When to move to 4 layers?

Jay Kickliter

I've been working on a board for a while now, and no matter what I do, I can't get a decent layout on 2 layers.

Quick specs

2 Layer Board, ~1.2 by 2 inches

Top:

64 0603 LEDs in a column-row array, with 8 limiting resistors

Bottom:

Propeller with 4 local 0.1u caps
CR2450 Rechargeable Lithium Battery
MAX1555 Charge Controller
USB Mini-B Connector
FT232R
NPN Transistor for RESET
Voltage Regulator (Haven't found a model with a low enough dropout, but will probably be SOT-223)

I can get freerouting.net's autorouter to route it with far fewer VIA's than I can by hand, but I don't really like the results. And that is at a board size a bit larger than I wanted. I tried using QFN versions of the Prop and FT232R, but that was actually worse, since the router couldn't use the area under the chips for via's.

Is it it lazy engineering to make a 4 layer board for something this simple? I'm probably going to get 20-40 boards made in China, so the price for 4-layer wouldn't bee too bad.

thanks,

Jay

kwinn

It's not lazy engineering if the extra cost of the 4 layer boards is less than the cost of your time to do the layout by hand, it's economic good sense (or maybe cents).

Leon

What design rules are you using?

Poor autorouters are a waste of time, you will get much better results with manual routing.

Leon

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Amateur radio callsign: G1HSM

mctrivia

part placement and good hand routing technics will probably get it done in 2 layers.

The reason to go to 4 layers is usually for EMI reasons. If you route everything on 2 layers except your power lines then you can vastly improve EMI by using the other 2 layers as your power traces. It is often not to expensive to shrink your tolerances but very expensive to add more layers. Will 7 or 6 mil traces/spaces help?

I can get a gold plated 4/4mil traces/space, and 8mil via for $280
silver plated 4 layer 7/7mil traces/spaces and 15mil viasfor $320
both are electrically tested but which is better is a matter of what I am doing. For BGA work I need the tight tolerances and gold plating. For other things tighter tolerances are not needed but need for a good ground plane will push me to 4 layers.

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24 bit LCD Breakout Board coming soon. $21.99 has backlight driver and touch sensitive decoder.

Jay Kickliter

Thanks for the advice guys. I'll stick with 2 layers, and instead of hiding everything other than the LED's on the back side, I'm putting most of the parts on the top. It's making it a lot easier to place parts logically. However, hand routing the LED matrix is pretty difficult, and the autorouter is doing it with a fraction of the vias than when I try.

Leon, did you mean for Eagle or freerouting.net? WHat do you recommend? I was following Sunstone's minimum requirements.

Leon

I don't know what Sunstone's requirements are but you could find another supplier with smaller design rules. It could be cheaper than going to four layers. I often use PCB Pool - they can do 6/6 mil and 0.3 mm drills on double-sided boards.

The Eagle autorouter doesn't have a very good reputation. I use the Pulsonix autorouter, it's actually Electra:

www.konekt.com/

It's a very good autorouter. A version for Eagle is available.

Leon

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Amateur radio callsign: G1HSM

Post Edited (Leon) : 11/29/2009 11:02:40 PM GMT