Solder paste is a critical material in PCBA manufacturing, combining metal alloy powders, flux, and solvents to enable reliable solder joints. The choice of metal alloy significantly impacts performance: traditional tin-lead (Sn63Pb37) alloys offer excellent wetting and low melting points (183°C), making them ideal for high-speed assembly. However, environmental regulations like RoHS have driven the adoption of lead-free alternatives, such as Sn-Ag-Cu (SAC305), which require higher reflow temperatures (217–245°C) but provide comparable reliability. Flux composition also plays a vital role; halogen-free fluxes reduce corrosion risks, while low-residue formulations minimize post-reflow cleaning. Solder powder particle size (Type 3–6) affects printability—finer particles (Type 4–6) suit high-density PCBs but are more prone to oxidation. To address these challenges, manufacturers optimize viscosity and tackiness for automated printing while ensuring long shelf life through vacuum packaging and refrigeration. Recent innovations include bio-based fluxes and recycled solder alloys, aligning with sustainability goals without compromising solderability.

Material selection extends beyond electrical performance. For automotive applications, high-reliability alloys like Sn-Bi (eutectic at 138°C) are used for low-temperature soldering, reducing thermal stress on components. Aerospace PCBAs often employ indium-based solders for cryogenic environments. Flux activity levels (R, RM, RA) are tailored to surface oxidation levels—active fluxes (RA) remove heavy oxides but require thorough cleaning, while no-clean formulations simplify processes. Advances in nanotechnology have enabled submicron solder particles, improving joint resolution for 0.3mm-pitch devices. Environmental controls, such as maintaining 23±2°C and 45±10% RH in storage areas, prevent flux degradation and particle agglomeration. By balancing cost, performance, and environmental impact, manufacturers achieve first-pass yields exceeding 99% in high-volume production.