Platinum Metals Rev., 1968, 12, (2), 46
Monolithic Ceramic Capacitors
Platinum Metal Electrodes in a Fired Multilayer Construction
There is a growing use of small monolithic ceramic capacitor chips in integrated hybrid and thick-film circuits. They provide the miniature capacitor elements in resistance/capacitance networks where the resistors may be fired patterns of screen printed resistive inks on an alumina substrate.
The ceramics used normally fall into two categories; “High K” and “NPO”. High K capacitors are those based on ceramics of dielectric constants generally in the range 600 to 5000 and provide exceptionally high capacitance per unit volume. NPO capacitors employ ceramics of lower dielectric constant whose chief merit is that of having a temperature coefficient of capacitance very near to zero over a wide operating temperature range.
It has been established over many years now that the platinum metals not only provide the most reliable electrodes in these components but that they also enable an economic production method to be employed. The manufacturing process first entails producing thin strips of “green” ceramic. An ink or paste incorporating finely divided platinum group metals is screen printed on to these strips in multiple arrays of rectangular areas. When the print is dry the strips are assembled one upon the other so that the metallised areas are superimposed. The assembly is topped with plain ceramic, and a light compression bonds the layers together.
The assembly is then diced into individual “chips”, each containing one series of superimposed metallic areas interleaved with ceramic dielectric. Fired at 1300°C, these chips become robust monolithic blocks.
Only the use of platinum group metals makes possible this air-fired, fully integrated construction. The process gives reliable and reproducible results in quantity production and the long-term stability of the components in subsequent service are as good as the ceramics themselves allow.
Matthey Printed Products Limited, of Burslem, have recently announced the introduction of a new range of miniature monolithic ceramic chips. These are their K1200 and NPO series. The type of construction and the choice of ceramic produce chips with a capacitance density of as much as 2.5 μ F per cm3 in the K1200 series, while the NPO series offers chips with a temperature coefficient of capacitance of zero ±30 × 10−6 per °C. The rated operating temperature range for these components is −55°C to +125°C.
In general, users of monolithic ceramic capacitor chips must be able to connect them into or mount them on integrated hybrid and thick-film circuits quickly and conveniently. For this purpose, the chips are supplied with their terminal ends metallised with silver so that they can be soft soldered with ease.
Conventional capacitors incorporating monolithic ceramic chips are manufactured by soldering wires to the metallised terminal ends and encapsulating the bodies in a synthetic resin that does not affect the properties of the chips but provides protection against humidity.