Electronic – Have I placed too much on this PCB layout


I am doing my first PCB layout (using Altium) and have finally gotten past the auto-router stage. The result is a mess and there are some missing nets and design rule violations. Have I packed too much on this board or do I just need to re-think my component placement?

The board is two layers.

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I am stuck with a very specific enclosure and won't be able to make the board bigger in the x-y axis.

This is a hobby board, but I have a full SMD soldering setup at home (nice scope and all). The connector placement is part of the enclosure (otherwise those would be the first things to move). It's a drop-in replacement for an older engine monitoring system. It takes measurements mostly from thermocouples and thermistors. The large chip in the center is an ATmega2560 running at 16 MHz.


Thanks for all of the input. I rearranged the board and moved to 4-layers. Then I routed it all by hand. It looks much better now!

New board layout

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Best Answer

I assume that you are using the autorouter because you think it will save your time. But I have some bad news: it is said that PCB layout is 80% component placement, 20% routing. You can't just slap down components, you need to think about how the signals connect and if you place the components right, the layout will "flow" from this placement. So if you have a good placement, you have your routing straight away and might as well do it yourself (or at least large parts of it) while doing this layout.

Autorouters are a pain. I've never seen people use them very successfully - especially the built-in ones like what you find in Altium (though they were showing a new tool recently, so that might help?). In addition, the placement of components is vital.

One problem with any automatic layout or circuit synthesis is that the program will only do what you tell it - and if you don't tell it everything, it will do stupid things. Your rules need to be perfect. Your constraints need to be complete. Every requirement you have needs to be put down in the form of rules and directives. Often you might not realize how much you implicitly know/require - Don't route the power signals all the way around the board - The connection between the decoupling capacitor and the supply pin of the chip has to be as short as possible and not go snaking around a bunch of analog circuit - the list goes on.

Your placement seems sloppy - take this example:

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If you were to flip R17 the trace that goes from R17 to R18 would not need to cross the trace going from R17 to D1. R19 seems to be in parallel with C12 - perhaps this is something you can use to simplify the layout, by physically putting them parallel to each other. Moving R19 above or under C12 would also make it easier to route C18 nicely. C17 also seems like it could be flipped 180 degrees such that it doesn't require crossing traces. Turning D1 90 degrees clockwise might make it easier to route that trace from the "center" pin to R17. And you have a bunch of unused space under these components, why not use it and move the entire assembly down a bit? Remember that thing I said about 80% placement, 20% actual routing?

In addition, it seems like your autorouter just gave up. Take for example: Odd error

There is a lot of space to move these traces apart. This shouldn't be a problem, and anyone can see that you just have to move the left trace a fraction to the left, and the error would be fixed.