Going Vertical
I am writing to you today from the summit of Enchanted Rock, a large outcropping of pink granite in the Texas Hill Country, about 100 miles northwest of Austin. The peak is about 1800 feet tall, well above its flat surroundings. At the top, through shimmering walls of heat rising from the bare granite, you can see a number of other similar, mesalike formations—places where the earth is pushed up into massive, flat-topped hills.
The roads below run like ribbons, undulating across small creeks and depressions. Scrub oaks and mesquite dot the landscape. Cattle cluster underneath the trees seeking relief from the midday sun. It's a good hike to the top; fortunately, I brought some water.
Sitting here just taking a rest, I can really appreciate the power of visual perspective. Everything on the ground looks so tiny. In the distance, a long, low bridge crosses a wide, dry gully. It reminds me of a pc trace, suspended on pillars at a constant height above a solid reference plane.
I wonder if this is the view that an ant would have from the top of a big processor (Figure 1). Perched on top of the QFP heat sink, the ant would be about 1/2 in. (500 mils) off the board. If the traces were stacked 5 mils above the nearest ground (or power) layer, the ant would be 100 times higher above ground than the traces. Just like I am about 100 times higher above ground than that bridge.
Figure 1—An ant surveys the territory surrounding a main processor.
As the ant climbs about the board, he might notice several things not immediately apparent from our perspective. For example, climbing down from the QFP, the ant would notice how large the QFP is compared with the traces. Surface-mounted parts protrude at least 50 or 100 mils into the air, but traces stay a comparatively modest 5 mils from the nearest ground (or power) layer. Surface-mounted parts look huge compared with the traces.
The ant, if he's smart, might wonder whether the electromagnetic emissions from the surface-mounted parts are larger than the emissions from the traces. Yes! In many cases they are. To see why, think about where the currents flow on the board. The same currents that flow in the traces also must flow through the ICs. The path for this current goes from the die, through the I/O pins of the chip package, out onto the board and then returns to the chip package through its power or ground pins. If the total loop area of this path is larger than the loop area between the trace and its nearest reference plane, the chip radiates more than the trace.
Do all traces radiate the same? No. Given two exposed traces (microstrips) of equal length, each carrying the same signal, the one higher above the nearest solid reference plane radiates more. For example, a microstrip 10 mils above the nearest plane radiates 6 dB more than a microstrip at a height of only 5 mils. In general, the higher an object is above the planes, the more it radiates.
Uh-oh. Looks like a thundercloud is going to pass overhead. I can smell the rain. There could be a giant electrostatic discharge sometime soon. I'd better get down off this peak. As everyone knows, the higher the peak, the more susceptible it is to lightning. By the way, a similar effect applies to circuit boards. Emissions and susceptibility are related. The higher you fly your traces above the planes, the more susceptible they are to the disruptive effects of ESD.
Keeping your traces close to a solid, uninterrupted reference plane is one of simplest, most effective things you can do to reduce electromagnetic radiation and harden your product against ESD.