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Uses of copper barrier capacitors in medical field

Posted: 08 Oct 2013 ?? ?Print Version ?Bookmark and Share

Keywords:medical electronics? passive electronic components? capacitors? MLCC? MRI scanner?

Rapid technological developments in medical electronics are generating significant demand worldwide for a range of passive electronic components, capacitors in particular, for use in a growing and diversified range of medical equipment.

Two parallel trends are prompting further demand through increased volume requirements of this equipment in both developed and developing countries. First, the prevalence of ever smaller and low cost electronic devices is facilitating the development of affordable, portable medical equipment. Second, a fast developing wireless infrastructure worldwide, is supporting the rapid deployment of this low cost, portable medical equipment in telemedicine applications.

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 the brain, heart, lungs, 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. The field strength can be inadvertently increased not only by magnetic components inside the scanner tunnel, but also in ancillary equipment. For these reasons, it is imperative that components, such as capacitors, within and surrounding the MRI scanner, are non-magnetic.

Surface mount Multilayer Ceramic Capacitors (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 in MRI scanner 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. Typically, such minute traces were previously regarded as insignificant, but under these extremely sensitive conditions, this is no longer the case.

Nickel alternatives
Historically, there have been applications in various industry sectors where nickel has been unacceptable. One often used alternative is a non-magnetic 'fired' silver/palladium (Ag/Pd) termination. 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. The common solder alloy used being 62%Sn36%Pb2%Ag, variously known as 62s or LMP solder alloy.

More recently however, with the enforcement of EU Directive 2002/95/ECRestriction of Hazardous Substances (The RoHS Directive), the use of certain materials found in electrical and electronic products is now prohibited, except in a few special cases. Lead (Pb) tops the list, with widespread impact across electronics designers and manufacturers, requiring a complete re-think on the use of Pb in solder alloys. All applicable products placed on the market in the EU after July 1, 2006 have had to be 'RoHS compliant'.

The removal of Pb from solders used in the assembly of electrical and electronic equipment has forced the move to tin based solder alloys. These have higher melting temperatures: a typical SnPb solder has a re-flow temperature of 179ºC, whereas typical Pb-free solders have re-flow temperatures in excess of 217ºC.

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