Ceramic Capacitors : Construction and Applications

Ceramics are among the most versatile dielectric materials, and were among the first to be used for capacitors.  They exhibit a wide range of dielectric constants, operate over wide frequency range, and also work under large temperature range. Ceramic capacitors were made in 1900, invented by Lombardi in Italy. Around 1930s, it was found that Barium titanate could be added to ceramic to double its dielectric constant. Ten years later, Barium titanate was found to have excellent insulation properties, paving the way for cheap ceramic capacitors.  They were made in number of geometric forms, including tubular structures and holed discs, and the first tubular capacitors were made in 1936. These were phased out over time, and today disc type and MLCC capacitors are the mainstay of ceramic capacitors. They carry benefits like small size, large capacitance, mass production suitability as also heat resistance.

A semiconductor type ceramic with low resistivity was developed in 1950s, which had very high dielectric constants, and these helped miniaturization of capacitors with low voltage ratings. Advent of ICs computers, electronic devices, mobile phones and other portable electronic devices gave a big impetus to ceramic capacitor market.

Ceramic capacitors are fixed value capacitors with ceramic materials as dielectric.  Two types are ceramic are in common use – disc capacitors and multilayer ceramic capacitors (MLCC). Dielectric constant of ceramic varies widely with nature of ceramic used, and can vary from 20 to 20,000. This gives a huge possibility of large range of ceramic capacitor sizes and voltage ratings. Capacitance in the same volume varies widely with ceramic material and process used.

Ceramic capacitors are usually made in small values from as low as 0.5 pF to 1μF, while a few may have values up to 100 μF. Voltage ratings vary widely, starting from 2.5V and going up to 20 kV. Ceramic capacitors are available in several types and sizes depending on construction and usage. Today they are available as disc type capacitors, or multilayer ceramic capacitors (MLCC). Further, tubular feed-through and discoidal (disc-shape) ceramic capacitors are also available.

Smallest physical sized MLCC capacitor (made by Kyocera Corporation, Japan) has dimensions of just 0.125 mm x 0.125 mm x 0.250 mm (type X5R, rated 2V). Applications of ceramic capacitors cover a wide spectrum, ranging from mobile phones, tablets, computers and most electronic circuits, serving several functions. While most MLCC are rated not over 5 Volts, Samsung has come up with 16 V rating for automobile industry, considered world’s highest under this category.

Multilayer Ceramic Capacitors (MLCC) are manufactured in mind bogglingly large numbers of over 10¹² numbers (one trillion) annually. About 80% of all capacitors manufactured worldwide are chip type ceramic capacitors. Following table gives some idea of MLCC capacitors going into each unit of these modern gadgets.

Application	Number of capacitors
Computer memory chip	16, 777, 216
4 – wheeler car	3000 – 10,000
Smartphone	400 – 500
Notebook / tablet	800

Challenges in manufacturing of Ceramic capacitors

Ceramics have the advantage of very high dielectric constants, ranging from 20 to as high as 20,000 or even more. Packing maximum capacitance in smallest sizes is by having large dielectric constant and as thin a dielectric as possible. Manufacturing process of these capacitors involves powder mixing, forming, sintering and metallization need strict quality standards and control, like temperature, pressure, process times etc.

Challenges in manufacturing involves

• Producing thin ceramic sheets
• Precise temperature control of kiln during firing,
• Control of plating process of nickel and tin layers. Nickel acts as barrier and tin prevents oxidation.
• Terminations demand precision with mixing of metal powder and solvents, and to ensure proper adhesion and functioning.

Benefits of ceramic dielectrics

• High dielectric strength and low leakage,
• High dielectric strength,
• Low loss factor
• Good hardness, mechanical strength, high temperature resistance, corrosion resistance,
• High thermal conductivity- this makes them suitable for power electronics,
• Wear resistance. Compact and robust physical structure
• High stability
• Non-polarized nature makes them well suited for AC signals.
• RF interference (RFI) and Electromagnetic Interference (EMI) suppression is better.
• Low cost.

Limitations of ceramic capacitors

• High capacitance values not possible. Capacitance values largely limited to below 1μF
• Large temperature coefficient, i.e. variations over temperature range are large.
• High tolerances, making them less precise than many other types of capacitors,
• Leakage currents may not be acceptable for some applications
• Mechanically brittle nature of ceramics means good care needed during manufacture and use.
• Cannot handle higher voltages, and easily breaks down under over-voltages.

Ceramic Disc capacitors

These capacitors use ceramic disc, on which silver coating is deposited on both sides, which act as electrode plates. These capacitors are made from 10 pF to 100 μF and voltage ratings from 16 V to 20 kV or above. A single disc of 3-6 mm is generally used for low capacitance values. Several layers can be used for higher capacitance values. Ceramic discs were originally made with holes in center, a design now obsolete. Disc capacitor have voltage ratings from 16 V to several kV. The capacitors are encapsulated and hermetically sealed in epoxy resins.

Multilayer Ceramic Capacitors (MLCC)

MLCC capacitors are made by creating several layers of ceramic material, interspersed with alternating metal layers, thus forming a stack of several capacitors in parallel. The layers are extremely thin, in micron levels, capacitors so formed are very small, and end connections are directly soldered to printed circuits. MLCC may use over 500 layers or more, using thin film technology and multilayer technology, with thickness as low as 0.5 microns. With improvement in technology, thinner layers and smaller sizes have become possible. These capacitors can replace electrolytic capacitors in number of places, where the nonpolar nature of MLCC is an advantage. Developments over decades have brought down costs to pennies level, and sizes to unimaginable low levels.

Applications of ceramic capacitors

Being most commonly manufactured in large numbers, ceramic capacitors have a wide range of applications. Capacitors of the size of a grain of sane are available. Few of the applications of ceramic capacitors are as follows

• High power high precision resonant circuits in transmitters
• Printed Circuit Boards
• Robotics
• DC-DC converters
• Automobile industry
• Mobile phones
• Computers

The list is endless. From coupling / decoupling, filtering, resonance and so many functions are served by these capacitors in vast array of applications all over the world.

High voltage ceramic capacitors

Large ceramic capacitors can handle large power and high voltages. Power ceramic capacitors range from 2 kV to 100 kV. They have advantage over film capacitors when it comes to small values. While film capacitors are not made below 0.1 μF, high voltage ceramic capacitors are available even as 0.5 pF or 1 pF. Another advantage is small pitch of 2.5 mm, 5 mm, 7.5 mm etc., not possible with film capacitors.

Some of the applications of high voltage ceramic capacitors are:

• Measurement & control circuits
• X-ray machines