A Medical Electronics Manufacturing Fall 1997 Feature

ELECTROMAGNETIC COMPATIBILITY

Joseph E. Butler

Board-level shielding, laminates, and enclosures can resolve EMI problems and help designers comply with international medical equipment standards.

The importance of effective EMI shielding of medical devices continues to escalate, due in large part to two European regulatory requirements that apply to all medical equipment manufacturers that sell products in Europe. The European Union's (EU) Electromagnetic Compatibility (EMC) Directive, which came into force in January 1996, and the Medical Devices Directive (MDD), which becomes mandatory June 1998, together are driving the medical industry's increased focus on EMC. The MDD, which is currently in a transition period, is centered on the international standard IEC 601-1-2, Medical Electrical Equipment, Part 1: General Requirements for Safety. 2. Collateral Standard: Electromagnetic Compatibility­Requirement and Tests.

Radiated EMI Requirements

When establishing shielding needs, designers need to understand the radiated emission requirements of IEC 601-1-2 as well as those under consideration for the standard's second edition revisions.

Emission. Radiated emission requirements extend from 80 MHz to 1 GHz. In the case of the emission requirements, EMI shielding needs can be determined by analyzing the circuitry within the medical device and its operating frequency range. However, basing shielding selection solely on an analysis of the circuitry and the allowable radiated emission levels will result in a somewhat limited shielding effectiveness envelope, in terms of the frequency range. The proposed revised radiated immunity requirements are likely to expand both the frequency range and the level of shielding required.

Immunity. Radiated immunity requirements of IEC 601-1-2 are also from 80 MHz to 1 GHz. The immunity level specified in this standard is 1 V/m, but the requirement increases to 10 V/m for life-support systems operating at frequencies above 800 MHz. The standard also requires field-level modulation of the applied signal, with the standard default modulation of 80% AM, 1 kHz. For devices that control, monitor, or measure a physiological parameter, the modulation requirement is 2 Hz. Medical devices tested with patient simulation equipment with simulation frequencies > 1 Hz or < 3 Hz must use the 2-Hz modulation. For the frequency ranges of 80 to 960 MHz and proposed for from 1.4 to 2 GHz only, the signals are swept frequencies. The radiated immunity requirement also requires a dwell time at test frequencies to ensure that the device under test can function properly in all operational modes.

The detailed radiated immunity specification increases the likelihood that EMI shielding will be needed. Even if a medical device does not operate at a level that would make it susceptible to high-frequency signals, modulation of high-frequency radiated immunity levels may allow potentially interfering signals to enter the device. A testing situation involving modulated rather than straight continuous wave signals is generally more difficult for devices to pass. For this reason, mechanical and electrical designers must address the highest frequency radiated immunity signal to which the equipment is to be tested. The shielding effectiveness envelope requirements derived from radiated emission analysis will indicate only a small part of a device's shielding needs. Radiated immunity requirements will ultimately dictate the degree of shielding necessary.

Other External Radiated Immunity Concerns

Further complicating radiated EMI requirements for medical devices is that several handheld wireless communication devices and some fixed transmitting devices in hospitals can produce radiated field strengths in excess of those specified by the radiated immunity requirements of IEC 601-1-2. Table I lists some of these devices and their associated field levels and frequency ranges.

Table I. Wireless communications devices with associated field strengths above IEC 601-1-2 radiated immunity limits.

When comparing the field levels shown in Table I with the specification level of 3 V/m, it is apparent that there is legitimate concern about the adequacy of shielding levels established for medical devices based solely on compliance with IEC 601-1-2 (see Table II).

The differences in field strengths shown in Table II indicate that the shielding levels established by IEC 601-1-2 could be from 13 to 31 dB less than what is truly needed to ensure adequate protection against real-world radiated electromagnetic environments. For this reason, EMI shielding of medical devices should err on the conservative side. Further complicating the issue is that the transmitting sources will have harmonics and thus generate electromagnetic fields at frequencies significantly higher than those shown in Table I. Fortunately, many commercially available shielding products were developed with an eye toward both high frequency (e.g., 10 GHz) and relatively high levels of shielding (e.g., 70 dB or greater). The most recently available EMI gaskets are one example of this trend.

Table II. Difference in decibels between 3 V/m and other wireless source field strengths.

Shielding Solutions for Medical Devices

Board-Level Shielding. Proper printed circuit board layout is easily the most cost-effective approach to dealing with radiated EMI issues, whether the concern is radiated emissions or radiated immunity. There are several different commercially available software programs that analyze potential EMI problems at this level. However, using software is not infallible, and prototype board-level testing and iterative board relayouts are often needed for design optimization. Ultimately, it is less expensive to solve radiated EMI problems at the board level than at a later stage. For this reason, it is beneficial to allocate time for resolving potential EMI problems in the board-layout stage. More important, given the amount of electronic circuitry and the higher clock speeds in digital devices, it is rare that simple board layout can solve the EMI shielding problems without some form of suppression. The suppression at the board level can take the form of filtering, decoupling, low-impedance grounding, or of board-level shielding.

Board-level shielding is quite common in handheld devices, and usually involves soldering metal cans over certain components. One shortcoming of this shielding technique, however, is that the components on the board are sometimes exposed to a solder reflow condition when the cans are installed, affecting long-term component reliability. Board repairability is also more difficult with metal can shielding. However, this method may eliminate the need for enclosure-level shielding.

Laminates. If board-level EMI shielding is not possible or cannot produce the desired results, one might consider employing various applications of metal foil and dielectric film shielding laminates. While laminates require a degree of empirical testing, they can be relatively low in cost.

Ground Planes. Under one approach, a thin aluminum or copper foil with dielectric backing is placed under the printed circuit board as a ground plane so as to provide capacitive coupling between the printed circuit board and laminate. Instead of radiating off the board, the electromagnetic radiation transfers into the laminate, where it is dissipated as heat. For radiated immunity, this capacitive reactance limits the amount of external radiated electromagnetic signals that can be effectively picked up by the traces on the board. This type of laminate application works in much the same way as does a multilayer board in reducing board emissions and limiting radiated signal pickup.

Shadow Shields. Another technique involves using laminates as shadow shields. Under this approach, the laminate shadows a component or subassembly to provide limited EMI shielding. The laminate is grounded, but not at all points along its periphery. The decision as to where and how the laminate is grounded is critical, and needs to be determined in an EMI test lab to measure the grounding's effectiveness. Laminates constructed of aluminum foil with a conductive pressure-sensitive adhesive (PSA) and a releasable Mylar dielectric allow selective removal of the dielectric. This construction further allows for kiss cutting of the dielectric portion, so that it can be removed while the foil remains intact. The conductive PSA provides a good low-impedance ground connection.

Faraday Shields. When using a laminate as a Faraday shield, an attempt is made to provide a six-sided enclosure that can be assembled inside a device enclosure, typically a plastic one. Once again, the effectiveness of the laminate depends upon how well it is grounded, which can only be determined in an EMI test lab. Another challenge presented by this approach is making the six-sided laminate using kiss cuts and prefolded creases so it can be shipped flat and then simply installed inside the device enclosure.

Enclosure Shielding. Another option for EMI shielding of medical devices is to use metal or plastic enclosures. Metal enclosures offer the basic shielding envelope for the device with the concern limited to proper treatment of slots, seams, apertures, and cable entry. EMI gaskets, as well as shielded air ventilation panels, windows, and cables are typically required.

Plastic enclosures, used more commonly in the medical industry because of cost and weight considerations, usually require conductive coatings, platings, or paint to provide EMI shielding. Low-impedance grounding along the periphery of the shielding media affects the degree of shielding realized within the enclosure. This design requirement cannot be overemphasized; failure to adequately address grounding of the media will result in a loss of available shielding by tens of decibels, not merely a decibel or so. While this design aspect is relatively simple and inexpensive, it is often underutilized.

Enclosures made from electrically conductive plastic material are also available but cannot always produce high enough shielding effectiveness values at higher frequencies (e.g., 1 GHz). Again, a design challenge of creating an effective ground connection to the shielding media in the plastic is presented because the resin concentration at the edges of the mold prevents good electrical contact. Work continues in this area since the potential rewards for an effective solution are significant.

Conclusion

Before effective shielding of medical devices can be addressed, designers must understand the radiated immunity requirement and the real-world electromagnetic environment where the equipment will be operated. When possible, it is advisable to deal with radiated emissions and immunity at the printed circuit board­layout stage. If board-level shielding does not solve the problem, it may be necessary to incorporate shielding laminates. Using laminates to dissipate heat or create a shadow or Faraday shield will often provide the desired protection. Should a medical device require enclosure-level shielding, the simple concept of creating good low-impedance ground connections is one of the most cost-effective design parameters to address.

Joseph E. Butler is a market manager for the Chomerics Div. of Parker Hannifin Corp. (Woburn, MA).