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Alumina Substrate and Medical Equipment Resistors

Fri, 05/17/2013 - 4:13pm
Michael West, Director of Engineering, Ohmite Manufacturing

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Thick film on alumina technology offers precision and high resistance values in defibrillators

Ohmite’s TFS Series of alumina-based resistors offer values from 100 Ω to 100 KΩ, power rating of 0.5 to 2 W, and voltage rating of 3.0 to 11.0 Kv.Surge resistors are a crucial component in many electrical devices, providing protection and reliable operation in applications ranging from industrial power supplies to telecommunications. While this reliability is important in every industry, it is arguably most critical for medical equipment, where dependable operation can literally constitute the difference between life and death. Medical equipment manufacturers have utilized a variety of resistor types to provide this essential surge protection, including thick film and solid composition-type resistors, but the two most prevalent are solid composition-type and wirewound for common surge applications.

Thick film resistors, consisting of conductive, resistive, and insulating pastes deposited and fused onto a ceramic substrate base, have proven popular in medical applications because of the excellent precision and high resistance values they offer. One of the most effective and often-used substrate bases is alumina (Al2O3). Alumina substrate technology has been around for decades, but it has enjoyed something of a renaissance in recent years, as the use of microcircuits and hybrid microcircuits has increased. As opposed to competing technologies, such as PC Boards, alumina substrate technology allows resistors and other passive components to be printed directly onto the ceramic base, saving space and opening the door for creative component configurations. In addition, as a ceramic, alumina also offers high thermal conductivity and excellent heat dissipation characteristics. Manufacturers can therefore proceed without worrying about error due to hot-spotting or other irregularities. Using thick-film-on-alumina substrate technology, these resistors can be provided in the precision tolerances, high voltage ratings, and high resistance values that this technology is known for.

Wirewound resistors, most often composed of a rod-shaped ceramic or fiberglass core wrapped in metallic wire, also offer distinct advantages for safety-critical medical equipment. Wirewound resistors typically do not need to specify a maximum voltage in the way that thick film resistors do, because there is less concern about potential conduction from typically non-conductive traces. They are also better suited to manage excessively high current pulses, due to the surface area afforded by the wrapped wire. For this reason, some manufacturers may prefer to use wirewound resistors in medical device applications such as defibrillators.

However, although wirewound resistors can offer some advantages in handling current pulse, resistors constructed from thick film on alumina substrate can provide wider, more encompassing benefits in the design of medical device equipment. This article will walk through the various characteristics of these resistors, including high resistance values, high voltage handling, small size, resiliency, and more, to demonstrate how they might aid in the design of medical devices.

High Resistance Values
One of the chief advantages of thick film on alumina substrate resistors is their high resistance values. As mentioned previously, with this technology, resistivity is achieved by depositing and fusing a resistive paste to the alumina substrate base. These pastes can range in strength from 10 Ω/square to 10 MΩ/square, and different degrees of resistivity can be achieved by selecting the appropriate strength paste. The shift to a higher or lower resistance value is therefore relatively easy from a design perspective. It involves a change in paste selection rather than a change in geometry, size, or another more involved aspect of the resistor.

Achieving high resistance values is a bit more involved when dealing with wirewound resistors. In order to achieve the desired resistivity, manufacturers need to fabricate a smaller and smaller wire diameter for higher and higher values. In addition, high resistance values are inherently less valuable in wirewound construction due to the fine wire involved. Occasionally, other issues can also evolve, such as potential circuit interference caused by inductively wound wire. However, although there are some difficulties surrounding high resistance values, wirewound resistors often offer easier handling of high current pulse than thick film on alumina resistors.

High Voltage and High Current Pulse Handling
High-stress medical devices like defibrillators and their accompanying monitoring equipment need to withstand both high pulse and high voltage in the course of operation. The defibrillator itself endures a great deal of electrical strain when administering shock to the patients, with some modern machines capable of putting out nearly 2,000 V. (While this capacity is rarely exercised on patients, it is relevant for designers.) In addition, the monitoring equipment faces its own voltage and current challenges. These machines must absorb any pulse before it travels into the reading system, or the strong current will burn up the system.

Pulses in medical equipment can vary greatly from manufacturer to manufacturer. For example, defibrillators can experience current pulses as low as 20 J or as large as 300 J. For this reason, pulse is of varying concern for different applications. When designing for demanding circumstances, the choice between wirewound and alumina can end up seeming like a simple trade-off. In general, wirewound is more naturally suited to handle high current pulse while thick film is better equipped to endure high voltage. The question then remains, “Which is easier to modify?”

Wirewound resistors can handle high current pulses so well because of the way they are constructed. The wire wrapped around the core, though fine and thin, provides a great deal of surface area to absorb the pulse. When the current jumps, the lengths of wrapped wire can handle the sudden increase.

Ohmite’s new TP Series can handle up to 100 joules of energy in a 1% tolerance, feature higher pulse currents than standard parts, and were designed with more area for the current to flow through.Thick film resistors are not as ideally suited for current pulse, because they are smaller, with less overall surface area to work from. To equip an alumina substrate resistor for high current pulse, electrical designers cannot make the resistive deposit much thicker. Instead, they need to make it very wide in order to increase surface area. The more current that must pass through, the wider the square will get.

In contrast, achieving high voltage handling in wirewound resistors can become difficult relatively quickly. For voltages of up to 150 V, it is easy and inexpensive enough to simply decrease the diameter of the wire so that it is fine enough to handle this new voltage. However, most defibrillators will endure a voltage of around 2,500V. At that point, the diameters required can become restrictive.

In short, it is generally easier to design an alumina resistor to handle the current pulse than it is to design a wirewound resistor to handle the extreme voltages experienced by defibrillators and their monitoring equipment.

Small Size and Dependability
That same small size that can cause high current pulse handling concerns can also serve as an asset for alumina substrate-based resistors. In fact, their small size and resiliency have actually contributed to advancements in the medical field. Public access defibrillators, also known as automated external defibrillators (AEDs), are designed for use by a layperson outside of a hospital environment. With the advent of this technology, victims of cardiac dysrhythmias, ventricular fibrillation, or pulseless ventricular tachycardia no longer have to wait until arriving at the hospital for treatment, and this reduction in wait time is life-saving. In order for these AEDs to be viable, however, they need to be miniaturized to a small portable device and withstand harsher temperatures and humidity than their counterparts in hospitals.

Alumina substrate based resistors have provided the miniaturization necessary to make this possible. As mentioned previously, with alumina substrate, manufacturers can print resistors directly onto the surface of the substrate. These resistors are very thin, approximately the same height as the copper conductive traces on a PC board, and can be fitted into a space-saving planar package. Manufacturers can also place components directly on top of these resistors while still making conductor traces between them, doubling the space utilization. While components such as transistors and diodes must still be surface mounted on alumina substrate, the space saved on resistors adds up to big benefits.

Alumina’s ceramic properties also contribute to its compactness. With wirewound resistors, the entire resistor is wrapped in conductive wire, so designers must be careful about placing other elements too close to the resistor. If a conductive component is placed too close to the wirewound resistor, it may arc or cause a fault. Alumina, on the other hand, will be conductive on one side (the side where the conductive and resistive pastes were deposited) and non-conductive on the other. As a result, designers can place other elements very close to the backside of the resistor without worrying about a fault or arc. Should a fault occur, the failure mode of an alumina resistor is still an open circuit.

Conclusion
In summary, alumina substrate technology offers a variety of benefits to medical device designers and engineers, especially for the optimization of defibrillators and their monitoring equipment. As one of the most commonly used resistors in the industry, it offers distinct advantages that include high resistance values, excellent high voltage handling, small size, and tight tolerances.

For more information, visit Ohmite.

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