The use of advanced solder ball interconnect designs for board-to-board applications enables integrators to overcome co-planarity mismatch issues and greatly improve solderability across a wider process window. 
In PCBA assembly process when SMD device or IC is placed on a flat surface it will rest on its three lowest points known as seating plane of the device. During reflow, the surface tension properties of liquid solder cause the solder to wet between the pin and pad. The solder bridges any physical gap between them to form a fillet. If the co-planarity mismatch is too great for solder deposited, some pins may not make contact to form an acceptable solder fillet, resulting in an electrical open circuit. Please refer www.interplex.com for more information on this subject.
The best approach is to get the correct amount of solder where needed. Interplex Solder ball addresses these issues by incorporating a precise amount of solder directly on to the interconnects. The solder ball design consists of an inner contact of copper surrounded by an outer ring that assists in automated pick-and-place as well as seating and standoff distance. The module-end of the solder ball interconnect can be either through-hole or SMT.
The solder ball is formed on the tip of the inner contact to provide the subsequent interface to the mother-board. During reflow, the solder ball interfaces with the wet paste that is on the PCB, which 
puts the solder ball into a molten state and allows the module to settle on to the motherboard as the module’s weight exceeds the buoyancy of the molten solder. This process automatically compensates for co-planarity variances. Each solder ball can provide automatic adjustment flexibility that is equal to the distance by which the diameter of the solder ball extends beyond the end of the pin inside the ball.
Please refer www.interplex.com for more information on this subject.
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It is the small changes that make the largest changes.
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