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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.

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    \$\begingroup\$ If you're not willing to change your technology choices, then your only options are to be more clever about the routing, or more clever about designing a schematic that is easier to route. Since none of us can read your mind, we can't really be more detailed than that. \$\endgroup\$ – The Photon Feb 24 '17 at 16:57
  • \$\begingroup\$ Your comment and the other answer was exactly what I was looking for. I need to upgrade the board house/router package, etc. \$\endgroup\$ – mark b Feb 24 '17 at 19:12
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You have a few options.

You neglected to mention which PCB house you are using. Change to a PCB house with tighter tolerances, copper filled microvias, and smaller pad to drill size.

You neglected to mention which router you are using. A better router may help.

Look at the density of the routing, most CAD has a density map. Open up the power plane where the density is high.

You could also switch to a hash pattern with the power and ground leaving some area between the traces for signal routing. This could also be done just in the high density areas.

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    \$\begingroup\$ I am using Diptrace's router but it stinks so I'm doing it all manually. This helps. Thank you. \$\endgroup\$ – mark b Feb 24 '17 at 19:11
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    \$\begingroup\$ It's been a long time (15+ years) for me, the last packages I used were PADS-PCB and Protel 98 (now Atium). I have a very simple project a sting of 16 LEDs and the LED driver. I was thinking of trying Eagle, never heard of Dip Trace until now. Cannot find any comparison review between the two. Maybe you can help me out. Why did you not use Eagle? I am not implying Eagle is better. \$\endgroup\$ – Misunderstood Feb 24 '17 at 19:33
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    \$\begingroup\$ I used to use Eagle but the commands and flow was un-intuitive. Diptrace is cheap enough and everything just acts right and is intuitive. And Eagle was bought out and now has to phone home over the internet every once in a while to remain activated. \$\endgroup\$ – mark b Feb 24 '17 at 20:08

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