At 10:51 AM 1/21/2002 -0500, Matt Polak wrote:

>         It seems that a majority of you are doing some very dense, 
> high-speed layouts with 4-6 layers being quite a common occurrence. I'm 
> just wondering how much you typically route by hand, and how much you let 
> the auto-router whack away at.

I don't know about the majority, many of us are doing simple designs. We do 
both, but the vast majority of our designs are hand-routed. Sometimes, 
perhaps, we should be autorouting these boards, but we have a preference....

>         Being primarily self-taught in the ways of Protel, and with the 
> help of a few 'older school' engineer friends here and there, I've done a 
> number of successful design layouts thus far, but these have been 
> relatively simple 2 and 4 layer designs without many small-pitch/high 
> pin-count devices. I'm moving more towards laying out more high-speed 
> designs in the near future where a lot of stuff needs to be fit into a 
> small place, and all connect together without traces and vias meandering 
> all over.

It may be some time before you see an autorouter that will be fully 
satisfactory. At the present time, autorouters are especially good with 
boards that are complex in terms of numbers of connections but do not 
require space and layer optimization. In other words, they have room. 
Perhaps a company wants to make a test fixture, it has lots of parts but 
only a few are going to be made. In that case, the improved layout quality 
possible with hand routing is not cost-effective.

It's not just a matter of aesthetics. Good manual routing will, given the 
present state of the art, typically have shorter track lengths and fewer 
vias. As autorouter technology advances, I expect this advantage to be 
lost. How long it will take, I do not know.

>         When I look at sample six layer boards (such as the 5407 EVM 
> reference design Motorola has released) the bussing and interconnects are 
> extremely elegant and efficient in appearance. For fun, I unrouted the 
> 5407 board and then let the autorouter chew on it. It immediately made 
> 'via swiss-cheese' out of the board and created little more than a large 
> mess. I'm GUESSING quite a bit of these sort of designs are laid out by 
> hand, or at least pre-routed to give the auto-router a sense of direction?

Probably. But Protel's current router, under some conditions, can make 
fairly pretty boards. And that board may have been routed, say, with 
Specctra, which can cost, by itself, substantially more than the whole 
Protel suite.

>         Can anyone offer some basic pointers to getting started into 
> planning and laying out PCBs for multi-layer, high-speed designs such as 
> these? I have no idea where one would really even start with something 
> like this. It seems most of the important knowledge gets passed directly 
> from engineer to engineer; there are certainly no university classes 
> (that I know of, anyway) that teach you how to lay out a dense, 6 layer 
> board. :/ I appreciate the patience and wisdom of those who remember once 
> being where I am now, who are willing to take the time to pass some of 
> the tips and tricks down the engineering family tree.

I'll try to pull something out of my head.

I tend to place and route at the same time, placing components that seem to 
naturally go together -- perhaps they share a bus or busses -- and making 
the placement as tight as might be reasonable. At least I do enough 
routing, if any is necessary, to satisfy myself that this section of the 
board is going to route properly and efficiently. I might reassign gates or 
I/O ports at this time. These sections are then arranged in the board 
space. As a result, you might see, with one of my designs, tight space and 
empty space.

This is because tighter, in general, is better. One can always, if it makes 
a difference, spread parts out, though usually it is not worth the effort. 
But making a design tighter can be anywhere from time-consuming to 
impossible. So aim tight. It's also typically better, when done 
intelligently, from a noise perspective.

[EMAIL PROTECTED]
Abdulrahman Lomax
Easthampton, Massachusetts USA

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