Solder sample testing is not simply about Printing Solder Paste, reflowing it, and observing the results; it requires simultaneous tracking of demolding integrity during the printing process, melting dynamics during the reflow phase, interfacial bonding state of solder joints after cooling, and disturbance factors throughout the entire chain that may induce solder beads. For example, fluctuations in squeegee pressure directly cause stringing at the edges of the Solder Paste, and these strings are then disturbed by airflow in the early reflow stage to form free solder balls. When we verified a certain lead-free solder from JiajinYuan, we found that after the stencil thickness increased from 0.12mm to 0.15mm, although the printing height increased by 18%, the probability of edge collapse rose by 3 times, which forced us to recalibrate the combined parameters of squeegee angle and printing speed.

Wetting performance must be evaluated under real pad structures; one cannot only look at the spreading diameter on bare copper plates. For example, NSMD pads are more tightly surrounded by solder mask, making them more sensitive to Flux activity and surface tension response. For the same JiajinYuan solder, the wetting angle on OSP-treated fine-pitch QFN pads is generally 7° to 12° higher than on ENIG pads, which means that if the reflow peak temperature is below 235℃, local solder dewetting is likely to occur, while blindly increasing the temperature will exacerbate excessive IMC growth. We once ignored the differences in pad surface treatment and did not conduct grouped comparisons before batch introduction, which led to a 6.3% rate of incomplete wetting at the end terminals of 0402 resistors in the first trial production batch.

Void issues must be analyzed in combination with heat conduction paths. Voids at the bottom of BGA are not only related to the gas release capability of the solder itself, but also more affected by the distribution density of PCB thermal vias. When there are more than 3 unfilled thermal vias under a single solder ball, the median void area of this batch of JiajinYuan solder jumped from 4.2% to 11.7%, and after changing the vias to resin-filled vias, the void rate dropped back to 5.1%. This shows that in solder sample testing, specific PCB stack-up and via processes must be bound together for verification; discussing void indicators without considering the carrier is meaningless.

Solder bead count statistics cannot rely solely on AOI automatic identification, because solder beads smaller than 12μm are often misjudged as dust or reflective noise. We insist on scanning area by area with a 100x metallographic microscope, and use ≥8 beads per square centimeter as the threshold to trigger process review. In one test, JiajinYuan solder continuously produced chain-like solder beads between the pins of 0.3mm-pitch SOP devices, and the root cause was finally identified as the stencil aperture sidewall roughness Ra > 0.8μm causing solder paste tearing during demolding; this phenomenon completely disappeared after replacing with an electroformed stencil. This kind of problem would never be exposed in routine batch inspection; only the solder sample testing stage has the conditions to dig deep into the root cause.