Fan-out interposers convert fine pitch BGAs to a larger pitch
eliminating the additional costs associated with microvias and underfilling on the PCB.

When a fine pitch BGA is driving your design to additional layers, finer line & space,
or microvias, a cost analysis will determine if a ‘standard’ PCB with an interposer
is more cost effective than a higher technology PCB without an interposer.

For example, the best IC for your new system is primarily used for cell phones
and only available in a 0.5mm pitch uBGA package. This single component would force
your motherboard into microvia technology.

Also, IC manufacturers recommend underfill, which is a non-standard manufacturing process.

Fan Out Interposers allow the designer to continue using the optimum IC for the design,
the fine pitch BGAs, and reduce overall system costs.

For example, utilizing a fan-out interposer, a 0.5mm pitch Micro BGA can be converted
to a much more usable 1mm pitch, which falls in line with standard design rules
providing a more cost-effective solution.

The underfill process is much more efficient at the interposer assembly level
rather than on a manufacturing line configured for traditional PCB assembly.

Reduces motherboard complexity:

Fan-Out Interposer White Paper.PDF
BGA Adapters