Medical
Non-magnetic capacitors and components attract the attention of
the medical market
Imaging systems represent the largest and most vibrant sector of the medical electronics industry. Among the wide range of imaging modalities in continuing development, magnetic resonance imaging (MRI) scanners are one of the most significant. MRI equipment uses a powerful magnetic field to create an image, enabling doctors to visualise conditions inside organs, joints and elsewhere. The magnetic field in the tunnel of an MRI scanner must be uniform to parts per million. The resolution is dependent on the strength of the magnetic field, which can be inadvertently affected by magnetic components inside the scanner tunnel and also in ancillary equipment. For these reasons, it is imperative that components within and surrounding the MRI scanner, are non-magnetic
K
nowles Capacitor brands Syfer and Voltronics support the MRI industry with a wide range of non- magnetic components including surface mount MLCC (Multilayer Ceramic Capacitors), Ribbon Leaded, High Power MLCC and precision Trimmer capacitors. Levels of magnetism are carefully controlled with the use of specialist materials and tight supply chain control to ensure that performance, even at cutting edge Tesla levels, is maintained.
Surface mount MLCC, used extensively in the
electronics industry, are typically supplied with a nickel barrier finish. This consists of a silver base layer over plated with nickel, which provides solder leach resistance. A plated top layer of pure tin or tin/lead is used to protect the nickel from oxidisation and maintain a readily solderable finish. Nickel however has magnetic properties which renders it unsuitable for MRI applications. With such high field strengths involved in this type of equipment, careful selection of the dielectric material (in this case ceramic) is also critical as trace elements of magnetic material (Ni, Fe etc.) can be present along with some ceramics being Neodymium Titanate based. Historically, the industry solution for a non-magnetic termination material was silver/palladium (Ag/Pd), however the solder leach resistance of this termination type is inferior to that of the nickel barrier. This option therefore required the use of low melting point solders, typically lead-based, doped with a small amount of silver to prevent silver leaching out of the termination. To meet the demands of the growing medical market for non-magnetic components and ensure compatibility with the RoHS compliant soldering
processes, Knowles Capacitors have developed a ‘magnetic free’ range of MLCC products. The devices are constructed using selected non-magnetic C0G/NP0, High Q and X7R dielectrics and a non-magnetic ‘copper barrier’ plated finish. They are also available with strip line ribbon lead terminations and in high RF power versions to provide a solution for the full spectrum of non-magnetic requirements. In supplement to non-magnetic MLCC, Trimmer capacitors from Voltronics also meet the strict non- magnetic requirements of the MRI and NMR industries. Careful selection of raw materials (standard materials often contain trace amounts of iron from recycled swarf) means that these parts are used in high field strength areas without impacting performance or image quality. Generally available measuring equipment, for example portable magnetometers are unable to measure accurately enough to determine the suitability of a material for use in high field MR environments. Limits for relative permeability tend to be in the region of 1.0004, claimed measurement capabilities can be as low as an er of 1.00001 but with an error of ±0.001 this cannot be relied upon for absolute measurement. As a result of this it is common in the industry to establish the magnetic suitability of raw material and components using a very simple technique. A rare earth magnet is attached to a string to form a pendulum, the sample is then brought near and any movement of the magnet constitutes a fail. In order to verify and quantify performance Voltronics have undertaken some high accuracy measurements in conjunction with a University. Using a Vibrating Sample Magnetometer a 3mm cube raw material sample was evaluated at varying field strengths. The magnetic moment created in a field strength of 1000 Oe was +54 µemu, at 7000 Oe was +42 µemu and at 50000 was - 127 µemu. This translates to an er of 1.00000016, 1.000000018 and 0.9999999925 respectively, many factors lower than required. Voltronics are specialists in high-voltage high-power
multi-turn trimmers with in-house RF test capabilities, able to prove performance against common industry frequencies and power levels. Tuning ranges of up to 100pF and voltage up to 20kV DC and Q factors over 2000 are available, with new technologies under development to increase this capability further. Using materials knowledge built up over several decades of serving the industry Voltronics also are also able to offer common hardware such as screws, nuts and bolts with the same non-magnetic properties as our Trimmers.
www.knowlescapacitors.com www.cieonline.co.uk
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