Voiding is a phenomenon primarily caused by encapsulated out-gassing of solvents.

The voids are often located at non-wetted spots between the solder joint and the pad or component lead. But also flux can be encapsulated in the middle of the solder joint.

Cause:
A poor reflow soldering profile often cause solder joint voiding.
An inadequate period of time in the profile preventing the solder paste solvents to evaporate and the flux to burn off, tend to trap these gasses within the solder joint.

PCB's showing poor solderability can increase voiding while the gases tend to get stuck in poor solderable spots.

Solution:
V oiding can be minimized by increasing preheat time, decrease the preheat ramp-up rate and shortening of the soldering time.

The use of nitrogen in the reflow process can also minimise voiding.

Repair:
Since v oiding cannot be seen from the outside of the solder joints but only is found using an X-ray microscope or by destructive micro-sectioning it is a difficult and costly failure to find.
Solder joints containing small voids do not require any repair. And if the PCB suffer from massive voiding the cost of repair will in most cases be to high.

However, if only a few solder joints, found in X-ray, contain large voids, repair should be done.
The defective solder joint are removed using a professional soldering iron. To remove the solder, add a little flux to the solder joint, place a de-soldering braid on the solder joint, place the soldering iron tip on top and heat-up the solder alloy until the solder is melted into the braid and lift. Repeat until all solder is removed. Now, re-solder the component terminal using solder wire. Clean if necessary using isopropanol, a cotton bud or brush and lint free cloth until all flux is removed.

Note, that each time a solder joint is re-soldered the intermetallic layer will increase in thickness and thereby shorten the joints lifetime.