Incoming Solder Paste inspection is not a mere formality; it is the critical lifeline that determines whether the production line can run continuously. Every day upon receipt, we first verify the consistency between the batch number and the MSDS, then simultaneously conduct viscosity testing and visual inspection. If slight stratification or surface skinning is found, we immediately isolate the material and initiate the supplier nonconformance feedback process, as such phenomena often indicate instability in the Flux system or uncontrolled storage conditions.

Viscosity testing must use a rotational viscometer of the same model, with the rotor immersion depth strictly controlled at 12 mm. The measured value must fall within ±15% of the supplier's nominal range; otherwise, even if the appearance is normal, it will cause solder paste trailing or missing prints at the stencil openings. The metal content is determined by the ignition method; when the measured value is below 88.5%, the consistency of print thickness will deteriorate significantly, especially on 0201 and smaller components, where insufficient solder or even cold solder joints are prone to occur.

Particle size distribution is measured with a laser particle size analyzer; D50 must be between 20 and 25 μm, and D90 must not exceed 40 μm. Incoming solder paste outside this range is highly prone to clogging or edge tearing during fine-pitch QFN printing. We once had a batch of solder paste with a D90 of 43.6 μm, which caused the NG rate to exceed 12% for three consecutive shifts. After rework, we confirmed that the root cause was the omission of particle size retesting during the solder paste inspection process.

The slump test involves screen printing the solder paste onto a glass plate, letting it stand for 15 minutes, then observing the degree of deformation under a stereomicroscope. If the height loss exceeds 18%, the material must be rejected. Such incoming solder paste is prone to bridging before reflow, especially in the peripheral areas of 0.4 mm pitch BGA, where short-circuit defects will occur in concentrated bursts.

Wettability verification uses the copper dip method: immerse the copper sheet in a constant-temperature solder bath at 245°C for 3 seconds, then remove and cool it. If the solder joint coverage area is below 92%, it is considered nonconforming. This indicator is directly related to the void rate after reflow and the uniformity of IMC growth. We have compiled data from the past six months: for every 1 percentage point drop in the wettability pass rate, the proportion of BGA solder joints with X-ray voids >25% rises by 0.7 percentage points.

All inspection items must be completed within 4 hours of incoming material and recorded in the MES system. Delayed inspection will cause the solder paste temperature to rise, leading to viscosity drift. Especially in winter when the room temperature is below 18°C, pay extra attention to letting the solder paste stand at a constant temperature for 30 minutes before sampling; otherwise, low viscosity readings will cause misjudgment. In the high-temperature and high-humidity environment of summer, focus on spot-checking whether the solder paste surface has whitened, which is a typical sign of rosin crystallization, indicating that the incoming solder paste has undergone microscopic phase separation.