SMD capacitor basics

Surface mount capacitors are basically the same as their leaded predecessors. However instead of having leads they have metallised connections at either end.

This has a number of advantages:

• Size:   SMD capacitors can be made very much smaller than their leaded relations. The fact that no wired leads are required means that different construction techniques can be sued and this allows for much smaller components to be made.
• Ease of use in manufacturing:   As with all other surface mount components, SMD capacitors are very much easier to place using automated assembly equipment.
• Lower spurious inductance:   The fact that no leads are required and components are smaller, means that the levels of spurious inductance are much smaller and these capacitors are much nearer the ideal component that their leaded relations.

Multilayer ceramic SMD capacitors

The multilayer ceramic SMD capacitors form the majority of SMD capacitors that are used and manufactured. They are normally contained in the same type of packages used for resistors.

Construction:   The multilayer ceramic SMD capacitor consists of a rectangular block of ceramic dielectric in which a number of interleaved precious metal electrodes are contained. This multilayer structure gives rise to the name and the MLCC abbreviation, i.e. Multi-Layer Ceramic Capacitor.

This structure gives rise to a high capacitance per unit volume. The inner electrodes are connected to the two terminations, either by silver palladium (AgPd) alloy in the ratio 65 : 35, or silver dipped with a barrier layer of plated nickel and finally covered with a layer of plated tin (NiSn).

Ceramic capacitor manufacture:   The raw materials for the dielectric are finely milled and carefully mixed. Then they are heated to temperatures between 1100 and 1300°C to achieve the required chemical composition. The resultant mass is reground and additional materials added to provide the required electric properties.

The next stage in the process is to mix the finely ground material with a solvent and binding additive. This enables thin sheets to be made by casting or rolling.

For multilayer capacitors electrode material is printed on the sheets and after stacking and pressing of the sheets co-fired with the ceramic compact at temperatures between 1000 and 1400°C. The totally enclosed electrodes of a multilayer capacitor ceramic capacitor, MLCC guarantee good life test behaviour as well.

Tantalum SMD capacitors

Tantalum SMD capacitors are widely used to provide levels of capacitance that are higher than those that can be achieved when using ceramic capacitors. As a result of the different construction and requirements for tantalum SMT capacitors, there are some different packages that are used for them. These conform to EIA specifications.

Tantalum SMD capacitor 

. Note the bar across one end indicating the polarity

Electrolytic SMD capacitors

Electrolytic capacitors are now being used increasingly in SMD designs. Their very high levels of capacitance combined with their low cost make them particularly useful in many areas.

Often SMD electrolytic capacitors are marked with the value and working voltage. There are two basic methods used. One is to include their value in microfarads, µF, and another is to use a code. Using the first method a marking of 33 6V would indicate a 33 µF capacitor with a working voltage of 6 volts. An alternative code system employs a letter followed by three figures. The letter indicates the working voltage as defined in the table below and the three figures indicate the capacitance on pico-farads. As with many other marking systems the first two figures give the significant figures and the third, the multiplier. In this case a marking of G106 would indicate a working voltage of 4 volts and a capacitance 0f 10 times 10^6 pico-farads. This works out to be 10µF 

SMD capacitor codes

Comparatively few SMD capacitors have their values marked on their cases. This means that great care must be taken when handling them to ensure they are not misplaced or mixed. However a few capacitors do have markings. The capacitor values are coded. This means that it is necessary to know the SMD capacitor codes. These are simple and easy to decode.

A three figure SMT capacitor code is normally used as there is usually little space for anything more. In common with other marking codes the first two indicate the significant figures, and the third is a multiplier.

SMD capacitor code

Here the two figures 47 indicate the significant figures and the 2 indicates the multiplier of 2, i.e. 100.