Solderpaste

Solder paste is a semi-solid mixture of metal solder powder and flux used to join electronic components to printed circuit boards (PCBs) in surface-mount technology (SMT) assembly. It is typically deposited onto PCB pads through a stencil or dispensed by jet printing, components are placed on the paste, and the assembly is heated in a reflow oven to melt the solder and form metallurgical joints.

Composition varies with alloy and flux chemistry. Common alloys include traditional tin–lead (e.g., Sn63Pb37) and lead-free alternatives such as SAC305 (tin-silver-copper). Flux types include rosin-based, water-soluble, and no-clean formulations; flux activity and residue considerations influence cleaning and reliability requirements. Solder powder is classified by particle size (Type 1–5), with finer powders used for finer pitch components but presenting greater sensitivity to oxidation and handling.

Process control and storage are important for quality. Stencil design, print parameters, and paste rheology affect deposit volume and uniformity. Reflow profiles must be matched to the alloy and assembly to avoid defects such as tombstoning, voids, or solder balls. Solder paste typically has limited shelf life and is stored refrigerated; it should be brought to room temperature before use and mixed if separation occurs.

Safety considerations include using proper ventilation to control fumes, personal protective equipment, and adherence to waste and lead handling regulations when lead-containing pastes are used. Proper process controls and material selection contribute to reliable solder joints in electronic assemblies.