During reliability verification, we found that after uncleaned Solder Paste residue was stored for 168 hours in an environment of 40°C and 93%RH, the surface resistance dropped by as much as 3 orders of magnitude. Among them, RMA-type solder paste is more prone to moisture absorption than no-clean type, because its rosin matrix contains hydroxyl structures with strong hydrophilicity. Although no-clean type is labeled as "no cleaning required", its organic acid activators will still slowly hydrolyze under high temperature and high humidity to generate conductive ions, directly threatening insulation reliability.

A common type of failure in the workshop is passing functional testing but failing after aging, and the root cause is often hidden in solder paste residue. For example, a batch of WiFi modules had excessive standby current after aging for 96 hours at 70°C/85%RH. Cross-sectional analysis confirmed that solder paste residue at the bottom of the BGA absorbed moisture and formed micro-conductive paths, while the cleaned areas on the same board showed no abnormalities. This indicates that the chemical composition and residue thickness of solder paste jointly determine the moisture-induced leakage threshold, which cannot be simply avoided by a one-size-fits-all "whether to clean" approach.

We have tracked the performance of three mainstream solder pastes under the same temperature and humidity conditions, and found that paste residue with a solid content >85% is denser, with slow initial moisture absorption but accelerated degradation once the critical humidity is exceeded. Pastes with a solid content <70%, due to low resin crosslinking degree, have a fast and irreversible moisture absorption rate, leading to obvious differences in insulation reliability degradation curves. Therefore, when selecting solder paste, one cannot only rely on the Tg or halogen content provided by suppliers, but must match the moisture absorption kinetics of the residue in combination with the storage cycle and environment of the own production line.

Production line measurements show that for uncleaned solder paste residue within 24 hours after reflow soldering, if stacked in a warehouse with humidity >60% for more than 7 days, MΩ-level leakage current can be measured between QFP pins with adjacent spacing <0.3mm. If the PCB substrate water absorption rate >0.15% is further superimposed, the leakage risk directly doubles. Therefore, it is recommended to enforce cleaning processes for high-density, high-reliability products, and bind the solder paste batch with cleaning parameters into the MES system to avoid insulation reliability fluctuations caused by mixing different batches.

A colleague once tried to use IPA wiping instead of ultrasonic cleaning, but under the microscope, solder paste was still visible penetrating into the capillary gaps of the solder mask. After these residues were naturally stored for three months, more ion migration paths were precipitated during 85°C baking, resulting in worse insulation reliability than before treatment. This shows that cleaning is not just about performing the action, but must match the polarity of the solder paste, resin softening point, and PCB surface energy; otherwise, although it looks clean, it actually lays a greater hidden danger of leakage.