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Tips on routing this high density/high current board? I'm stuck - any ideas?

I am building a display consisting of a series of seven 5x7 LED dot matrix displays, butted up against one another. The displays are of the 0.7" variety (about as small as they come). I will be driving them with PWM to control brightness, using shift registers to control the 35 horizontal elements, and switching the 7 rows sequentially to paint the display. The pulse current on each dot is 100ma, and times 35 horizontal dots, that's a potential max of 3.5A.

Problem is, I am trying to keep the board small. The traces required to handle 3.5A are pretty fat - too fat to allow the close spacing I need for the project.

Anyone have any tips?

Some ideas....

I could mount the through-hole displays in DIP to SMT adapters and surface mount them, freeing up the back of the board to route my traces. Downside is cost and increased complexity, and the board would be thicker

I can seperate out the segments into 2, 2 and 3 and they'd be 1A, 1A,

1.5A, which I can fit the traces. However now I am faced with routing three fat traces to each of my power transistors that switch the rows, and I'd definitely have to increase the board size to accomodate

I can use a 4-layer board... probably the most suitable solution, but more expensive

I am already putting all my shift registers and microcontroller on a sub-board, so I've trimmed the components on the display to the bare minimum.

Any other ideas? Routing this board is driving me nuts!

Thanks!

Corp.

Reply to
ferrari.secret.santa
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On Thu, 16 Nov 2006 20:31:20 -0800, ferrari.secret.santa wrote: [about high-current traces]

Can you just order thicker copper plating?

Good Luck! Rich

Reply to
Rich Grise

There are two things you have to take into account with low-frequency, high current lines. One is DC resistive voltage drop, and the other is heating. For the first issue, peak currents are the main thing to look at. Depending on the application, you might be able to improve on that by putting your drivers closer to where they are needed, and/or using capacitors to store charge locally (i.e., think of you system as a series of unregulated switch mode power supplies). Other than that, the tracks should be as thick and straight as possible, with multiple tracks on different layers.

For the heat issue, it's the average current that is most important. There are also plenty of tricks that can be used to improve heat dissipation - splitting the track in two and running each half on a different board layer (especially the outer layers), using polygon pours where there is space, adding extra tracks that don't carry current anywhere but act as cooling fins, and no solder mask so that the tracks get a layer of solder when you produce the card (solder is not nearly as good a heat conductor as the copper, even with a thin track, but will give you better air cooling).

Reply to
David Brown

Perhaps use bus bars. Strips of plated copper with legs. They stand vertically and can be bent to go around corners.

Reply to
CWatters

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