Package on Package or PoP Gaining Greater Ground
The new era of package on package or PoP technology ushers in new and different ways of dealing with sub-assembly PCB design and assembly. By not abiding by these new and evolving design guidelines and assembly procedures, chances are pretty high an OEM’s product can significantly fall short of its performance objectives.
Maintaining high reliability for BGA-based PoP board design starts at the trace level. A good guideline to follow is to have only one trace between two BGA pads and not two because two traces pose manufacturing difficulties. Also, board fabrication challenges arise when running multiple and thin 2 to 3 mil traces between two BGA pads. Further issues surface if the length of those thin traces is exceedingly long, and those types of traces are spread throughout the board.
Ideal traces are 5 mils or thereabouts. Generally, they are highly acceptable as good manufacturing practice. However, in today’s world of miniaturization, those 5 mil traces may not be practical or possible. In the small PCB world requiring PoP, high reliability and excellent signal integrity demand that a single trace be no more than 2.5 to 3 mils between BGA pads, as shown in Fig. 1.