Thermal damage to BGA chip bodies during the ball mounting process has long been a yield bottleneck, especially in fine-pitch packages below 0.4mm pitch. Traditional Sn96.5/Ag3.0/Cu0.5 Solder Paste requires a peak temperature above 230°C to achieve reliable reflow, while the CTE mismatch between the chip molding compound and substrate causes thermal expansion incompatibility. If combined with multiple rework cycles, invisible microcracks are highly likely to form at the edge of the silicon wafer. Such defects often only surface after aging testing, directly extending the customer complaint cycle.

Low-temperature solder paste is not simply about lowering the melting point, but reconstructing the metallurgical reaction window

The active system of JJY LTS-215 solder paste has been verified through three production line iterations, ensuring sufficient precipitation of the SnBiAg ternary eutectic phase at a peak temperature of 185±3°C. Its Flux volatilization curve and heating ramp rate are strictly matched to the oxygen content decay rhythm under a nitrogen atmosphere, which not only avoids bridging but also prevents sphere oxidation and blackening. After a certain customer replaced their original high-temperature solution with this solder paste in CSP ball mounting, the void rate detected by X-ray decreased from 12.7% to 3.1%, and the AOI false alarm rate dropped by 40% simultaneously.

Squeegee pressure and release speed must be adjusted in tandem

The viscosity of low-temperature solder paste is more sensitive to temperature. If the original 2.5kgf squeegee pressure is used with a 15mm/s release speed, trailing solder paste is prone to remain at the stencil aperture, leading to post-ball-mount sphere diameter dispersion exceeding ±0.04mm. By reducing the squeegee pressure to 1.8kgf, increasing the release speed to 22mm/s, and simultaneously fine-tuning the stencil tension from 38N/cm² to 42N/cm², we achieved more uniform shear flow of the solder paste within the apertures, with the measured ball height CV value stably controlled within 5.3%.

Stencil aperture design must inversely match the rheological properties of the solder paste

The thixotropic index of JJY LTS series solder paste is 4.8, far higher than the 3.2 of conventional solder paste. This means it has high structural strength at rest and is prone to collapse during printing. Therefore, inverted trapezoidal apertures must be used for 0.3mm diameter ball mounting, with an upper aperture of 0.32mm, a lower aperture of 0.28mm, and a taper angle controlled at 8°±0.5°. Otherwise, after 200 consecutive prints, more than 3 spheres will shift on the 198th board, with the shift concentrated in the 15~22μm range, which falls exactly at the edge of the AOI recognition threshold.

There is a strong coupling relationship between reflow oven chain speed and nitrogen purity

When the oven temperature is set to a 185°C peak, if the nitrogen purity is below 99.992%, residual oxygen will accelerate the surface oxidation of Bi elements, causing the sphere surface roughness Ra value to suddenly increase above 0.85μm. At this point, even reducing the chain speed from 120cm/min to 90cm/min cannot improve the situation. It is necessary to increase the nitrogen purity to 99.995% and simultaneously extend the soaking zone time by 8 seconds to ensure sufficient atomic diffusion at the Sn-Bi interface, keeping the shear strength fluctuating around the 32MPa baseline by no more than 1.2MPa.