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Solder paste is a compound that typically consists of a fusible metal alloy and some type of deoxidizing flux. Different pastes can have a variety of compositions, though a typical formula consists of powdered solder mixed in with a gel-like flux material. In many applications, solder paste will be used to hold components in place prior to soldering, and also provides the fusible alloy that is heated to permanently bond them. This type of solder is most commonly used in the reflow soldering of surface mount devices (SMDs). It is often applied using some type of screen printing method, though it may also be dispensed manually.
There are several different types of solder paste, and it is often classified by the size of the metal balls that make up the powdered metal. These solder balls are typically uniform in size to facilitate the printing process. Each size category is based on both the distribution of the balls throughout the flux material and the physical size of each solder particle. Ensuring a uniformity in both mesh and size tends to result in better printing, especially when a stencil is used. Irregularly sized particles of solder may clog a stencil, while a nonuniform mesh may result in areas of oxidation.
Solder paste can typically be obtained in a variety of different alloys, each of which can be well suited to a particular application. A classic eutectic mixture of tin and lead is often used for electronics, though paste that contains a tin, silver, and copper alloy may be used instead. Solder paste containing tin, silver, and copper is typically referred to as a SAC alloy, and is often used due to health and environmental concerns over lead. Paste containing tin and antimony may be used if high tensile strength is desired, and other variations can be useful in other circumstances.
A variety of methods can be used to apply solder paste to a circuit board. It is commonly printed over a stencil using a pneumatic process, though other methods operate similarly to an inkjet printer. Solder paste may also be applied using a series of needles that are dipped first into the flux mixture and then pressed on to the circuit board in the desired pattern. Regardless of the method used to deposit the paste onto the circuit board, it will typically act as an adhesive to hold any electronic components in place until the completion of the soldering process.