The ball mounting process is extremely sensitive to the rheological properties, metal content and particle size distribution of Solder Paste. T5 (average particle size 5-12μm) and T6 (average particle size 3-8μm) ultra-fine solder pastes have shown obvious advantages in actual measurement on Jia Jinyuan's production lines. With high proportion of fine powder and narrow particle size distribution, combined with a Flux system with low thixotropic index, they can quickly release stress under the shear action of the squeegee, enabling the solder paste to fill the stencil openings fully and release smoothly, thus providing a stable basic form for subsequent solder ball formation.

The stencil opening design must match the physical properties of T5/T6 Solder Paste

We found that when using T6 solder paste, if the 1:1.2 opening width-to-thickness ratio corresponding to T4 Solder Paste is still used, the solder paste residue rate will increase and edge trailing will worsen. This is because T6 solder paste has lower viscosity and finer particles, requiring the width-to-thickness ratio to be tightened to 1:1.05 and the stencil thickness to be reduced to 80μm simultaneously, to suppress the collapse trend while ensuring the release rate. T5 Solder Paste can achieve better aspect ratio compatibility under a 90μm stencil. In actual debugging, the volume variation coefficient CV < 8% detected by SPI must be used as the judgment benchmark.

The heating rate during the reflow phase directly affects the roundness of solder balls

After adopting T5/T6 solder paste, the reflow oven chain speed and temperature zone settings must be recalibrated. If the heating slope of the first temperature zone exceeds 1.8℃/s, the intensified oxidation on the fine powder surface will lead to uneven liquid spreading, and the solder balls are prone to eccentricity or bimodal phenomenon. If the holding temperature is lower than 150℃ and the duration is shorter than 60 seconds, the flux activity cannot be fully activated, which will cause solder ball coalescence or size dispersion. Jia Jinyuan's on-site verification shows that holding at 155℃ for 75 seconds with a peak temperature of 235℃ is the optimal window.

Fluctuations in ambient temperature and humidity will amplify the edge collapse risk of T6 solder paste

When the workshop relative humidity exceeds 65%, the volatilization of the solvent on the surface layer of T6 solder paste slows down, and microscopic flow is prone to occur while the solder paste stays on the stencil. At this time, even if the printing parameters remain unchanged, AOI will frequently report excessive solder ball diameter and edge burrs. Therefore, the ambient humidity must be locked at 45%±5%, and the solder paste must be put into use within 2 hours after opening, to avoid rheological performance degradation caused by moisture absorption. Although T5 solder paste has slightly stronger moisture resistance, it still needs to implement this control measure synchronously.

The adaptability of the cleaning process determines the long-term stability of ball mounting yield

Micron-scale metal particles in the ultra-fine solder paste residue are more likely to embed into the stencil hole walls. If the cleaning agent has insufficient dissolving power for rosin-based resin or the spray pressure is lower than 0.25MPa, opening blockage will occur after three prints, causing local solder shortage. Jia Jinyuan recommends using a composite water-based cleaning solution containing 20% isopropanol, and extending the cleaning cycle from every 10 pieces to every 6 pieces, to maintain the opening permeability of T5/T6 solder paste during continuous operation.