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Determining the Most Effective Approach to EMI Shielding for Flexible Circuits and Cables

Thu, 06/24/2010 - 8:45am
Doug Walz

When electronic devices are being used in the medical field, electromagnetic interference is not merely a nuisance, it is a serious hazard that threatens the lives of the patients whose health relies upon these devices. This article presents specific advances made in the production of protective shielding to provide long-term protection against harmful disruptions.

Electromagnetic compatibility, or EMC, means that a device does not cause interference in its electromagnetic (EM) environment and does not emit levels of EM energy that cause electromagnetic interference (EMI) in other devices in the vicinity. A medical device can be vulnerable to EMI if the levels of EM energy in its environment exceed the EM resistance to which the device was designed and tested. EMI problems with medical devices can be very complex, not only from a technical standpoint, but also from the view of public health issues and solutions.

The problem is acute, with ever-increasing wireless communication networks in the form of cellular phones, police networks, and private communication networks where the EMI generated may cause the medical devices to malfunction or to indicate false readings. Because of a lack of understanding and management of EMI issues within the healthcare industry and environment, medical devices malfunction resulting in serious injury and even death. Additionally, systems fail to provide critical patient status and alert information, and lack of coordination and management result in short term fixes, not solid solutions.

Meanwhile, over the past decade, the use of flat flexible cables (FFC), polymer thick film circuits (PTF), and flexible printed circuits (FPC) in electronic devices has been growing steadily as electrical and mechanical engineers accommodate increased density. Flexible circuits and cables have been introduced across industries in everything from simple devices such as cameras, flip phones, and calculators to more complex interconnections for global positioning systems and lap-top computers. As they continue to become more prevalent in both consumer applications and within medical devices, the proper shielding of these components becomes increasingly critical. The shielded flat and flexible conductors allow more surface area than the shielded round conductors for handling higher frequency EMI noise and for the intelligent signals to be protected, making them more effective in application design.

EMI shielding in flexible circuits is like a fence that plays “good neighbor” by stopping the propagation of electromagnetic radiation from entering, or preventing it from propagating off an electrical conductor. The material and thickness of this flexible shielding would depend on the amplitude and impedance (Z) of the electromagnetic radiation wave, which is determined by the ratio of the Electric near Field (E) to the Magnetic near Field (H), or Z = E/H.

An E field is typically a high voltage, low ampere source with Z greater than 377 ohms that can capacitively couple to a susceptible conductor. The shielding best suited for E field radiation is made of conductive shielding materials like Silver, Copper, Gold, Chromium, and Aluminum.

An H field is typically a low voltage, high ampere source with Z less than 377 ohms and with an unwanted inductive coupling to a susceptible conductor. The shielding best suited for H field radiation is made of permeable magnetic shielding materials like Conetic, Mumetal, and Steel.

The most common EMI issue to require protective shielding is the E field radiation. Stray environmental E fields are the most likely to disrupt electronic medical devices. The shielded versions of the flat printed flexible circuits are best suited for providing this conductive shield. This feature is similar to the shield of a coaxial cable protecting the inner conductors from the outside capacitive coupling of the unwanted external EMI noise. The flexible shield will couple the bulk of the EMI energy to the device ground, away from the critical conductors. The balance of the EMI will reflect away from the E-Field Shield material and some will be absorbed by the shield material (heat).

The three boundary regions of an E-Field Shield material can protect the susceptible conductors or have a significant impact on reducing the corruption of a critical signal or data bus by the amplitude of the unwanted EMI noise energy. Like a coaxial cable, the susceptible conductors are within a flexible E-Shield enclosure, allowing the E-Shield to reflect, absorb, and conduct the unwanted EMI energy to a common ground. This protects the integrity of the signals that are critical in health monitoring or maintenance.

The E-Field shield will reflect some of the external EMI at the first boundary region surface. Some of the EMI energy will penetrate into the second boundary region of the shield material, reflect off the internal face of the third boundary region, and reflect within the second boundary region until the EMI energy is converted to heat energy. Some energy may or may not penetrate the third boundary region and could capacitively couple to the susceptible conductor, but at a much lower level of energy.

If the unwanted external EMI radiation couples to the susceptible conductor, it can mix in with the “intelligent” signal, changing the leading edge of a clock frequency. The signal interference can cause a system fault and prevent a critical function from being completed, or “crash” a system program. This is especially a concern with battery operated, hand held portable devices that may use radio frequency (RF) communication protocols to a central system. In some devices with inductive electromagnetic loads, damage of the low power electronics over time can occur with repeated transient energies induced on a susceptible unprotected data bus.

Conclusion

As medical devices become increasingly portable with low power electronics, it is evermore critical that these devices be protected from a variety of potentially harmful EMI environments. As a part of Johnson Medtech, Johnson Electric’s medical products group Parlex is a global leader in this technology and has developed subsystems for many medical devices, offering technical assistance in the proper material selection and application. Advances in flexible technology make it possible to achieve effective shielding and impedance control while minimizing thickness in order to maintain maximum flexibility of the FFC, FPC or PTF. Processes that balance cost and innovation make EMI shielded flexible circuits and cables possible for high volume, dynamic markets.

Choosing the best suited technology for their customer’s application, Johnson Medtech is able to deliver optimal solutions that enable customers to quickly introduce smaller and smarter products to the market.

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