The Rise of the Ultra-Miniature Magnetic Reed Switch



A HARM switch is ideal for demanding applications in battery-powered medical devices.

The widely used magnetic reed switch has been around for over 70 years, since it was invented by Bell Labs to replace the clunky electromechanical relays then used in telephone exchanges. Despite being a simple passive mechanical device in an increasingly solid-state world, reed switches remain essential for applications requiring a combination of zero-power operation, relatively small size, high-power switching capability, low ESD susceptibility and a ten trillion times ratio between their on- and off-resistance. No active magnetically operated switch shares this unique combination of features.

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The RedRock™ HARM reed switch is 2 mm long, 1mm wide and 0.95mm high, about 8 times less volume than the smallest commercially available conventional reed switch,with closure sensitivities of 10mT to 30mT.

Over the years, the length of the glass envelope of the reed switch has shrunk from its original length of 50 mm to around 5mm (see figure). Despite this progressive shrinkage in conventional reed switch length, basic physics ordains that reed switches can’t shrink much more. The reason lies in the way a conventional reed switch is made. Nickel-iron wire is flattened with a press into two flexibleblades that are then sealed in a tubular glass envelope—focused infrared radiation or a laser beam is used to melt the ends of the glass tubing, forming a hermetic seal around the nickel-iron. The tips of the blades are coated with an inert precious metal such as ruthenium to provide contact-switching longevity.Herein lies the problem; during the sealing process, heat is conducted to the tips of the blades where it can damage or destroy the precious metal coating. Below about 5mm glass length (corresponding to an individual blade length of about 2mm), it becomes impossible to protect the blade tips from this damaging heat. This is what limits the length. Alternative strategies such as omitting the precious metal coating are unsatisfactory, since the reliability of a reed switch with only base-metal contact surfaces is unacceptable.

So how do you make a much smaller reed switch while avoiding these problems? Clearly, a low-temperature process is neededto avoid contact damage. One excellent solution is high aspect ratio microfabrication (HARM), a type of MEMS processing that is ideal for producing reed switches.With HARM, the positive features of conventional reed switches—zero-power operation, high-power switching capability, low closed-contact resistance, millions of megohms of resistance—are combined with the economies of scale and item-to-item part reproducibility inherent in MEMS processing. Furthermore, one specific benefit of the HARM process for building magnetic switches includes contact forces hundreds of times higher than those previously achieved by conventional planar MEMS switches, ensuring reliable switching over hundreds of millions of cycles.In turn, the higher contact forces have also increased the maximum carry current a thousand times, from about 100 uA in past MEMS switches to 100mA in this latest design. As for potential contact damage, the HARM process steps never exceed 200 °C, so no damage happens. Other benefits of HARMfabrication include wafer-level packaging and hermetic sealing, ensuring that the contacts are permanently protected from environmental contamination, and a surface-mount form factor that allows the switches to be shipped in tape-and-reel packaging and attached using standard pick-and-place reflow soldering techniques.

A HARM switch is ideal for demanding applications in battery-powered medical devices such as capsule endoscopes, hearing aids and insulin pumps where a low parts count, zero-power operation and extremely small size are mandatory. For further information, contact redrock@cotorelay.com

 


day_stephen

Stephen Day is retained by Coto Technology Inc. as an engineering consultant, specializing in introducing MEMS devices to the company’s product portfolio. He holds B.Sc. and Ph.D. degrees in physical chemistry and analytical instrumentation from the University of Bradford (UK) and did post-doctoral work at Rensselaer Polytechnic Institute in Troy, NY.

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