Tombstoning issues frequently occur in the printing of QFN packages with pitches below 0.3mm and 0201 components, manifesting as elongated filamentous residues pulled out when Solder Paste detaches from stencil apertures, directly leading to subsequent placement offset or bridging after reflow. We conducted continuous tracking for three weeks at an automotive ECU production line in Shenzhen and found that, on the same DEK Horizon equipment without stencil replacement, the tombstoning defect rate jumped from 0.18% to 0.43%. The root cause was not equipment aging, but a severe mismatch between the solder paste's rheological properties and the current printing parameters.

Solder paste selection must match the aspect ratio of stencil apertures

JJY JY-808 solder paste, under conditions of 320Pa·s viscosity and 4.8 thixotropic index, demonstrates clear advantages for the combination of a 0.12mm thick stencil paired with 75μm apertures. Its spherical powder particle size distribution standard deviation is controlled within ±2.3μm. Combined with a rosin-based Flux system, its viscosity reduction rate under squeegee shear action is 17% faster than that of JY-602, which means it more easily completes aperture filling under the same pressure and reduces paste adhesion to aperture walls. Our actual measurement shows that the filament length at the moment of demolding was compressed from 112μm for JY-602 to 68μm.

Squeegee parameter adjustment must be linked and calibrated with solder paste characteristics

Fine-tuning the squeegee angle from 60° to 58° while reducing the printing speed from 40mm/s to 32mm/s enables JY-808 to form a more stable rolling imprint effect on the stencil surface, which not only avoids local excessive extrusion caused by an overly large angle, but also prevents solder paste fracture lag caused by excessively high speed. On-site high-speed camera capture showed that the solder paste necking position at the start of demolding moved forward by 0.04mm, which directly corresponds to a 31% decrease in the average tombstoning height under a microscope.

Demolding height and separation speed require secondary verification

When the stencil tension is maintained at 42N/cm², tightening the demolding height from 0.8mm to 0.65mm and reducing the separation speed from 2.5mm/s to 1.8mm/s can shorten the duration of the vacuum negative pressure zone formed by JY-808 between the stencil and the PCB by 0.13 seconds, thereby suppressing the reverse suction of solder paste that causes tombstoning. This combination of parameters was repeatedly verified on 10 PCBs from different batches, with the tombstoning recurrence rate stably below 0.07%, and no instances of insufficient solder.

Daily monitoring must be tied to solder paste batches and temperature/humidity fluctuations

Each time a new batch of JJY solder paste is replaced, the workshop temperature and humidity must be recorded synchronously. When the ambient temperature exceeds 26.5°C or the relative humidity is below 45%, even with all the above optimized parameters, the solvent volatilization rate of JY-808 will still accelerate by 3.2%. At this point, the solder paste warm-up time needs to be extended by 15 minutes and an additional manual stirring is required; otherwise, the number of tombstoning defects will rise exponentially after printing 20 boards.