In essence, most of the voids in SMT solder joints are formed because the flux volatiles trapped by the molten solder joints during the reflow soldering process do not have enough time to discharge in time.
Under normal circumstances, the flux in the BGA solder paste will be driven out by the "polymerization force" of the molten solder during reflow soldering. If the flux is still trapped during the solidification of the molten solder, bubbles can be formed. If the formed bubbles do not escape in time, a void will form after the solder joint solidifies, as shown in the figure below.
Under what circumstances is it easy to retain the flux?
1) In the first case, the oxidation surface is seriously oxidized
If an oxide film exists on the surface of the soldered surface or the solder powder particles, on the one hand, it will hinder the wetting of the molten solder, and on the other hand, the flux will adhere. In essence, it acts as a "empty seed."
2) In the second case, the boiling point of the solder paste flux solvent is relatively low
Volatile fluxes tend to produce highly viscous flux residues, and it is difficult to remove these flux residues from the molten solder, which tends to form voids. The more volatile the solvent, the easier it is to “resist” the flux residue and the easier the solder joint is.
3) In the third case, the escape of flux volatiles in BGA solder joint solder paste is not smooth
The escape of volatile gases is related to the escape path or resistance around the solder joint. Like QFN and LGA packages, the distance between component package and PCB is very small. It is usually blocked by liquid flux residue and cannot be completely removed. It is difficult for the flux to escape and it is easy to form voids. This is also the reason why solder joints such as QFN are prone to voids.
If the exhaust passage is not smooth, there will be a lot of flux residue, so the effect of the exhaust passage on the void is essentially due to the amount of flux remaining.