On Tue, 2007-08-28 at 12:37 -0500, Matthew Mucker wrote:
> I know this is an open-ended question that is likely to draw many opinions, 
> but I'm going to ask anyway.
> 
> When is it appropriate to move to a 4 layer board from a 2 layer board?
> 
> I'm designing a circuit that's more complex than I've done before and want 
> to know some of the factors that go into this decision, and the 
> benefits/trade-offs in this decision.

There aren't any "hard" rules IMHO. 4 layer (or more) is quite important
if power and ground planes are of utmost importance, so if you're doing
anything high speed (say over 100MHz digital) then I'd recommend going
to 4 layer just for the fact that you can dedicate a whole layer to
ground. RF circuits can also benefit from 4 layers.

Aside from that, density is the biggest factor. In general, a 4 layer
board allows for tighter spacing between components due to the fact that
usually you dedicate one layer to ground and another to power. Since
power and ground are usually the "most connected to" on a board this
drastically increases your signal routing options.

Of course, going 4 layer is more expensive, so you have to weigh the
size issue. If size isn't that important then often the larger 2 layer
board will still end up cheaper then the equivalent 4 layer board
(sometimes marginally though). If size IS important then the added cost
of a 4 layer board is usually OK.

Best way to tell is to try place and routing your design using 2 layer
and then using 4 layer (you don't have to finalize the route, a few
dangling nets are OK, you just want an idea of "how much better" the 4
layer option will be.

TTYL

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