General tips for 4 layer boards

Question

For my Super OSD Pro project (Super OSD is divided into two versions: Lite and Pro) I'm going to be using a 4-layer board. Are there any gotchas to be aware of when using 4 layer boards?

I was thinking of having the most widely used nets (+3.3V and GND) on layers 1 and 2, and having layers 3 and 4 for carrying signals. I could also occasionally use layers 1 and 2 for carrying signals where 3 and 4 are full up.

One other thing I'm also concerned about is how to connect pins to the right layers? Say I want to connect a pin on the top to layer 2. I've always routed a trace from the pin to a via, then dropped the via to that layer, but is there a better way? I've heard of via on pad but I've also heard that can produce bad results and is only really for BGA's, not the TQFP's I'll be working with.

Answer 1

It's more usual to have the ground and power planes on the inner layers. It's best to keep them free of tracks.

Just use a short track and a via to connect leads to the other layers.

Answer 2

Ground and power planes are best to put on the inner layers.

You do not want vias in your pads except when you need them there for a specific reason, such as leading away heat or excessive solder paste from the middle ground pad on a BGA.

Answer 3

You'd normally put power/gnd planes on inner layers, mostly as these are the least likely to need track-hacking to fix errors or aid debugging. Almost all 4L PCBs you'll see do it like this. You will often have space available on the power plane to route a few signals. There can sometimes be some marginal benefits in EMC and track density (due to via size) by putting them on outer layers but you'd only do this if you'really struggling to avoid going to 6 layers, or need a little extra EMC performance due to the shielding effect of outer planes.