When selecting water-washable Solder Paste, it is necessary to simultaneously verify whether the spray pressure of the cleaning equipment reaches above 0.3MPa, whether the water temperature is stable at 55±3℃, and whether the concentration of the cleaning agent is maintained between 8% and 12%. Otherwise, even if the solder paste is nominally water-washable, white residue will still form at the bottom of QFN or around 0201 devices, and such residue will induce electrochemical migration after high-temperature and high-humidity aging, leading to functional failure. We once encountered a certain brand of water-washable solder paste with a yield of 99.7% when the cleaning machine parameters met the standards, but as long as the water temperature fluctuated by more than ±2℃, the bridging defect at the bottom of BGA soared to 3.2‰.

To judge whether no-clean solder paste is truly "no-clean", one cannot only look at the ionic contamination data in the manufacturer's TDS, but must conduct three-party verification using the actual stencil, squeegee and reflow oven on the production line. For example, a certain no-clean solder paste with a nominal ROSE value <0.78μgNaCl/cm2 indeed has no corrosion under nitrogen reflow, but after switching to air reflow, slight halogen residue appears in the gold finger area. The reason is that its rosin system oxidizes in an oxygen environment to generate slightly acidic by-products. More importantly, the thixotropic index of no-clean solder paste must be controlled in the range of 4.5 to 6.2. If it is lower than 4.5, it will cause post-printing collapse and short circuits, while if it is higher than 6.2, it will cause poor demolding and insufficient solder.

The matching between solder paste and stencil openings is often overlooked. For 0.3mm pitch QFP devices, if solder paste with a particle size D50 of 25μm is selected, even with a laser stencil, metal particles will get stuck on the side walls of the openings, resulting in insufficient solder for three consecutive prints. At this time, it is necessary to switch to fine-particle water-washable solder paste with D50≤20μm, but the fine-particle type is more sensitive to environmental humidity. When the workshop RH>65%, its viscosity decreases 1.8 times faster than conventional solder paste. Therefore, the solder paste warming time must be extended from 2 hours to 3.5 hours and forced stirring for 3 minutes. No-clean solder paste has slightly better humidity resistance, but is extremely demanding on the stencil wiping frequency. It must be dry-wiped every 15 prints, otherwise the Flux film accumulated at the bottom of the opening will block the solder paste from falling.

When using solvent to clean solder paste, one must never apply a general cleaning process. Rosin-based water-washable solder paste needs to be pre-rinsed with pure water at 60℃ for 30 seconds before entering the main washing section, otherwise the rosin will quickly solidify and block the nozzle when exposed to cold. If carboxylic acid no-clean solder paste residue is mixed into the water washing line, it may react with the amine components in the cleaning agent to form insoluble salts, which adhere to the surface of the track wheel and cause PCB scratches. We once found on a shared cleaning line that the same equipment cleaned water-washable solder paste normally, but after switching to a certain no-clean solder paste, obvious white crystals appeared on the track wheel within a week. The root cause was that the residues of the two solder pastes underwent a complexation reaction in the alkaline cleaning agent.