We have found in actual production lines that the acid value of Solder Paste directly determines its ability to remove copper surface oxide layers and the level of ionic contamination in post-solder residues. When encountering OSP-treated PCBs or 0201 resistors with silver-plated pins, low-acid neutral solder paste often exhibits poor wetting and an increased rate of cold solder joints. After switching to weakly acidic solder paste with an acid value of 0.8~1.2mgKOH/g, the number of solder balls decreased by more than 40%, provided that an ultrasonic cleaning process is used; otherwise, chloride ion residues will cause CAF failure.

The acid value of solder paste is not necessarily better the higher it is. A customer once mistakenly used a strongly acidic paste with an acid value above 1.5 when soldering ENIG (Electroless Nickel Immersion Gold) boards, resulting in selective etching of the gold layer and blackening at the solder joint edges. Obvious interfacial corrosion pits were visible under a microscope, and this phenomenon was particularly severe when the reflow peak temperature exceeded 235°C. Therefore, we require all acidic solder pastes to undergo board-level ion chromatography testing before going online to confirm that the total amount of Cl? and Br? is below 0.5μg/cm2.

Although neutral solder paste eliminates the cleaning step, it is more sensitive to stencil aperture design and squeegee pressure. For example, in 0.3mm pitch QFN printing, if a neutral paste with 68% solid content is used while retaining the 60° squeegee angle of acidic paste, the insufficient shear force will cause the skip printing rate to soar from 1.2% to 7.6%. At this point, the squeegee angle must be reduced to 45° and the release speed must be simultaneously increased to 40mm/s to restore yield.

The acid value of solder paste also has a coupling effect with solder powder particle size. We compared Type 4 (20~38μm) and Type 5 (15~25μm) solder powders paired with solder pastes having acid values of 0.9 and 1.1 respectively. Under the same stencil thickness of 120μm, the slump rate of Type 5 solder powder paired with high-acid paste was 2.3 times higher than that of Type 4. This means that for fine-pitch devices, medium-acid solder paste must be prioritized in conjunction with stricter temperature and humidity control.

The most commonly overlooked factor when changing solder paste models on the production line is stencil life adaptability. Chemical erosion of acidic solder paste on stainless steel stencils will increase the aperture size by 0.8μm after 30,000 consecutive prints, while neutral paste only increases it by 0.2μm. Therefore, when switching to acidic solder paste, the stencil replacement cycle must be shortened from 60,000 to 40,000 prints, and aperture tolerances must be spot-checked with a coordinate measuring machine every 20,000 prints.