Formula mechanism and application of solder paste
Release time:
2022-12-02 14:48
With the application of reflow welding technology, solder paste has become the most important process material in surface assembly technology (SMT), and has developed rapidly in recent years.
In reflow soldering of surface assemblies, solder paste is used to connect the leads or endpoints of surface assembled components to the pads on the printed board. Solder paste coating is a key process of surface assembly technology, which will directly affect the welding quality and reliability of surface assembly.
1 Composition of solder paste
Solder paste is a homogeneous mixture, made of alloy solder powder, paste flux and some additives mixed into a certain viscosity and good thixotropy paste. At room temperature, the solder paste can initially stick the electronic components in the established position, when heated to a certain temperature (usually 1830C) with the volatilization of solvents and some additives, the melting of the alloy powder, so that the welded components and the pad are connected together, cooling to form a permanently connected solder joint. The requirements for the solder paste are to have a variety of coating methods, especially with good printing performance and reflow welding performance, and have stability in storage.
2 Alloy solder powder
Alloy solder powder is the main component of the solder paste, accounting for about 85%-90% of the weight of the solder paste. Commonly used alloy solder powder has the following types:
Tin - lead (SN-Pb), tin - lead - silver (SN-Pb - Ag), tin - lead - bismuth (SN-Pb - Bi), etc.
The composition and ratio of alloy solder powder and the shape, particle size and surface oxidation of alloy powder have great influence on the performance of solder paste, so the manufacturing process is relatively high.
Metal composition and melting point of several common alloy solder powders.
Sn63Pb37 and Sn62Pb36Ag2, of which Sn63Pb37 melting point is 1830C, eutectic state, after the incorporation of 2% silver melting point is 179℃, eutectic state, it has good physical characteristics and excellent welding performance, and is not corrosive, wide range of application, silver can improve the mechanical strength of the solder joint.
Shape of alloy solder powder:
The shape of the alloy solder powder can be divided into spherical and oval (amorphous), and their influence on the performance of the solder paste can be seen that the spherical solder has good performance.
The particle size of common alloy solder powder is (200/325) mesh, and finer metal particle size is required for fine spacing printing. The surface oxidation degree of alloy solder powder is related to the manufacturing process, shape and size. Relatively speaking, the oxidation degree of spherical alloy solder powder is small, usually the oxidation degree should be controlled within 0.5%, preferably below 10%-4%.
Generally, the size and shape of the solder powder particles are selected by the opening size of the printed steel plate or screen plate or the caliber of the syringe http://www.xindalang.com. Different pad sizes and component pins should be selected with different particle sizes of solder powder, not all small particles, because small particles have a much larger surface area, making the burden of flux increased when dealing with surface oxidation
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