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Utilising copper barrier capacitors in MRI

28 Nov 2013  | Dave Beckett

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MRI equipment utilises 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 overplated 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 render it unsuitable in MRI scanner applications. With such high field strengths, 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 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%Sn, 36%Pb, 2%Ag, variously known as 62s or LMP solder alloy.

More recently however, with the enforcement of EU Directive 2002/95/EC—Restriction of Hazardous Substances (RoHS), 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 reflow temperature of 179ºC, whereas typical Pb-free solders have reflow temperatures in excess of 217ºC.

Significantly, the higher the percentage of tin in the solder, the more likely it is to leach the silver from the capacitor termination. It was quickly found that doping the Pb-free solder alloy with small amounts of silver does not prevent the leaching, as it does with SnPb alloys.

Non-magnetic solution
To meet the demands from the growing medical market for non-magnetic components, and to ensure compliance with the RoHS Directive, UK-based capacitor manufacturer Syfer 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. A key consideration during the product development process was to ensure that the plated finish is capable of meeting the requirements of the high temperature (260ºC) soldering reflow profiles as detailed in IPC 7351A (the land pattern design standard with guidance for lead free soldering processes, as well as reflow cycle and profile requirements) and J-STD-020 (the standard for moisture/reflow sensitive SMDs suitable for lead-free/high temperature processing).

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