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?
Best Answer
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.