Know About Solder Paste

Solder paste is a material used to connect the surface mounted components to pads on printed circuit boards. The solder paste is powder metal solder suspended in a thick medium called flux. Here, flux ingredients is generally known as Activators whose primary function is to remove oxidation not only from the surface of PCBs but also from the solder powder itself , Flux become active at soldering temperature

Solder Compositions used are Sn-Pb (Sn-63% & Pb-37%) , SAC alloys(Tin, Silver, Copper), Sn-Sb( Tin-Antimony) for high tensile and shear strength etc.

A solder paste typically has a shelf life of 6 months when refrigerated manufacturers recommend storing solder their pastes between 0 and 10 °C (32 – 50 °F), but customers should confirm this with each manufacturer’s specific instructions for storage. The best approach is to refrigerate solder paste immediately upon receipt and only thaw when needed, in amounts that will be completely consumed. Avoid thawing and re-refrigerating pastes as much as possible, in order to take advantage of the full shelf life.

Solder paste are available in two categories , one is leaded solder and another is lead free solder.Leaded solder paste contains composition of tin and lead (Sn-63% & Pb-37%) . A benefit of using leaded solder is its ability to heat up at a much lower temperature ( 183 °C) than lead-free solder (217 °C) thus posses less thermal threat to the components. Also it takes on a shinier appearance than lead free solder therefore it is easier to spot problems such as oxidation.The problem with leaded solder is the lead present in the paste which is poisonous and harmful to mankind and environment due to which this paste is not used in ROHS complied process or industries.

Solder paste grades

Depending upon the type of Assembly there are various grades of solder paste.The solder paste is graded according to the size of the small solder balls. The particle (mesh) size of the solder powder can also impact shelf life. As the powder size decreases, the surface area per volume or mass of powder drastically increases.

IPC TYPE DESIGNATION PARTICLE SIZE (ΜM) * Type 1            75 - 150 Type 2            45 - 75 Type 3            25 - 45 Type 4            20 - 38 Type 5            10 - 25 Type 6              5 - 15 Type 7               2 - 11 Type 8              2 - 8

According to IPC standard J-STD-004 "Requirements for Soldering Fluxes", solder pastes are classified into three types based on the flux types:

·         Rosin based solder pastes:   Rosin based pastes are made of rosin, a natural extract from pine trees. These fluxes can be cleaned if required after the soldering process using a solvent (potentially including chlorofluorocarbons).

·         Water soluble flux based solder pastes :   Water-soluble fluxes are made up of organic materials and glycol bases. There is a wide variety of cleaning agents for these fluxes.

·         No clean solder paste:   A no-clean flux is made with resins and various levels of solid residues. No-clean pastes save not only cleaning costs, but also capital expenditures and floor space. Although the no-clean flux based solder pastes sound attractive, they need a very clean assembly environment and may need an inert reflow environment.