Making PCBs EMC compliant

PCB board design is often governed by established cost aspects. If a designer comes up with an EMC-proof design (in theory difficult, if not impossible to achieve without actually testing it as a product), they are likely to hear that it’s going to be expensive to lay it out according to their wishes.

Therefore there’s a bit of a prayerful ‘she’ll be right’ attitude when it comes to compliance.

If during EMC testing, for example, the emission level is too high, effective encapsulation in a radiation-proof housing might collect things. One way is to use a metal box using high conductivity screws (rather than anodised screws) to hold panels together, to consider the use of high conductivity gasket material as intermediate between panels, limit access holes and take particular care with front panels.

Enclosure of boards and individual component groups may be a way of heading off EMC problems too, however, apertures can be very effective in emitting radiation by diffraction, created inside the box.

A seam in which a small gap occurs can act as a pinhole emitter. The gap doesn’t need to be more than 2-3 % of the wavelength. In electrostatic discharges, the front panel can act as an antenna, beaming component-destroying radiation within the box interior.

Don’ts, therefore, include the use of Teflon and other non-metallic fixturing, chosen more for appearance than functionality, and anodising front panels.

However, doing things afterwards is often expensive - and might even render the complete product unmarketable. Some designers hold that problems not solved at component level will increase costs at board level by an order of magnitude.

Sounds a bit extreme but a factor to be borne in mind.

Figure 1: Langer H-probe.

The use of mini-probes to measure H and E fields at pin level (see illustrations) at the very least provides clues as to how to prevent interference signals from spreading or affecting input signals.

Figure 2: Langer E-field probing.

Once again, if not solved at board level, expect another order of magnitude in correcting the problem at modular level. Generally there’s not much to choose with ICs - control of EMC characteristics not being considered an important design parameter.

FPGA and ASICs by their customised nature, allow designs that take account of immunity and emissivity aspects. EMC problems can arise in DSP circuitry because of the very high data rates being exchanged between ICs.

VLSIs pose a particular problem in that the core logic uses much smaller ‘devices’ than the output drivers and because of consequently higher internal frequencies, the signal edges are sharper and therefore carry the distinct possibility of RFI.

These transients can affect track lengths as short as 5 mm. The miniaturisation of packages also brings problems with it because of much reduced lead inductance.

Filtering options are confusing. First, let’s look at what’s available. The options include series R, series L, shunt C, RC combinations, LC combinations, resistive Ts, inductive Ts (series L, shunt C) as well as pi topology combining R, C, and L.

For frequencies above a few MHz, soft ferrites are preferred over inductors. It is best when laying out PCBs that selected common mode chokes are used rather than individual ferrites and resistors.

Because of the density of connectors, frequently encountered, it seems at first blush like a good idea on high-density boards to stagger ferrites, but this is actually a bad idea as it can increase the stray capacitance between filtered and unfiltered lines.

Currents flowing in metallic enclosures used to encapsulate RFI emitting or sensitive components can make them act as antennas and, in general, such currents cannot be avoided other than in perfectly reflecting box surfaces.

Traditionally, PCB shielding cans are constructed from sheet metal or beryllium copper. The use of clip-on lids with multiple fingers can assist in minimising the leakage of radiation.

Where an anodised front panel is being used and EMC problems persist, the rear surface should be abraded to form a good conductive path with the rest of the enclosure. The surfaces of the enclosure panels must be highly conductive because RF currents only flow on the surface (eddy current effect).

Where long runs exist, cross talk is inevitable. Separating tracks is ideal but real estate limitations will conflict. The worst scenario is a single ground return track.

Interleaving ground returns helps, but best is a ground return for each bus. Another way of limiting potential interference is the use of limited slew rate, line drivers and so-called ‘frequency dithering’ clocks (clocks that spread energy over a wider bandwidth).

DC to DC converters can produce high transient currents. Even an unbroken ground plane can offer significant impedance to these currents. This provides challenges to designers. The splitting of ground planes, although according to some EMC specialists considered as unnecessary and even potentially deleterious, is done to separate analog from digital portions. For segregated ground planes, zero ohm links (small capacitors) should be ‘stitched’ around the island plane (refer to diagram).

High switching speeds and the shrinking of IC sizes, necessitates decoupling of all power pins. This applies to analog as well as digital signals. Decoupling capacitors should be used whenever fluctuating currents are drawn from power rails. EMC compliance makes it necessary, in practice, to fit decoupling capacitors on off-board connectors whether they carry signal or power.

Apart from incoming power line filters, interior filtering is also often necessary. Capacitors and feed-through capacitors are effective RF filters. X-capacitors are used to attenuate symmetric interference (for instance on signal lines) and Y-capacitors for asymmetric (common mode) interference. Feed-through capacitors are particularly effective because they have low inductance. X-capacitors are suitable for the attenuation to about 1 MHz. Y-capacitors can work over a much larger range, eg to 30 MHz. The use of parallel-connected, smaller capacitors is preferred to a physically larger one that is likely to have a higher self-inductance.

Ferrite beads and toroids can also provide additional attenuation. Ferrites are ceramics of various metallic oxides and are characterised by extremely high permeability. Like all inductors, the impedance of ferrite cores below resonance is proportional to the square of the number of turns of wire passing around the core.

Ferrites are useful over a wide frequency range to 300 MHz and beyond.

Screened cables are an obvious way of limiting interference. However, screens do not necessarily provide 100% coverage of enclosed conductors. Closed screens provide coverage to frequencies well over 100 MHz.

Helicoil screens do generally provide poorer coverage by virtue of the overlapping metal areas not making good ohmic contact. Shielded cable connecting to a PCB shielded can must have the shielding material make contact for 360 degrees.

When filtering a conductor, its filter position is very important and, where possible, the filter’s mid point must be aligned halfway between the shielding can of the PCB - in practice perhaps not so easy, but more possible with small components such as ferrite beads.

Feed-through filters should be screwed in or soldered in correctly dimensioned holes on the shielding can. No mouse holes should be created as these can serve as antennas.