The Quick INQ guide to designing motherboards

WE'VE ALREADY had a bash at the process of getting a motherboard's schematics together in our previous article, so now let's look at the next step - working out how it will look.

A high-tech process
This might surprise you, in an age full of high-tech 3D modelling software, but for many companies, the next stage is a fantastically nostalgic regression to methods seemingly more suited to the 1900s - a pencil, paper and a pair of scissors!

Yes, trying to work out where the main components should go is perhaps a thousand times easier to do by pushing life-sized cut-outs of the chips and bits around on a drawing of your target PCB, say for example, an ATX board, complete with all the possible mounting holes and where the slots should go, than it would be in CAD.

What we want
There are a few things that you're trying to achieve here:

• Work out if everything fits! It might seem pretty obvious but if it doesn't, you'll need to go back to the drawing board and choose some smaller bits or get rid of some features. Best do it now before you start routing everything.
• Work out the practicalities of how everything fits from a physical point of view. For example, how do your power and IDE cables block things on the board when you plug them in? Can you unplug a cable if you have two graphics cards installed?
• Where are the hot bits on the board, how close are they to the other hot bits, are they in a place where they will have air moving past them, and is there likely to be a lot of copper under the board to take heat away from it (the PCB actually acts as a very good heatsink for most of the components!)
• Lastly, and perhaps most importantly - how easy will it be to get signals from A to B without having to go via Timbuktu!

Getting it right
Helpfully, chip designers tend to think of the motherboard guys when they put the pins or balls on their packages, grouping together similar signals in areas of the chip that will point in the right direction.

For example, on a north bridge, you would hope that the designer would put the CPU interface at the top right, the memory interface at the bottom right, the PCI express graphics at the top left, and the south bridge connection at the bottom left.

Let's look at the Intel P965 layout below to see how this works in real life!

And, as you'd hope, everything matches a typical ATX board nicely - so draw Pin A1 and the main interfaces on your piece of paper, and you know roughly where and in which direction to point the chip.

Get it wrong, and what happens is pretty nasty - you will end up with signals criss-crossing each other. Whenever that happens, you have to move from one electrical layer to the other, through something called a via - no relation to the chip company. These joints have both resistance and inductance, so delay high-speed signals. On a parallel bus, even if your stability remains OK, it still means that bit 1 may arrive after bit 4, and it restricts how fast you can clock it - overclocker's nightmare!

If you have lots of signals that need to change layers, you're also likely to need more layers overall. PCBs typically come in 2, 4, 6 or 8 layers, with a usually motherboard taking four (two signals, one power and one ground plane). Go from four to eight layers, just because you have placed things badly, and you'll nearly quadruple the cost of the PCB. Ouch!

Getting it all together
So when you've got the bits of paper all right, and are happy about how things will be routed, generally you'll use a CAD program to create a DXF file with the overall dimensions and all the major bits on. This gets combined with the netlist, the symbols and the bill of materials and dragged into your PCB design software so the nasty business of routing can begin. µ