Once the bottom circuit board has been designed, the assembly guidelines for package-on-package (PoP) versions of the OMAP35xx processor and the memory device must be considered.
PoP packages have an enormous number of variables associated with assembly. The following factors have a major effect on the quality and reliability of final assembly: the circuit board design, the solder paste characteristics, solder paste deposition, the reflow profile, and the fluxing or solder paste deposition onto the PoP memory prior to assembly.
There are two major techniques for mounting the PoP memory: one pass and two pass. Although both are discussed, the one pass process is recommended as it is the most economical and provides about the same assembly yields as the two pass.
The BeagleBoard, discussed in the companion article to this document, PCB Design Guidelines for 0.4mm Package-On-Package (PoP) Packages (SPRAAV1) - which will be referred to as Part I throughout the reminder of this document - forms the basis for the understanding of 0.4mm assembly guidelines and process parameters. This application report describes only one of many processes. Your assembly acility will likely have others; it cannot cover all of the possible variations. It is strongly recommended that you perform your own trial runs and studies in conjunction with your assembly house to optimize the PoP assembly process.