Most customer questions we encounter on the production line focus on the same issue: whether no-clean Solder Paste can truly be used without cleaning. This depends on whether the ionic content of its residue is below 0.5μg/cm² NaCl equivalent, whether it passes the L0 classification certification of IPC-J-STD-004B, and must also be judged in combination with the PCB surface cleanliness test results. For example, when an ion contamination tester detects chloride residue on the board surface exceeding 0.8μg/cm², even if it is labeled no-clean, a cleaning process must be added.

The residue morphology and distribution state of solder paste are more critical than simply looking at the model. In a medical motherboard rework case we handled last week, the same no-clean solder paste caused solder paste collapse under conditions of overly large stencil openings and insufficient squeegee pressure. The Flux was squeezed to the edge of the pads, forming piled residue. Although there was no obvious corrosion after reflow, it caused poor contact of ICT probes. This kind of problem cannot be solved by changing the solder paste model; printing parameters must be adjusted synchronously and X-ray verification must be performed.

Whether to clean or not must ultimately return to reliability verification data. We recommend conducting an 85℃/85%RH 1000-hour aging and power-on test on the first batch of trial production boards, combined with SEM+EDS analysis of halogen element enrichment at the solder joint interface. If bromine is found to be significantly aggregated above the IMC layer, it indicates that the thermal decomposition products of this batch of solder paste have not completely escaped. At this time, even if it is labeled no-clean, supplementary cleaning must be performed; otherwise, after mass shipment, it will induce leakage current or even short circuits in high-temperature and high-humidity environments.

The cleanability of flux residue is also greatly affected by the PCB substrate. Residue on FR-4 substrate is relatively easy to remove, while for high-frequency substrates such as Rogers 4350B, due to low surface energy and numerous micropores, the cleaning difficulty of the same concentration of solder paste residue increases by more than three times. At this time, the compatibility of the cleaning agent with the substrate must be evaluated to avoid delamination or dielectric constant drift after cleaning.

On-site engineers cannot only focus on the three words "no-clean" on the solder paste manual, but must actually measure the insulation resistance value near the solder joints after reflow. If the measured resistance value under a 50V bias is lower than 1×10⁸Ω, it indicates that the flux residue still has a conductive risk. Especially for power modules with an operating voltage higher than 24V, this kind of residue may cause soft breakdown during long-term operation.