Surface mount resistors are resistors that are specifically designed and manufactured to be soldered directly onto the surface of a printed circuit board. This is in contrast with the other class of resistors, through-hole resistors, whose leads are placed through the holes of the printed circuit board and then soldered onto the board. Surface mount resistors have gained tremendous popularity in the electronics industry and have largely replaced through-hole resistors because of their extreme efficiency and ease of use. Besides being smaller in size and saving circuit space, they allow for extreme efficiency in the automation process.
A key unique characteristic of surface mount resistors is that they have short leads or no leads at all. Typical resistors usually have two long leads attached to their ends so that they can be placed inside the sockets of a breadboard or be inserted through the holes of a printed circuit board for connection to a circuit. Surface mount resistors are connected to the board by being placed directly on its surface, so their leads are, thus, significantly shorter. Many surface mount resistors are, in fact, manufactured with no leads at all; instead, they have metallic tabs or contacts on their bottom surfaces to serve as electrical contact. This makes surface mount resistors advantageous because they are smaller in size and thus take up less space on a circuit board, leading to higher circuit density.
Surface mount resistors also are unique because they have a different encoding system to represent their resistance value. Instead of having a color code to encode resistance value, as through-hole resistors do, they follow a numerical encoding system. They typically have three numbers printed on them, with the first two numbers being the significant digits and the third number being the power of 10 that the first two numbers will be multiplied by to give the total resistance value. So, for example, if a surface mount resistor has "451" printed on it, for example, it has a resistance of 450 ohms. Two exceptions to this rule are resistors with values of less than 10 ohms, which use an ‘R’ to represent the placement of the decimal point, and precision resistors, which use four digits to represent resistance value, the first three being the significant digits and the fourth being the power of 10 that the first three numbers will be multiplied by to give the total resistance value.
Specialized resistor-inserting machines can easily place and connect surface mount resistors on a printed circuit board. This greatly reduces the labor costs of having an engineer manually solder the components onto the board, and it significantly boosts production rates. These advantages are why surface mount resistors have continued to gain increasing use.