Components attracting attention

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 increased 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.

Knowles Capacitor brands Syfer and Voltronics support the MRI industry with a range of non-magnetic components including surface mount MLCC (Multi-layer Ceramic Capacitors), Ribbon Leaded and High Power MLCC. Levels of magnetism are carefully controlled with the use of specialist materials and tight supply chain control.

Surface mount MLCCS 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 is also critical as trace elements of magnetic material can be present along with some ceramics being Neodymium-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, Knowles Capacitors has 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.

Trimmer capacitors from Voltronics also meet the non-magnetic requirements of the MRI and NMR industries. Careful selection of raw materials means that these parts are used in high field strength areas without impacting performance or image quality.