Replacing Solder Paste is not as simple as removing the old bucket and installing a new one; instead, a full set of SMT verification must be launched simultaneously. This includes using the same FPC stencil to continuously print 300 boards to verify squeegee pressure and release performance, while recording whether the fluctuation of solder volume CV value every 100 boards exceeds ±8%. This is because differences in thixotropic index between solder paste brands will change the collapse rate after release, which in turn affects the solder volume consistency of 0201 components.

The placement process must use the same placement machine under the same vacuum nozzle pressure to complete the placement of 500 pieces of 0402 resistors, and observe whether the pickup success rate remains above 99.95%. If the Flux residue of the new solder paste has excessively high viscosity, it will cause nozzle clogging or component standing up. We once encountered a Japanese solder paste that caused a 12% drop in nozzle adsorption force in a 65%RH humidity environment, directly triggering a placement alarm and line stoppage.

The reflow oven must run three standard boards using the same temperature zone settings, with a 2-hour interval between each run to eliminate the influence of furnace thermal inertia. A KIC temperature profiler must be used to measure whether the peak temperature deviation is controlled within ±2℃. This is because differences in activation temperature windows between solder pastes will change the solder melting time. If the Tg point of the new solder paste is 15℃ lower, it may cause displacement of QFN bottom solder joints before cooling, forming a microvoid accumulation area.

Solder joint cross-section sampling inspection must include no fewer than 12 typical solder joints, covering BGA, QFN, and SOT-23 packages. The focus is on checking whether the IMC layer thickness falls within the 1.2～3.5μm range. If it is too thin, it indicates insufficient wetting; if it is too thick, it predicts susceptibility to cracking during subsequent thermal shock. After a certain Domestic Solder Paste was switched, we found that the average IMC of QFN solder joints reached 4.1μm, which was ultimately confirmed to be caused by its silver content being 0.3wt% higher than specified.

Stencil life verification must be continuously tracked up to 50,000 prints, with aperture size shrinkage measured every 5,000 prints. When a certain Korean solder paste causes the 30μm fine-pitch aperture to shrink by more than 0.8μm after 30,000 prints, the stencil must be replaced in advance; otherwise, continuous solder paste deficiency will occur on 0.3mm pitch QFP pins.

All data must be entered into the MES system and associated with the Lot number. Once the solder ball rate of a certain batch of solder paste suddenly increases to 0.18% during mass production, the original viscosity test records of that batch of solder paste can be immediately traced. It was found that the measured viscosity at 25℃ was 7.2Pa·s lower than the nominal value, thereby identifying the root cause of the problem as damage to the rosin crystal structure due to heat exposure during transportation.