Source: Capacitor Faks blog
The performance characteristics of capacitors are mainly determined by the materials used to construct them. One of the key characteristics to consider when selecting capacitors for medical applications is the magnetic properties of the materials used.
MLC capacitors with materials that exhibit magnetic properties are not suitable for medical applications such as magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) scanners, hi-rel medical systems, test and diagnostic equipment, and laboratory analysis systems where any parasitic magnetics within the system adversely effects the output, as for example in the quality of an MRI or NMR image. Such applications require non-magnetic MLC capacitors.
Some of the materials that are commonly used for constructing terminations for MLCCs include nickel, copper, and silver/palladium (Ag/Pd).
Nickel exhibits magnetic properties, and is therefore not suitable for use as terminations in medical applications that require minimal parasitic magnetics. Ag/Pd terminations do not exhibit magnetic properties, and, while they can be used for non-magnetic applications, they require low melting point, typically lead-based, solders.
The regulations by Restriction of Hazardous Substances (RoHS) discourages the use of lead-based solders; RoHS compliant solders have high temperature requirements, and the use of high temperatures on Ag/Pb terminations causes solder leaching problems. Therefore, although these terminations are non-magnetic, silver/lead MLC capacitors are generally an unsuitable option for medical applications.
Like Ag/Pb terminations, copper terminations are non-magnetic but, unlike Ag/Pb terminations, they do not exhibit solder leaching problems, and are therefore more commonly used in capacitors for non-magnetic applications. These terminations are usually plated with silver or tin. Some of the dielectric materials that are commonly used for constructing capacitors for non-magnetic applications include Class I COG/NPO types, and Class II X7R dielectrics.
Apart from medical applications, non-magnetic MLC capacitors are also used in other sectors of the electronics industry, including the military, telecommunication, and aerospace industries.