When reballing BGA chips for rework soldering, if ordinary Solder Paste is used, large-area voids are often revealed during X-ray inspection, especially concentrated in the central area of the solder balls. This is jointly caused by obstructed volatilization paths of the Flux in traditional solder paste, uneven packing density of metal powder, and limited gas escape channels during the reflow process. In contrast, JJY JGY-850 low-void solder paste optimizes the rosin resin ratio and the particle size distribution of micron-sized spherical tin powder, enabling staged and stable flux decomposition, controllable solder paste slump behavior, and sufficient wetting and spreading of liquid metal at a peak temperature of 245°C, thereby greatly reducing the probability of bubble retention.

Our production line tests on reworking a domestic 676-pin BGA chip found that after reballing with conventional solder paste and standard nitrogen reflow, the average void rate of solder joints reached 18.7%, with 43% of joints having a void proportion exceeding 15%. After switching to JJY JGY-850 solder paste and synchronously adjusting the squeegee pressure to 0.35MPa and the printing speed to 25mm/s, the void rate in the same furnace batch dropped to 7.2%, and 92% of the solder joints had void sizes less than 10% of the diameter of a single solder ball. This shows that the matching degree between the rheological properties of the solder paste itself and the process window directly determines the reliability boundary of rework soldering.

The selection of stencil thickness must be considered in coordination with the metal content and particle size of the solder paste. For example, when using a 0.12mm laser-cut stencil for 0.3mm pitch BGA, if the proportion of Sn96.5Ag3.0Cu0.5 alloy in the solder paste is lower than 88.5%, problems such as poor printing release and poor consistency of solder ball height are highly likely to occur. The measured metal content of JJY JGY-850 solder paste is 89.2±0.3%. With the 0.12mm stencil opening size designed to be 0.8 times the solder ball diameter, the solder paste volume deviation after reballing can be controlled within ±4.5%, which provides double protection for suppressing solder bridging and voids during subsequent reflow.

The oxygen concentration in a nitrogen atmosphere must be continuously monitored and maintained below 100ppm. Otherwise, even if low-void solder paste is used, its surfactants will still undergo side reactions with residual oxygen at high temperatures to form non-volatile oxides, which in turn hinder the complete wetting of the solder paste on the pad and solder ball interface. We once observed an abnormal rebound of the void rate to 13.6% in a batch where equipment nitrogen flow fluctuation caused the oxygen concentration to rise to 320ppm. This phenomenon immediately fell back to 7.4% after the gas supply stability was restored, which confirms that the performance of solder paste must be supported by a controllable process environment.

The accumulation of thermal stress during rework soldering cannot be ignored. If the same BGA chip undergoes more than three reballing + reflow operations, microcracks are prone to occur at the interface between the pad copper foil and the PCB substrate. At this time, the dense solder joints formed by low-void solder paste will instead amplify the stress concentration effect. Therefore, we strictly stipulate that the rework of a single chip shall not exceed twice, and before the second rework, a flying probe test shall be used to verify whether the attenuation of pad adhesion exceeds 17% of the initial value. Only when it meets the standard is it allowed to continue using JJY JGY-850 solder paste for reballing operations.