The yield bottleneck of SMT Soldering for BGA/QFN chips often lies not in placement accuracy or reflow temperature profile, but in the inherent binding force of the Solder Paste itself against void formation. JJY JL-808 solder paste has a viscosity of 780 Pa·s at 25°C and a thixotropic index of 0.72. This rheological property enables it to fully fill the 0.2mm gap under QFNs under squeegee pressure, while maintaining sufficient cohesion in the early reflow stage to suppress the upward escape of Flux vapor.

Stencil Apertures Must Match the Rheological Properties of the Solder Paste

We once used the same JL-808 solder paste under identical printing parameters to compare 0.12mm and 0.15mm thick laser-cut stencils. We found that the 0.12mm stencil exhibited obvious insufficient solder volume in the QFN 0.4mm pitch area, resulting in a void rate of 19.7% for edge solder joints after reflow. In contrast, the 0.15mm stencil paired with 25μm trapezoidal apertures reduced the void rate to 7.2%. This indicates that finer solder paste does not necessarily mean easier filling; the key lies in the dynamic adaptation between the geometric dimensions of the stencil apertures and the yield value of the solder paste.

Nitrogen Atmosphere Is Not a Universal Solution, But It Can Amplify Solder Paste Differences

When using JJY JL-808 solder paste in a nitrogen reflow oven with an oxygen content of 200ppm, the average void rate of BGA 0.8mm pitch devices was 3.8 percentage points lower than that in an air environment. However, after switching to a competing solder paste, the nitrogen benefit was only 1.1 percentage points. It can be seen that the formulation advantages of low-void solder paste can only be fully realized in a controlled oxidation environment; blindly adding nitrogen may instead mask the inherent defects of the solder paste.

The Reflow Heating Ramp Must Allow the Solder Paste to "Catch Its Breath"

After slowing the preheat ramp rate from 1.8°C/s to 1.2°C/s, the void morphology formed by JL-808 solder paste under QFNs changed from interconnected large cavities to isolated microvoids. This is because the slower heating allows the flux solvent to volatilize in stages, avoiding concentrated vaporization at the critical point of solder paste collapse that would rupture the molten solder film. This change is clearly visible in X-ray cross-sections and does not rely on any additional process compensation.

Cleaning Residues Can Conversely Degrade Void Performance

The production line once mistakenly used a water-based cleaning agent for PCBA cleaning after JL-808 solder paste soldering. As a result, ring-shaped microcracks were found at the solder ball interface during BGA device rework. EDS analysis confirmed that this was caused by the secondary reaction between organic amine salt residues in the cleaning agent and halogen activators in the solder paste. After switching to a no-clean process, void repeatability improved by 23%, indicating that the final performance of low-void solder paste is highly dependent on the compatibility of the supporting process chain.