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I'm wondering at the moment with my Super OSD project whether to go to a 4 layer board.

At the moment, it consists of two micros and some additional circuitry such as a discrete audio amp and switch logic, and a serial EEPROM plus a temperature sensor. There are probably around 40 resistors, 10-15 caps and a few other components (all surface mount 0805 or 0603.) It might get tricky to route it.

Is it worth paying the extra bucks to get 4 layer boards for a relatively simple project?

I'm on the fence, I like 2 layer because it's cheap and if you've got the right equipment you can even do it at home, 4 layer is more expensive, but allows me to squash more stuff into each square millimeter. I've seen a couple of open source projects (Super OSD is open source) using 4 layers but many more using 2 layer.

So, opinions?

Thomas O
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3 Answers3

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Some reasons to move to 4+ layer:

  • Need additional routing space

  • Need power and ground supply planes due to current draw

  • Need to control trace impedance. You can only control the impedance of a trace if it has a consistent reference on the adjacent layer(s). This is critical for signal integrity of many signals but mostly associated with medium-high speed digital and/or long traces.

  • Need low noise performance, either analog or digital. Its almost impossible to pull this off without at least a ground plane in the stack up.

  • EMI: Unless you're able to devote the bottom to a full ground pour you have to be very vigilant with your ground return paths to avoid current loops.

I'm sure there are more, that's just what popped into my head.

I've personally never had a board fabbed that wasn't at least 4 layers so I can't say too much about the cost of 2 layer.

The move to 6 or 8 layers is usually what I have to fight with and most of the time the same rules I listed above are the driving forces, with the addition of needing to route very high speed signals between plane layers to reduce EMI.

Toby Jaffey
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Mark
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I agree Mark, but would like to add some info about cost. If you are looking purely at prototyping in low quantities you can get a 4 layer board from Advanced Circuits for $66 and $33 for a 2 layer board.

If you are looking further into the future and want to think about large quantity fab you will need to consider that PCB fab houses will look at everything, more then just number of layers, when they give you a cost. Some items include size, number of vias, special cuts, number of holes... So in this sense, if you are able to get a board to be half of the size and with half of the vias by going to a 4 layer board it may be well worth it.

Also, when you get to higher quantities you will probably need to be looking at FCC testing. Many times in order to pass FCC testing you would need to get a 4 layer board in order to get your unintentional radiators down.

So hopefully by now you can tell that there are many many factors that factor into which way is the smarter choice. It is hard to tell which way to go, but at some point you have to make it.

Kellenjb
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Put it this way, the cost of the time you save by being able to route on 4 layers (or having internal power planes to ease routing) will pretty much always outweigh the cost difference between a 2 and 4 layer board.

So its very worth it in my opinion

Fuzz
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    This is actually a hard thing to say that it is always worth it to go with a 4 layer board. For Hobbyist, time doesn't usually have a huge value where as money out of pocket does. For large quantity there is always a trade off between employee time and pcb cost. – Kellenjb Oct 18 '10 at 02:45
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    One example of this that I see a lot is the cheap AC USB chargers. Typically they will use a single sided pcb. In this case the company decided it was worth the time to figure out how to route on 1 side because of the quantity that they would be producing in. – Kellenjb Oct 18 '10 at 02:46
  • good point Kellenjb. I tend to base this on working with projects for the company, not from a hobbiests perspective. For me, making PCB's is a work thing to do, not a hobby haha – Fuzz Oct 18 '10 at 05:33
  • The boards are also single sided because it is easier to meet EMC compliance and safety rules (re isolation between traces) afaik. – Thomas O Oct 18 '10 at 12:27