Our production line processes 3200 BGA motherboards per day. For Solder Paste cleaning, we do not choose the most expensive Solder Flux cleaner, but the one that best matches our current process parameters. For example, when the board surface appears white after reflow and ICT frequently reports open circuits, it is most likely caused by uncleaned carbonized rosin-based Flux residues. In this case, using a 5% isopropanol + 95% ethanol compound cleaner is 40% more efficient than pure water-based solutions. However, the spray pressure of the cleaning machine must be adjusted from 1.2 MPa to 0.8 MPa simultaneously; otherwise, the uncured flux residue under the BGA will be washed into the gaps between solder balls.

No-clean flux may appear clean on the surface after reflow in a nitrogen atmosphere, but the organic acid residue level is still 0.12 mg/cm². Using an alkaline water-based solder flux cleaner with a pH of 8.3 combined with 60°C ultrasonic cleaning can reduce this to below 0.015 mg/cm². However, note that after cleaning, the board must be rinsed twice with deionized water for no less than 90 seconds each time; otherwise, invisible white crystalline salts will appear on the board surface after drying, which will directly induce CAF failure during damp heat aging tests.

Hydrocarbon cleaners have a fast dissolution rate for flux with a resin content exceeding 35%. However, after switching to a new batch of hydrocarbon cleaner, SMT pick-and-place machine nozzles became clogged for three consecutive days. Investigation revealed that the initial boiling point of this cleaner was only 142°C, and fluctuations in workshop ambient temperature caused it to condense and precipitate waxy substances inside the nozzles. The problem was later resolved by switching to a narrow-boiling-range hydrocarbon cleaner with an initial boiling point above 168°C. This shows that the boiling range of the cleaner is more important than simply looking at the ingredient list.

Cleaning effectiveness cannot be judged by visual inspection alone. We used an ion chromatograph to measure the residual sodium ion concentration of different cleaners: isopropanol-based cleaners averaged 1.8 μg/cm², water-based cleaners 0.7 μg/cm², and hydrocarbon cleaners were as high as 3.2 μg/cm². Therefore, even if hydrocarbon cleaners are used for high-reliability military boards, a secondary pure water rinsing station must be added; otherwise, the HALO test will not pass.

Our production line once used cheap industrial alcohol instead of dedicated solder flux cleaner, which accelerated the oxidation of OSP board pads. The first-article solder thrust dropped sharply from 4.2 N to 2.6 N. It was later found that the alcohol contained 0.3% methanol impurities, which underwent a transesterification reaction with the OSP film and destroyed the organic protective layer. Therefore, solder flux cleaners must provide a full SGS ingredient report, with particular attention paid to the impurity limits of methanol, acetone, and benzene series.

Higher cleaning temperature is not always better. If the water-based cleaner exceeds 65°C, the amide-based activators in some flux will prematurely crosslink into a network structure, making them harder to remove. If the hydrocarbon cleaner is heated above 55°C, it will significantly accelerate the aging of rubber sealing rings. Therefore, we control the temperature of the water-based cleaning tank at 58±1°C and the hydrocarbon tank at 48±1°C. This temperature difference is the optimal balance point measured over three years.