The soldering process in PCBA assembly is fundamental in determining electrical performance and reliability. Studies have shown an increasing trend in problems such as corrosion, short circuits, and open circuits caused by corrosion and electrical migration. Previously, our understanding of cleaning was inadequate, as we believed that the residual flux was non-conductive and benign, and would not affect electrical performance. However, in the current trend of miniaturization in electronic component design, with smaller components and smaller gaps between pins and pads, contaminants may get trapped in the crevices. This means that even small particles of residue between two pads may potentially cause short circuits. Key sources of PCBA contamination include:
- Residues from solder paste, solder flux, solder wire, etc., used in the soldering process during PCBA manufacturing, which can cause contamination on the PCBA surface.
- Contaminants from workplace environments such as dust, water and solvent vapors, smoke, fine particles, organic compounds, and electrostatically charged particles adhering to the PCBA.
- Fingerprints from manual soldering processes, as well as traces left by wave soldering, such as solder wave footprints and soldering tray imprints, may introduce various types of contaminants to the PCBA surface, such as flux residues, adhesive residues from high-temperature tapes, fingerprints, and airborne dust.
- Contamination from components constituting the PCBA and oxidation of the PCB itself can also contribute to PCBA surface contamination.
The function of solder flux during soldering is to remove oxides from the PCB surface, ensuring necessary cleanliness of the metal surface, disrupting surface tension of molten solder, preventing re-oxidation of solder and soldering surfaces, enhancing solder diffusion, and facilitating heat transfer to the soldering area. The main components of solder flux are organic acids, resins, and other substances. Residues are often composed of polymers, halides, and metal salts formed by reactions with tin-lead, which have strong adsorption properties and poor solubility, making them difficult to clean. Contaminants may get trapped in crevices, and even small particles of residue between two pads may potentially cause short circuits.