Non-magnetic, copper barrier capacitors attract the attention of the medical market
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 MLCC (Multilayer Ceramic Capacitors), 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/EC – Restriction 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. 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).
Early development work focused on establishing a specification, and to this end, the leach resistance properties of nickel barrier products was used as the minimum acceptable standard. The goal for new devices with a Copper Barrier finish was to at least match this performance.
During the development of the plating process Syfer experimented with copper solutions from a number of suppliers. Both acid and alkaline plating solutions were tried before finalising the selection of the optimum plating solution. It was important that the material is fully compatible with the dielectric and the underlying termination, and provides the most stable plating platform to ensure component reliability.
Stamp of approval
Syfer has been working closely with a number of key customers in the medical equipment sector. Several customers have specified the non-magnetic Copper Barrier products and reported great success! They are now purchasing these in large volumes. The components have received full approval and production quantities have been ordered across a wide range of chip sizes and capacitances.
Evolving range
Syfer has a philosophy of continual product development and innovation. Consequently, the Copper Barrier range is constantly evolving. Already available are multilayer ceramic chip capacitors, in C0G/NP0, High Q and X7R dielectrics. With the C0G/NPO and High Q dielectrics, parts are offered across the capacitance range from 0.1pF to 15nF (50V to 3kV). In addition, X7R capacitors range from 47pF to 6.8µF (16V to 2kV).
New versions manufactured using the X7R dielectric now incorporate the award-winning FlexiCap™ polymer termination as standard. FlexiCap™ is a proprietary flexible epoxy polymer termination material, applied to the device under the plated barrier finish. This can accommodate almost double the degree of board bending than conventional capacitors, and makes them more resistant to damage under stress and temperature cycling extremes.
The Copper Barrier devices are packaged in case sizes from 0402 to 2225, depending on value, and the operating temperature range is -55 to 125oC, making them eminently suitable for medical equipment and other applications requiring non-magnetic multilayer chip capacitors.
About the author
Dave Beckett is Process & Engineering Manager at Syfer Technology – www.syfer.com
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