Choose the right flux. There are many reasons for welding balls, but flux is the main reason.
Generally speaking, fluxes with low solid content tend to form solder balls when the underside SMD elements require double PCBA wave soldering. Because these additives are not designed to be used for long periods of time. If the flux sprayed on the PCB has been used up after the first wave crest, there is no flux after two peaks. Thus, it cannot play the function of flux and help reduce tin balls.
One of the main ways to reduce the number of welding balls is to choose a flux that can withstand longer periods of heat.