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