What is a Surface Mount Capacitor?

Capacitors are common components in electronic circuits. They block the effects of direct current but allow the effects of alternating current, thus giving them many uses, such as filtering noise and smoothing electrical signals. A surface mount capacitor lacks traditional wire leads. Instead, it has metallic ends that can be soldered directly onto circuit boards.

Practical capacitors have been around since the 1930s. They have found use with tubes, transistors, and, more recently, computers and microwave communications devices. While the capacitor has always had a place in electronics, the form it has taken has changed over the years, mostly because of new materials from which to build it, as well as more precise manufacturing capabilities. The surface mount capacitor is its most recent incarnation.

In the 1960s, a new format for electronic components was developed. Called surface mount technology, this new standard allowed electronic components to be mounted directly onto printed circuit boards either by hand or with automated equipment. At first, the new technology did not catch on, even though it found use in systems such as the Apollo program’s Saturn V rocket system. It was not until the 1980s that surface mount technology began to see widespread use and many electronic components, such as surface mount capacitors, became publicly available.

The operation of capacitors centers on two different internal components: at least two metal plates and a dielectric. In a traditional capacitor, each of the two plates is connected to a metal lead resembling a wire. A material called a dielectric separates the plates. Dielectric material acts as an insulator, which prevents electrical current from passing directly between the plates.

When an electrical signal is applied to the first plate of a capacitor, the two plates of the capacitor have two separate polarized charges, which cause an electromagnetic field to build between them. This field can pass through the dielectric material and will build until it reaches its maximum potential. Once that happens, the field will induce an alternating current in the capacitor’s second plate, which will then pass through the capacitor and on to the rest of the circuit.

A surface mount capacitor functions just like any other capacitor. The primary difference is that the plates are not connected to metal wire leads. Instead, they connect to small metal pads on the ends of the capacitor. These pads allow the capacitor to be soldered directly onto the surface of the circuit board without the need for the through-holes required by other capacitors.
Surface mount capacitors, however, have one advantage over standard capacitors not intended by the creation of surface mount technology. Because it lacks wire leads, a surface mount capacitor is far less likely to create or be affected by many of the negative effects of high-frequency signals. This has allowed surface mount capacitors to find use in even more high-speed, high-frequency applications.