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What Are SMD Capacitors?

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  • Written By: Andrew Kirmayer
  • Edited By: Allegra J. Lingo
  • Last Modified Date: 10 September 2014
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Surface Mount Device (SMD) capacitors are common components used on printed circuit boards. After SMD resistor components, they are the most common type of part for these boards. A capacitor includes conductive metal plates, which are built parallel to one another and are separated by an insulator. When voltage is applied to the capacitor, the insulating material serves to store energy which it gathers from displaced electrical charges taken from an electric field. Other SMD components include SMD inductors, SMD diodes, and SMD transistors.

SMD capacitors come in various forms, and the most common is the SMD ceramic capacitor. They come in the same package types as resistors do, and are designated with a four digit number according to the size of each type. Each set of two numbers corresponds to the dimensions of the capacitor expressed in hundredths of an inch (about 0.25 mm). For example, the smallest size is labeled 0201, which is 0.02 x 0.01 inches (0.6 x 0.3 mm). Other sizes include 0402 (1.01 x 0.51 mm), 0603 (1.52 x 0.76 mm), 0805 (2.03 x 1.27 mm), 1812 (4.57 x 3.05 mm), and 1206 (3.05 x 1.52 mm).

Ceramic is a preferred material for SMD capacitors because it can tolerate the heat applied during soldering. Materials such as polystyrene and polypropylene are more easily damaged by heat and therefore are not used for SMD processes. Silicone dioxide and sapphire are sometimes used, and are viable for use with microwave capacitors.

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SMD capacitors are generally more resistant, when compared to an SMD non-encapsulated capacitor, to mechanical stress both during manufacturing and while operating. They are also less likely to have internal cracks or contact tears, and are made with flame-retardant materials. Each size dimension is defined according to standard manufacturing practices, so it is easier for SMD capacitors to be ordered as well as placed on circuit boards using robotic assembly in fabrication plants.

Durability is important from the time the capacitors are first manufactured because they are heated at temperatures up to 2,372°F (1,300°C). This ensures that the SMD capacitors feature the desired chemical composition at the end of the production line. There is a lot of mixing and grinding of material before electrode materials are printed on sheets and stacked, and during these processes the capacitors are exposed to even higher temperatures. Such a manufacturing process ensures long lasting and reliable SMD products.

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