Growing interest in smaller and more functional handheld devices in applications ranging from point-of-sale devices to military communications systems creates demand for more heavily populated PCBs form the OEM.
A response to this has been a technique called package-on-package (PoP) which allows board manufacturers to build up rather than out.
PoP technology involves stacking semiconductor components on top of each other, rather than the traditional 
method of placing them side by side on the board, thereby reducing circuitry and board space required by up to 43%.
PoP assembly techniques are well illustrated by the example of the Texas Instruments OMAP microprocessor, which is available in configurations to support the use of PoP technology, using components stacked vertically and topped with a memory chip.
Broader applications for PoP technology see it enabling high performance system-on-module 
units containing processor and memory subsystems, input output units, and communications.
These compact, fully-contained processing engines can be plugged into larger systems and have the advantage of modularity and scalability.
To support PoP technology it is necessary for the manufacturer to characterise the assembly process by experimenting with a variety of different solder pastes and tack fluxes to bond the stacked components together, as well as soldering the base package to the PCB and have settled on a solderpaste printed base package followed by a solderpaste dipped upper package; before reflowing the entire assembly to achieve the finished solder joint connections.
It is also necessary to refine the placement method for stacking the components.
At present we are working with two stacked components but, as the technology develops further we foresee customers asking for three or four components stacked vertically, something Japanese manufacturers are already developing.
Author is Gareth Beckett, senior medical account manager at Axiom Manufacturing Services
See more at: Southern Manufacturing 2010 February 10th & 11th, stand F18, FIVE, Farnborough