1. The Solder Paste model must strictly match the BGA solder ball diameter. For example, when using φ0.3mm solder balls, if a high-activity, low-viscosity RMA type solder paste is selected, collapse or deviation is highly likely to occur before reflow. In contrast, matching with medium-activity, good thixotropy no-clean solder paste can maintain stable post-printing profile, while balancing oxide film removal capability and residual controllability.

2. The stencil thickness is usually selected as 0.12mm or 0.15mm. When the BGA pad pitch is less than 0.8mm, a laser-etched + electropolished stencil must be used; otherwise, edge burrs will cause lateral solder paste trailing, which in turn leads to adjacent solder ball bridging. Especially on 0.4mm pitch CSP devices, the recurrence rate of this problem reaches over 73%.

3. The aperture size is set to 92% to 96% of the pad diameter. If the upper limit value of 96% is uniformly adopted, it can improve the primary ball mounting yield when the pad surface oxidation is slight and the PCB flatness is better than ±25μm. However, for boards with uneven electroless nickel immersion gold layer thickness or FR4 board warpage >0.75mm, local excess solder paste will cause sphere fusion or increased void rate.

4. The squeegee angle is set between 55° and 65°. An excessively small angle will result in insufficient solder paste filling, while an excessively large angle will aggravate stencil wear and induce solder paste extrusion splashing. We once experienced consecutive three batches of BGA ball mounting cold solder joints on a certain medical equipment production line due to long-term use of a 68° squeegee; after replacing it with a 62° squeegee, the yield rebounded from 89.3% to 99.1%.

5. The printing speed is controlled within the range of 25mm/s to 40mm/s. A speed lower than 25mm/s easily causes the solder paste to stay in the stencil aperture for too long and dry out and form a skin, while a speed higher than 40mm/s leads to a decrease in solder paste transfer rate. Especially in winter workshops with ambient humidity lower than 30%RH, a solder paste constant temperature and humidity box must be activated simultaneously and the single-board printing interval must be shortened.

6. The heating rate in the Reflow Soldering preheating zone should be maintained at 1.0℃/s to 1.5℃/s. A rate lower than 0.8℃/s will cause the rosin component in the Flux to carbonize in advance, while a rate higher than 1.8℃/s will cause the metal particles in the solder paste to enter the soaking stage before they have time to wet the pads, ultimately resulting in poor solder ball sphericity and insufficient solder climb height.

7. The soaking zone temperature is set between 150℃ and 165℃, and the time is controlled between 90s and 120s. An excessively short time will cause the organic carrier to not fully decompose and remain at the interface, while an excessively long time will accelerate the oxidation of the pad nickel layer. When we tested different Ni/Au thickness combinations, we found that when the nickel thickness was <120μin and the soaking time exceeded 135s, the probability of discontinuous IMC layer growth increased by 41%.

8. The reflow peak temperature must be 20℃ to 25℃ higher than the liquidus of the solder paste used. For example, the liquidus of SAC305 solder paste is 217℃, so the peak temperature must be set to 237℃ to 242℃. If it is only set to 230℃, although thermal stress can be reduced, the solder ball wetting angle is generally >45°, and X-ray inspection shows that the void rate exceeds the standard by 58%.

9. The residence time in the reflow zone must be guaranteed to be 45s to 65s. Insufficient time will cause the internal Sn-Pb eutectic structure of the solder ball to not be completely rearranged, while excessively long time will aggravate the precipitation of brittle Cu6Sn5 phases. We found in the rework of a certain base station main control BGA that the fracture rate of solder joints with a 72s post-peak residence time was 3.2 times higher than that of the standard 60s group after 500 cycles of -40℃ thermal shock.

10. The cooling rate should be controlled at 2.5℃/s to 4.0℃/s. A rate lower than 2.0℃/s easily forms a coarse grain structure and reduces fatigue resistance, while a rate higher than 4.5℃/s may induce solder ball microcracks. An automotive electronics customer once adjusted the cooling section fan air pressure from 0.35MPa to 0.42MPa for this reason, increasing the thermal cycle life of BGA solder joints to over 1200 cycles.

11. Reflow must be completed within 4 hours after ball mounting. For boards that have not been processed for more than 6 hours, the stencil must be re-cleaned and solder paste must be re-printed; otherwise, solvent volatilization in the solder paste will cause viscosity to rise, printing release force to increase, and the measured release failure rate will soar from the normal 3% to 27%.

12. It is strictly prohibited to mix BGA devices with different solder ball diameters in the same reflow oven. For example, if φ0.3mm and φ0.5mm solder balls are soldered in the same oven, the small balls will melt and collapse in advance due to low heat capacity, while the large balls have not been completely wetted, ultimately resulting in a solder joint height dispersion of the entire board >35μm and an AOI false judgment rate increased to 19%.