right click on it and select change track width. You can see that it has become thicker. You can do the same thing for all the Vcc lines.

For the ground tracks, I would like to also make it thicker. Taking the available space into account, I will choose to make the Ground tracks 0.381mm wide. Again, choose this value from the Track Width drop down menu, then right click on the Ground tracks and select Change Track Width.

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This GND track is adjusted to 0.381mm width.

That should be okay. I will do a check in a second. Same thing for this. It will change the whole track not just the segment. If you want to just change the segment then you can just choose, change segment width.

In this chapter we decided on the track width for the Vcc and GND tracks by simply guessing what values might be ok. There is not much science in this. In the next chapter, I will show you how to use Kicad’s built-in track width calculator to calculate, instead of guessing, the appropriate track width.

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