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EMBED THIS PC

PART 4: Designing Peripherals In
by George Martin

Start ı The Hardware Interface ı The Artwork ı Sources and PDF

THE ARTWORK

Finally, letıs talk about the artwork for your design. If you attempt to put as much circuitry on one printed-circuit board (PCB) as possible, youıll undoubtedly end up in surface-mount technology (SMT). Thatıs not so bad, if you keep the use and implementation of that technology under control.

SMT mounts parts on the pads of the PCB surface instead of using through holes for components to be inserted into. Through-hole spacing is 0.100", which lets you put component leads on a 0.100" grid. Look at any dual inline package (DIP), and youıll see what Iım talking about. With 0.100" pad spacing, a pad of 0.050" diameter, and a track of 0.012" thickness, you could route one track between pads and keep a 0.013" clearance.

Tracks can also be routed on a 0.025" grid with that same clearance maintained. But, this layout commonly uses 12 mil lines and 12 mil spaces. Engineering design practices extended this to 10 and 10 by switching to a 0.010" line and a 0.010" grid, which lets you put two lines between IC pads.

In contrast, SMT lead spacing can be 0.100", 0.050", 0.020", or even smaller. With the smaller spacing, you canıt route tracks between the pads until the spacing gets to 0.020". Actually, it is possible, but 5 and 5 design rules are expensive when you get into production. Just so PCB costs donıt get out of hand, itıs usually best to keep to 10 and 10 or 8 and 8.

All the devices I talked about in this series are available in a DIP or 0.050" SMT package, with the SMT packages typically referred to as SOJ or PSOP. Be careful if you see a TSOP package type. Thatıs the 0.020" category. But, the TSOP-2 (a.k.a., TSOP type 2 or TSOP-II) uses larger spacing and might work for your design.

You have the same headaches with resistors and capacitors. Package types, such as 1206, 0805, and 0603, will come flying across your desk. Just duck. Because the numbering convention indicates package size, the 0805 is approximately 0.080" by 0.050", and so on.

Pick shapes small enough to fit your design, but donıt go so small that you drive your manufacturing costs out of sight. I chose the 0603 package size for a recent design because I thought I might need pull-up resistors on a bus. I was going to run a track between the IC pads, which had 0.050" centers. If each pad needed a pull-up resistor, then the resistor needed to be small enough to permit the track to pass. Resistors larger than 0603 block the routing lanes.

BALANCING DESIGN FACTORS

The point Iım trying to make about the artwork is that you need to balance the size with the cost of the parts, PCB, and manufacturing. Thereıs no such thing as a great design if the design rules you used are so unnecessarily complicated that you canıt afford the PCB.

Iıve run out of general guidance for embedding a ı486. If you have questions about this series or specific issues, post a question in the Circuit Cellar news groups. Currently, Iım beta testing for 32-bit versions of various tools sets, and Iıll write about those soon. This is an industrial/commercial grade design that can be customized and licensed. If you have any interest, please contact me.

Next month, letıs embed the Internetıalmost.

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