The water-washable Solder Paste process appears to simplify the downstream cleaning process, but in mass production it frequently exposes typical problems such as excessive solder paste residue, whitening of PCB pads, and false ICT test judgments. The root cause is that the cleaning step lacks a quantitative closed loop for controlling solder paste residue. For example, when the cleaning agent concentration is below 1.8%, even extending the cleaning time cannot effectively dissolve rosin-based Flux residue, while a concentration exceeding 2.5% will aggravate erosion of the OSP film layer. Therefore, the concentration must be stably controlled within the range of 2.0% ± 0.2%, and the cleaning tank solution temperature must be simultaneously monitored to ensure it is maintained at 42°C ± 3°C, because when the temperature is below 38°C, the movement of surfactant molecules slows down, reducing the stripping efficiency of micron-level solder paste residue by more than 40%.

Improper squeegee parameter settings are a pre-trigger cause of solder paste residue. When the printing pressure is set to 6.5 kgf, the squeegee angle is 58°, and the printing speed is 18 mm/s, it will cause solder paste to form trailing-like accumulation at the bottom of the stencil opening. These solder paste particles torn by shear force are extremely easy to embed into the micro-gaps of the stencil hole walls, and in the subsequent water washing stage, due to the small contact area, they are difficult to be fully wetted by the cleaning agent. Therefore, we require that after every 25 boards are completed, a combined action of dry wiping of the stencil bottom + IPA wet wiping must be performed, and the dry wiping must use a lint-free cloth to apply pressure in one direction for more than three times; otherwise, the residual solder paste will continuously accumulate during continuous printing and eventually fall off onto the PCB pads.

The centrifugal drying step is often overlooked, but it is precisely the key node that determines whether solder paste residue forms secondary pollution. When the centrifuge speed is lower than 1200 rpm or the spin-drying time is less than 90 seconds, the cleaning liquid remaining at the root of component pins and under the QFN thermal pad cannot be completely discharged. After the water evaporates, ion residues such as sodium and chlorine will precipitate, causing excessive leakage current in subsequent high-temperature and high-humidity aging tests. Therefore, we mandatorily stipulate that all water-washable Solder Paste Production lines must be equipped with centrifugal equipment with speed closed-loop feedback, and the parameters must be written into the MES work order for locking, to prevent manual adjustment by operators.

The conductivity test must be performed in conjunction with microscopic re-inspection. Relying solely on the post-cleaning PCB surface conductivity < 1.2 μS/cm cannot be used as a release basis, because this value only reflects the total amount of macroscopic ions and cannot identify local solder paste residues in blind areas such as under the BGA and inside the shielding cover. We require that the first and last pieces of each batch must be scanned with a 200× metallographic microscope at at least 5 risk points, focusing on observing whether there is translucent gelatinous substance at the edge of the pad. Once found, the line must be stopped immediately to trace the cleaning agent batch and centrifugal parameter records used in the previous 3 hours.