I am routing a board that I cannot make larger. It is fairly dense with lots of SMD parts. I am trying to route traces to and from a 100 pin uController to various chips on the board.
It is a 4 layer board with GND on layer 2. Mostly power planes on layer 3 so I have layer 1 and 4 for the signal traces. But layer 1 is really dense with parts.
I have routed about 80 percent of the lines but now I'm finding I'm running out of space to route the remainder. I'm finding the need to make long winding traces for the rest of the lines.
On two of the sides of the uController there are some traces going to a 20 mhz crystal and a 32768 khz crystal. There are grounded copper pours oing around those crystals so that takes up some board real-estate there.
In the middle of the chip there are fan-outs of lots of vias and I have run out of room there.
What techniques are there for routing these things short of going to more layers?
I've gotten the trace width and clearances down to the minimum the board manufacturer will allow me (.006)
I've used the smallest vias I can, but I cannot make blind, buried vias nor do I want to put vias inside of the larger pads.
I cannot put parts on the underside of the board. I cannot make the board bigger.
How much of an issue will there be if I use a part of the layer 2 (GND) for this? The highest frequency item on the board is the 20 mhz crystal. Everything else runs at 1/4 that speed. I have a buck converter running at around 1.2 mhz but that's in an area that I'm done routing.
Or would it be best to rip up the lines and try re-routing from the uController chip out? I didn't want to do that since my uController allows a lot of pin re-arrangement in software.
I'm using Diptrace.
