The voiding issue on the bottom thermal pad of QFN packages is never an isolated defect, but rather the result of coupled out-of-control conditions involving Solder Paste printing thickness, oxygen content in the reflow atmosphere, and PCB pad surface finish. During the introduction of JJY SG-880 solder paste, we found that when the stencil aperture area ratio was increased from 85% to 92%, although the printing release rate improved by 3.2%, the void rate instead surged from 11.7% to 23.4%, indicating that simply enlarging the aperture does not solve the root problem.

Stencil thickness and squeegee pressure must be calibrated synchronously

The average solder paste volume printed with a 0.12mm stencil at a 35° squeegee angle and 0.45MPa pressure is 0.028mm³, while the volume jumps to 0.041mm³ with a 0.15mm stencil under the same parameters. However, if the squeegee pressure is not simultaneously reduced to 0.38MPa, the solder paste will be excessively squeezed and flow to the outer side of the pad, blocking gas escape channels during reflow. This increases the probability of voids concentrated in the central area of the pad with sizes exceeding 80μm by 4.6 times.

Reflow nitrogen purity is strongly correlated with the solder paste activity window

The Flux activation onset temperature of JJY SG-880 solder paste is 152℃. When the nitrogen dew point inside the furnace is higher than -40℃, residual moisture reacts with the flux during the 180℃ soak zone to generate hydrogen bubbles. These bubbles cannot be completely expelled before the 225℃ peak temperature, and are eventually encapsulated by solidified solder to form regular circular voids. After stably controlling the nitrogen dew point below -45℃, the void area ratio of the same LOT of solder paste decreased from 14.3% to 6.8%.

Deviation in PCB pad OSP film thickness directly amplifies solder paste volume errors

When the OSP film thickness fluctuates from 0.3μm to 0.55μm, the wetting and spreading speed of the solder paste in the early reflow stage decreases by 37%, causing the actual coverage area of the same volume of solder paste to shrink by 19%. This physical shrinkage forces molten solder to pile up at the pad edge and block gas escape paths. In one project, after we tightened the OSP film thickness control to 0.32±0.03μm, no single-point voids larger than 120μm appeared even when using a 0.15mm stencil.

Stencil cleaning frequency affects void consistency more than solder paste batch

After 8 hours of continuous printing, flux residue accumulated at the bottom of the stencil apertures reduces the actual opening area by 11%. At this point, even if a new batch of JJY solder paste is replaced, the void rate is still 5.2 percentage points higher than that of the first article. Therefore, we have mandated that the bottom of the stencil be wiped every 2.5 hours with a non-woven cloth dipped in JJY special cleaning agent W-22, and that the opening status of 3 typical pads be spot-checked under a 200x microscope.