The formation of solder balls is never caused by a single factor, but is the combined result of excessive Solder Paste printing volume, geometric mismatch of stencil apertures, and uncontrolled Reflow Soldering temperature ramp profile. In our application of Jia Jinyuan solder paste, we found that when the stencil thickness is 0.12mm, if the aperture aspect ratio is less than 1.5 and the area ratio exceeds 0.65, the volume of solder paste after printing will exceed the capacity limit of the pad. Especially around small-sized CHIP components such as 0402 and 0201, excess solder paste is squeezed to the outside of the pad during the preheating stage. After entering the reflow stage, it cannot retract due to insufficient surface tension, and finally solidifies into independent spherical objects.

The reflow heating rate is the key switch that triggers the manifestation of solder balls. Measured data shows that when the heating slope in the 150℃ to 183℃ range exceeds 2.8℃/s, the Flux volatilizes sharply and violently, and the solder paste particles are splashed by the airflow. If there is also uneven solder paste thickness at this stage, such as local accumulation caused by squeegee pressure fluctuations, the splashing probability increases by more than 3 times. At this time, even if the stencil aperture is compliant, dense solder ball clusters can still be observed at the four corners of the QFN thermal pad.

The rheological properties of the solder paste itself are also deeply involved. The viscosity of Jia Jinyuan SN100C solder paste at 25℃ is 720Pa·s. If the workshop temperature and humidity are not controlled within the range of 22±2℃ and 55±5%RH, the thixotropic recovery time of the solder paste will be prolonged, and it is easy to draw strings and break during printing demolding. The remaining fine solder paste strings fuse and shrink into balls during reflow. This type of solder ball is usually smaller in size (30～60μm) and irregularly distributed, but extremely numerous, requiring a 20x magnifying glass to accurately identify.

Insufficient stencil cleaning frequency will exacerbate the problem. After using nitrogen purging instead of wet cleaning, residual flux carbide accumulates in the stencil apertures, leading to coexistence of incomplete solder paste filling and edge trailing in subsequent printing. The trailing part collapses under heat during reflow, separates from the main solder and forms independent balls. This type of solder ball often adheres below the projection area of the stencil support ribs, with highly repetitive positions. During troubleshooting, the stencil support structure diagram can be directly compared for positioning.

Field verification shows that adjusting the squeegee angle from 60° to 55° and reducing the printing speed from 40mm/s to 32mm/s can effectively reduce the shear stress of the solder paste and make demolding cleaner. Combined with reducing the heating slope of the reflow soldering 150～183℃ section from 3.2℃/s to 2.4℃/s, the solder ball defect rate can be reduced from 1200 per million points to below 210. However, the premise is that the stencil aperture must be implemented according to the recommended parameters of Jia Jinyuan, otherwise any process adjustment will only delay the outbreak rather than eradicate the hidden danger.