Selection depends on what properties are desired, electronic standards that need to be met, and what types of electronic device are being made. It is important to be aware of any interactions the alloy selection can have with the base metals it is joining as this can impact the security of the metal joint. Some of these metals include but are not limited to antimony, bismuth, indium, and nickel. These are only two of the most common alloys that are used in solder paste, however, there can be other alloys that have a combination of different metals and percentage levels to improve the quality of the solder joint. Copper reduces the melting point of the alloy and aids in the wetting of the molten solder while silver adds mechanical strength but is less ductile than lead. SAC305 is mostly made of tin, silver and copper are added to supplement the strength and lower melting temperature to improve the properties of the alloy. Compared to eutectic solder, this SAC305 has a higher melting point. This has led to the emergence of lead-free alternatives, the most common of them being SAC305, which is comprised of tin, silver, and copper. However, lead is considered both a health and environmental hazard so there has been a push in the industry to produce products that use less hazardous materials. These benefits are a reason as to why a sizable percentage of alloys for solder paste use a eutectic base. Additional metals can also be added to further change the mechanical properties of the alloy.įor example, lead lowers the overall melting temperature and forms strong joints with other metals such as copper and aluminum which are often used for PCB pads or component leads. Both alloys are comprised mostly of tin which has a low melting point and high tensile and shear strength.

Solder powder can be composed of a variety of alloys, the most common being a combination of eutectic (tin and lead) and lead free, typically SAC305 (tin, silver 3.0, copper 0.5). This guide aims to provide information on the material properties of solder paste, its role in an SMT (surface mount technology) production line, and an overview of common defects. The combination of these components determines what subcategory of flux it falls under, which dictates the rheological properties of the material and how the flux removes oxides.

Flux is made of a combination of components such as rosins, activators, rheological additives, and solvents. These properties determine the time above liquidus, level of oxides, strength of the intermetallic bond, minimum aperture size, and the inner diameter for dispense printing. Solder powder has two important properties, the alloy type and the particle size. At this temperature, flux is activated and will remove oxides from the surface of the leads, metal pads, and surface of the solder powder, allowing the solder powder to melt effectively and form solid metal joints. In solder reflow applications, the solder paste is heated to an above liquidus temperature. This results in high speed and volume throughput for production.

The paste forms a tacky consistency that prevents movement of components after placement on a printed circuit board (PCB) during SMT production. This combination produces a thick material, which allows for easier deposition, typically through stencil printing or deposition. Solder paste is made with a combination of solder powder and a thick flux which is preblended to form a paste.

Soldering paste use