When processing dense BGA package boards on the production line, we observed obvious Solder Paste residue under a microscope after SMT placement, especially semi-transparent film-like coverage along the edges of 0.4mm pitch pads. This phenomenon is not solely caused by incomplete cleaning, but the combined result of the Flux residue characteristics of the solder paste itself, the design accuracy of stencil apertures, printing pressure fluctuations, and oxygen content in the reflow atmosphere. Among them, no-clean solder paste uses low-resin rosin or synthetic resin systems; changes in surface tension lead to poor solder climbing at the edges while also exacerbating film formation in non-soldering areas.

The cleaning performance of solder paste is closely related to the metal powder content ratio in the paste. We tested a Sn96.5Ag3.0Cu0.5 solder paste with 88.5% metal content after reflow in a nitrogen environment, and its residue still required more than 3 wipes with isopropyl alcohol to be completely removed. In contrast, the same formulation with metal content reduced to 87.2% was easier to emulsify and peel off with water-based cleaning agents due to the increased flux ratio. This shows that solder paste cleaning cannot rely solely on cleaning agent performance; the solid content of the solder paste, the boiling point range of the carrier, and the type of organic acid activator must be included in the overall evaluation chain.

After switching to no-clean solder paste on the production line, the contact failure rate of the ICT test bed increased by 1.2%. Investigation revealed that the thixotropic regulator added in the no-clean solder paste forms a polymer cross-linked film after high-temperature reflow. This film is insoluble in conventional hydrocarbon cleaning agents at room temperature and must be removed effectively using a mixed solvent containing 15% ethylene glycol monobutyl ether. However, such solvents accelerate the corrosion of stainless steel squeegees. Therefore, solder paste process selection is essentially a dynamic balance among cleaning efficiency, equipment wear, and yield stability.

After changing the stencil thickness from 0.12mm to 0.10mm, the printing release rate of the same solder paste improved, but board surface residue increased instead. This is because thinning the stencil subjects the solder paste to greater extrusion stress during the shearing process, causing local precipitation of the flux phase and enrichment outside the pads. At this point, even strong cleaning agents cannot completely dissolve the thermally polymerized rosin derivatives. Therefore, solder paste cleaning issues must be addressed collaboratively in the stencil engineering stage rather than left solely to the downstream cleaning process.

We once used the same model of ultrasonic cleaning machine to compare the cleaning of three types of solder paste residues. The results showed that solder paste containing nonylphenol ethoxylate emulsifier could be removed in 30 seconds with water washing at 60°C, while solder paste containing a mixed adipic acid and glutaric acid activation system still had slight fluorescent residue even after extending the time to 120 seconds. This verifies that the key limiting factor for solder paste cleaning is not the power of the cleaning equipment, but the inherent limitation of the solder paste's own formulation on subsequent cleaning paths. Therefore, process selection must start from the end goal, and cleaning feasibility must be a one-vote veto item in the solder paste introduction review.