SMD capacitors

SMD Capacitor

- an overview of the Surface Mount Device (SMD) capacitor, its performance, construction, mechanical details and other useful information.

SMD capacitors are used in vast quantities. After SMD resistors they are the most widely used type of component. There are many different types of SMD capacitor ranging from ceramic types, through tantalum varieties to electrolytics and more. Of these, the ceramic SMD capacitors are the most widely used.

Ceramic SMD capacitors

The 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.

• 1812 - 4.6 mm � 3.0 mm (0.18" � 0.12")
• 1206 - 3.0 mm � 1.5 mm (0.12" � 0.06")
• 0805 - 2.0 mm � 1.3 mm (0.08" � 0.05")
• 0603 - 1.5 mm � 0.8 mm (0.06" � 0.03")
• 0402 - 1.0 mm � 0.5 mm (0.04" � 0.02")
• 0201 - 0.6 mm � 0.3 mm (0.02" � 0.01")

Construction:   The SMD capacitor consists of a rectangular block of ceramic dielectric in which a number of interleaved precious metal electrodes are contained. 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 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.

• Size A   3.2 mm � 1.6 mm � 1.6 mm (EIA 3216-18)
• Size B   3.5 mm � 2.8 mm � 1.9 mm (EIA 3528-21)
• Size C   6.0 mm � 3.2 mm � 2.2 mm (EIA 6032-28)
• Size D   7.3 mm � 4.3 mm � 2.4 mm (EIA 7343-31)
• Size E   7.3 mm � 4.3 mm � 4.1 mm (EIA 7343-43)

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 (m F), and another is to use a code. Using the first method a marking of 33 6V would indicate a 33 mF 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 picofarads. 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 picofarads. This works out to be 10 mF