EMI in low-cost circuit boards

Electromagnetic News Report, Nov/Dec 2002 by Kimmel, William D, Gerke, Daryl D

EMI in Low-Cost Circuit Boards

With microcomputer speeds well past the GHz range and still going up, we sometimes lose sight of the fact that the overwhelming majority of circuit boards manufactured today are built on one or two copper layer boards. Hard to believe? Not when you consider the sheer number of circuit boards used in consumer products, including microwave ovens, washers and dryers, toys, greeting cards, and especially automobiles.

The boards are often small, with relatively simple dedicated function. The production quantity of each board may run well into the millions, making piece part cost a major consideration in the design - even fractions of a cent are significant. The most conspicuous cost to be avoided is the circuit board - one copper layer is the holy grail, two layers if absolutely necessary, a multilayer board is blasphemy.

Even at that, we wonder if the one- or two- layer board is always less costly. For a start, figure on expending lots of development time and money - meeting modem EMC requirements with a board made of baling wire is a real challenge. And after lots of design and testing, you still may not get there. Then consider extra components that are added to compensate foia low-cost board and you may find your gains disappearing.

Should you spend $100,000 more in engineering costs and add an additional three months to your design schedule to save the cost of a multilayer board? If your projected quantities are in the millions, it may be worthwhile.

Grumbling aside, we recognize that low-cost boards are the only choice for many engineers. We can't solve everyone's problems, but we can identify some common constraints.

What Are the Key Elements?

First and foremost, ground impedance must be kept to an absolute minimum. This is easy with multilayer boards, but what about one- or two- sided boards? This is certainly not easy, but that's life in the fast lane.

It is exceedingly difficult to overcome the deficiencies arising from high ground impedance. In the 100 plus MHz range, even a short trace has three orders of magnitude higher impedance than a ground plane. As an example, a one cm trace has an inductance of about 8nH, which has an impedance of about 15 ohms at ESD frequencies. In contrast, the plane has an impedance of about 5 milliohm at the same frequency. Looked at from an ESD standpoint, one amp of ESD current will bounce the ground of a one cm trace about 8 volts while bouncing the ground of a plane only SmV. We would expect a SmV bounce to be tolerable in most digital applications, but 8 volts of ground bounce will almost assuredly cause a data error - ESD currents must be kept off the board.

Loop area plays a similarly important role. Antenna efficiency (either for emissions or reception) is proportional to the loop area (for small loops). Traces routed above a continuous ground plane always have a small loop area, but the two-sided board has serious loop area problems (see Figure 1). Achieving ten-- fold reduction in loop area over a ground plane is commonplace.

The number of copper layers is not the issue. The miracle occurs with the presence of the first ground layer. Additional layers are icing on the cake, but by no means mandatory.

Having said we need a ground plane, where do we get it? Figure 2 shows a technique we like to call "micro-island." The concept is to allocate copper under the key circuits. As shown, copper is under the chip and filter terminations. For simple cases, it is feasible to implement this on a two-sided board, and it is not inconceivable that this could be implemented on a one-sided board.

It is important to have the ground plane be continuous -- cuts in the plane degrade performance very quickly. If you can't make the plane continuous, add jumpers to minimize the slot size.

Suppose we can't even allocate copper for a ground plane? Well, it's like losing the queen in a chess game - we have lost a major piece, but we can still give it a try. You start by minimizing the loop areas of critical traces. This includes the power/ground path, clock traces and input strobes. Keep the ground traces as wide as you can to keep impedance down. Once you have a layout, fill as much as possible, and connect the fill to ground to keep loop areas down. Finally, bridge adjacent ground paths with jumpers - slice and dice the ground paths to reduce loop areas and ground impedance.

There are other options, but they involve cost, detracting from the gains you were seeking. Chip shields were designed for shielding high frequencies directly from the chip, but also work well for providing low impedance grounds at the chip level. Conductive paint on one side of the circuit board is also effective.

Protect the Periphery

Having done as much as we can on the internals of the circuit board, we need to turn our attention to the periphery. Circuit boards are not particularly prone to radiation (inbound or outbound) until you get into the hundreds of MHz. At lower frequencies, the problem almost always involves the wire connections, including power, signal and ground wires. This mostly calls for filters.