When performing BGA repair, we never select Solder Paste Flux based on the activity level marked on the packaging. Instead, we first disassemble the faulty board to confirm the oxidation level of the chip solder balls, whether the PCB pads have suffered OSP film failure due to repeated thermal shock, and the type of previous soldering residue, then make a comprehensive judgment combined with the heating uniformity and nitrogen protection capability of the rework equipment. For example, for the same Xilinx Kintex-7 chip, rework in the high-humidity environment of Shenzhen in summer is more inclined to use RMA-2 grade Flux with a slightly higher rosin ratio than in dry workshops in northern China, because its activation temperature window is wider and its tolerance to moisture is slightly better.

The essence of flux activity is the reducing ability of acidic components to metal oxides at a specific temperature, but what really works in BGA soldering is the active layer less than 5 microns thick on the surface of the molten Solder Paste. Therefore, controlling the squeegee printing thickness at 120±10μm, designing the stencil opening to 0.85 times the solder ball diameter, and stabilizing the reflow peak temperature at 235±3℃ are the prerequisites for the activity to be exerted. Otherwise, no matter how high the activity level is, it will volatilize completely before the solder is fully melted, which will instead cause bridging or an increase in void rate.

JJY's production line statistics over the past three years of BGA rework failure cases found that 67% of the problem root causes are not the flux activity itself, but the mismatch between the flux and the solder paste alloy system. For example, using SAC305 Solder Paste with RA type flux with excessive chloride ions will lead to obvious abnormal thickening of the IMC layer at the solder joint interface after reflow, and X-ray inspection shows that voids are concentrated in the 1/3 area at the bottom of the solder joint. After switching to halogen-free RMA-1.5 grade flux, the void rate dropped from 22% to below 8% under the same process parameters.

In actual operation, we require that the flux must be used up within 24 hours after opening, the storage environment temperature is strictly controlled at 22±2℃, and the humidity is 45±5%RH. Before each use, stir slowly along the container wall with a squeegee for 30 seconds to ensure that the resin and activator are re-homogenized. If the paste becomes stringy or its thixotropy decreases, stop using it immediately, because this indicates that the organic acid has partially decomposed. At this time, even if the nominal activity remains unchanged, the actual activation efficiency near the 183℃ eutectic point has attenuated by more than 40%.

The test of BGA soldering on flux is far more than the activity value. The thermal decomposition temperature of its carrier resin must be at least 15℃ higher than the reflow peak temperature, otherwise residual carbides will be embedded in the solder joint to form stress concentration points. We once encountered a domestic flux with a nominal RMA-2 grade, but thermogravimetric analysis showed that its resin began to lose weight at 220℃, which led to a 31% decrease in solder joint shear force during the high-temperature aging test after Intel CPU BGA rework. The problem was completely solved after switching to an imported RMA-1.8 grade product with a thermal decomposition onset temperature of 252℃.