Solder Paste sample evaluation is not a procedural formality for order approval; instead, we run three or more rounds of printing followed by reflow on production line equipment using real stencils, real PCBs, and real squeegees. We require at least five sets of parallel tests for each batch of sample solder paste, including initial printability verification at 25°C ambient, re-printing stability testing after 48 hours of room temperature storage, and continuous operation adaptability assessment under simulated workshop temperature and humidity fluctuations (20°C~32°C, 40%RH~75%RH). Only these data can reflect the performance of the sample solder paste on a real SMT production line.

Printability testing must use the stencil aperture dimensions and thickness from the customer's actual mass production, and standard test boards cannot be used as a substitute. If the solder paste transfer rate for 0201 component pads under a 0.1mm thick stencil is lower than 72%, it indicates that the thixotropic index of the sample solder paste is mismatched. If bridging or insufficient solder occurs in apertures corresponding to QFP-100 pitch under a 0.12mm thick stencil, immediately check whether the particle size distribution of the sample solder paste exceeds the 5~25μm range, and simultaneously use a microscope to observe whether the number of solder ball residues exceeds the upper limit of 3 per square millimeter specified in IPC-J-STD-005.

Solder slump testing uses a stainless steel cylindrical mold with a diameter of 10mm and a height of 2.5mm. After dispensing on a 25°C constant temperature stage, the mold is left to stand for 15 minutes before measuring the spread diameter. The spread ratio of qualified sample solder paste should be controlled between 1.35 and 1.65. A value below 1.35 indicates excessively high viscosity leading to poor release, while a value above 1.65 predicts a high risk of wicking or bridging during reflow. This value must be strongly correlated with the squeegee settings of 0.35MPa pressure, 45mm/s speed, and 1.2mm/s separation speed; discussing slump without equipment parameters is meaningless.

Viscosity testing must be measured using a Brookfield cone-and-plate viscometer at dual rotational speeds of 0.5rpm and 5rpm. If the viscosity of the sample solder paste is higher than 120Pa·s under low shear and lower than 35Pa·s under high shear, it indicates abnormal shear thinning characteristics. This type of solder paste is prone to stringing the moment the squeegee starts, and difficult to fully release during the separation phase after printing. We have already reworked three batches of BGA packaged boards on the Fuji NXT production line due to this issue.

Wettability verification must be performed with ten reflow cycles each on bare copper boards and OSP boards specified by the customer, with a peak temperature of 235°C and a time above liquidus of 65 seconds. A solder joint wetting angle smaller than 35° is considered passing, but more critically, observe whether the solder joint surface appears dull and lusterless or locally whitened. This often indicates an imbalanced rosin ratio in the sample solder paste or an excessively low activation agent decomposition temperature; even if the contact angle is qualified, the batch cannot be released.

All solder paste evaluation data must be jointly signed and confirmed by the Process, PE, and SQE teams. If any single item fails, the batch of sample solder paste is judged as not passing, and no "concession acceptance" is allowed. Because solder paste is the first gate of soldering quality, if it has problems, all subsequent processes are only patching leaks.