Voids develop through contraction during the cool down phase of molten metal. In a solder joint, it is usual that two solidification fronts are formed. One front originates from the pcb, the other from the component. In between these two fronts, the solder is the longest in the molten stage. During solidification of the fronts, the solder contracts, thereby pulling material away from the still liquid area. If the surface area of the liquid portion has solidified, further cooling of the inside of the joint will give rise to voids, also referred to as blowholes. During solidification of tin/silver/copper alloys, the blowholes frequently reach up to the surface of the fillet. During visual inspection, this may be, erroneously, recognized as a crack.