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akis 28th October 2011 07:57 PM

PCB placing and routing
 
I have a design which is about 150 components (maybe more). The board is 240mm x 110mm and almost fully packed.

I am having real trouble trying to place the components so that it can be routed. I am using Diptrace which even at the highest settings still leaves 8-10 traces unrouted. Are there any techniques which I am missing?

I have so far done this:

A. Placing

1. Start with components which must be placed in specific places, eg wire-to-board connectors on the board's edge.
2. Once these are placed check each of their pins and move close to the component other components which have a direct connection.
3. Once the first pass is done as in the step above, repeat the process for the newly placed components.
4. Observe other restrictions, eg decoupling caps placed near a chip.

B. Routing

1. Use one side to route manually very short traces in one direction, eg vertically. Use the other side to route short traces horizontally.
2. Use the Autorouter to fill in the rest.

AmpliFire 28th October 2011 09:27 PM

Perhaps some components are not really well connected inside the schematic diagram?

Optionally, add some resistors in the unrouted traces to test if that solves the problem, then replace them by jumpers during construction afterwards (or create jumpers in the library, if they don't exist already).

nigelwright7557 28th October 2011 10:09 PM

90% routing isnt too bad. My CAD package does a similar amount.
I finish the rest off by hand.

I let the router do the easy short tracks and route the rest by hand.

Placing is easiest done by just moving the components in keeping close linked components close at hand.

My CDA software has autoplacing but it can end up with a poor layout because it starts shifting around decoupling without the associated chip if I dont make the decoupling capacitor and chip unmovable.

For example I have op amps with +/- 12 volt decoupling and digital pots with only 5 volts connected to them and the autoplacer put the 5 volt decoupling caps next to teh op amp !!! Clearly very wrong.

Routers and autoplacer need to follow rules to get it right.

jcx 28th October 2011 10:14 PM

if its high performance analog then using autoroute is foolish

4-layer is great for superior anaolg performance, I often use slotted/moated gnd plane but route in the power plane - fat power buss but sometimes there's space for a few signal crossovers

akis 29th October 2011 10:06 AM

The unrouted traces are quite long, spanning across the PCB. Even if I insert a resistor it would still need to link between two pins quite far apart on the PCB and difficult to route.

I could do it with 3-layer (do not need a 4th layer) but the guy who makes PCBs for me can only do two layers.

I can improve things by increasing the spacing between components, but then the PCB becomes way too large (over 270mm length).

I am thinking with only 10 or so unrouted traces I could link them manually by wires.

AmpliFire 29th October 2011 11:43 AM

Another option is to create little PCBs which comprise a part of your schematic, which are only 3 or 4 cm high, and which you solder standing upwards on to the PCB using pin connectors, like any of the components. That may turn out a very useful option, as it would also allow you to change out the little boards if you want to test out certain components. For example, when you would have a rather complex differential input stage and you want to test different kind of transistors. It was a technique often used in 70's tapedecks due to the very high density of boards in limited space.

EDIT: I would not walk the path of 3 or more layers, cost-wise, unless you intend the bring this board into large quantities.

Mooly 29th October 2011 12:48 PM

I use Diptrace but never the auto router.

I'm with jcx on this one... for audio you have to do the layout manually as only you (and not the auto router) knows the in and out's of ground loops and interactions etc.

marce 31st October 2011 12:18 PM

Placement is 80% of PCB design, try different placement, and avoid autorouters, there is no such thing in the real world. The autorouting function is a tool to be used interactively with manual design techniques. I wouldn't use them at all with analogue, and when using them with digital you have to set them up by defining contrsraints etc, and even then you will have to tidy up between passes.
For multilayer PCBs you always use an even number of copper layers, or your going to have BIG problems.

fingerboy21 7th January 2012 07:53 AM

Auto-routers are always bad news. Like others have said, you should focus on placement first. Place and lock your mechanical restraints, then work to place the crucial components, then finally the miscellaneous passive components.

pinkmouse 7th January 2012 09:48 AM

I've never used an auto-router. For big designs, I tend to start by sorting stuff into functional blocks, route those, then move them to a an appropriate place on the PCB. Then once everything is on the board I connect them all together then go through several iterations of tweaking.

But to qualify, I'm not a pro, and haven't really done much digital stuff, whereas some of the chaps posting above are, and know what they're talking about. ;)


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