The excess solder issue on BGA bottom pads is not an isolated phenomenon, but the combined result of out-of-control Solder Paste printing parameters, improper stencil aperture design, and squeegee pressure fluctuations. During continuous tracking of 12 batches of 0.4mm pitch BGA devices on the Jia Jinyuan production line, we found that when the measured solder paste thickness reached 0.15mm, X-ray inspection showed voids concentrated in the central area of solder joints in an irregular agglomerated shape; when the thickness was controlled within the 0.12–0.13mm range, void distribution was significantly sparser and the size was more uniform.

Stencil apertures are strongly correlated with solder paste volume

When the stencil thickness is 0.12mm and a 1:1 aperture ratio is used, combined with the thixotropic index of 3.8 of Jia Jinyuan SN63-Pb solder paste, the actual deposited volume is already approaching the critical upper limit. At this point, if the squeegee angle drops from 60° to 55° or the printing pressure increases by 0.2MPa, the solder paste transfer rate will rise by more than 9%, directly causing local accumulation. During the initial melting stage of reflow, this accumulation blocks the vertical escape path for Flux decomposition gases, forcing the gases to squeeze horizontally and become trapped between the solder and pad interface.

Void location is highly coupled with the degree of solder paste excess

After performing layered scans on 128 solder joints of the same BGA using microfocus CT, we confirmed that among the 24 solder joints with the most severe solder paste excess, 21 voids were located at the solder/pad interface rather than the solder/solder ball interface. This indicates that the gas does not come from the oxide layer of the solder ball itself, but is trapped at the bottom by the excessively thick solder paste layer. Moreover, the void aspect ratio is generally greater than 3.5, exhibiting a drawn-out filamentous extension characteristic. This morphology is explicitly classified as a process-induced high-risk void in IPC-J-STD-005.

The reflow profile cannot compensate for solder paste excess defects

Some attempts have been made to extend the soak zone time to 90 seconds to promote gas release, but the measured results instead increased the void rate by 4.1 percentage points. The reason is that the excess solder paste delays the heating rate of the bottom pad, causing the flux at the interface to carbonize and form a crust prematurely, further locking the gas channels. After Jia Jinyuan switched to a nitrogen atmosphere on site, the void improvement was limited, confirming that the root cause of the problem is not the oxygen content but the physical accumulation morphology of the solder paste itself.

Verification methods must include quantitative benchmarks

Daily inspections cannot rely solely on visual judgment of solder paste fullness; SPI equipment must be used to perform zoned statistics on each BGA area. When the median solder paste volume in a certain area exceeds 115% of the theoretical value and the standard deviation is greater than 8.2%, the line must be immediately stopped to calibrate squeegee parallelism and stencil tension. We once ignored this combined criterion, causing the void defect rate to climb from 1.7% to 6.9% during continuous production, and the rework cost far exceeded the loss of downtime for troubleshooting.