In our circuit assembly production line, process defects that occur after wave soldering operations are common. The defects include insufficient fill of the PTH and large voids within plated-through holes. There are several things that could cause insufficient fill of the PTH.
Insufficient flux coverage: Check your flux application to be sure that there is sufficient volume and area coverage over the complete PCB.
Insufficient pre-heat: Solder flows to the heat. If the PTH's have not reached the exact temperature, the solder will not flow as well, while an exact temperature depends on your operation. As a rule of thumb, the assembly should reach approximately 150C to 170C just before contacting the wave.
Contamination: Probably the most common cause of insufficient solder coverage is contamination. Verify that the boards have been handled properly before the assembly process and during the process.
Any oxidation, oils, etc., can cause solder problems. Clean several boards and then put them through your assembly process. Make sure the circuit boards are out of the sealed packages for a minimal amount of time before they are processed.