SMD surface mount capacitor are the most widely used for of capacitor today - being small, leadless and easy to place on a PCB they are ideal for high volume manufacture. Their performance is also very good, some performing particularly well at RF.
SMD surface mount capacitor are used in high volume manufacture - quantities used are numbered in the billions. They are small, leadless and can be placed onto modern printed circuit boards using pick and place machines used in modern manufacturing.
There are many different types of SMD capacitor ranging from ceramic types, through tantalum varieties to electrolytics and more. Of these, the ceramic SMD capacitor are the most widely used.
SMD capacitor on a printed circuit board
SMD capacitor on a PCB
Surface mount technology
SMD capacitor are just one form of component that uses surface mount technology. This form of component technology has now become commonplace for manufacturing electronic equipment as it enables much faster and more reliable construction of electronic printed circuit boards.
Note on the Surface Mount Technology:
Surface mount technology offers significant advantages for the mass production of electronic equipment. Traditionally components possessed leads at either end and these were attached to either terminals or later they were mounted through holes in a printed circuit board. Surface mount technology does away with the leads and replaces them with contacts that can be mounted directly onto the board enablong easy soldering.
SMD capacitor basics
Surface mount capacitor 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:
Ease of use in manufacturing: As with all other surface mount components, SMD capacitor are very much easier to place using automated assembly equipment.
Size: SMD capacitor 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.
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 capacitor are much nearer the ideal component that their leaded relations.
Lower cost: Not only can these components be used more easily in production reducing the production costs of the final product, but they also lend themselves more easily to their own high volume manufacture. The lack of leads makes their manufacture easier. In addition to this, the enormous volumes in which they are manufactured has resulted in significant cost reductions in their production.
Multilayer ceramic SMD capacitor
The multilayer ceramic SMD capacitor form the majority of SMD capacitor that are used and manufactured. They are normally contained in the same type of packages used for resistors.
MULTILAYER CERAMIC SMD CAPACITOR DIMENSIONS
SIZE DESIGNATION MEASUREMENTS (MM) MEASUREMENTS (INCHES)
1812 4.6 x 3.0 0.18 x 0.12
1206 3.0 x 1.5 0.12 x 0.06
0805 2.0 x 1.3 0.08 x 0.05
0603 1.5 x 0.8 0.06 x 0.03
0402 1.0 x 0.5 0.04 x 0.02
0201 0.6 x 0.3 0.02 x 0.01
SMD ceramic capacitor selection
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 multil layer capacitor 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.
SMD electrolytic capacitor
Electrolytic capacitor 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 capacitor 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 of 10 times 10^6 pico-farads. This works out to be 10μF.
ELECTROLYTIC SMD CAPACITOR CODES
LETTER CODE VOLTAGE
SMD tantalum capacitor
Tantalum SMD capacitor are widely used to provide levels of capacitance that are higher than those that can be achieved when using ceramic capacitor. As a result of the different construction and requirements for SMD tantalum capacitor, there are some different packages that are used for them. These conform to EIA specifications.
SMD tantalum capacitor top and bottom views
SMD tantalum capacitor
TANTALUM SMD CAPACITOR DIMENSIONS
SIZE DESIGNATION MEASUREMENTS (MM) EIA DESIGNATION
Size A 3.2 x 1.6 x 1.6 EIA 3216-18
Size B 3.5 x 2.8 x 1.9 EIA 3528-21
Size C 6.0 x 3.2 x 2.2 EIA 6032-28
Size D 7.3 x 4.3 x 2.4 EIA 7343-31
Size E 7.3 x 4.3 x 4.1 EIA 7343-43
SMD tantalum capacitor were for many years the only type of high value SMD capacitor available. It took some years before SMD electrolytic capacitor were developed, because of the requirement for SMD capacitor to be able to withstand the high soldering temperatures, and as a result, tantalums were widely used. Nowadays, SMD electrolytic capacitor are the main type used, although tantalums are still used in large quantities as their performance tends to be better in some respects.
SMD capacitor codes
Comparatively few SMD capacitor 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.
Advantages & disadvantages of SMD capacitor
As with any technology there are advantages and disadvantages to the use of a particular technology and the same is true for SMD capacitor.
Surface mount capacitor are used in their billions within facilities that mass produce electronics equipment. Their size and the ability to be placed onto a printed circuit board enable them to be used with ease. As a result, surface mount capacitor are used in virtually all positions on mass produced electronic equipment.