Sep 27, 2010 
As mobile and other electronics devices become smaller with more sophisticated functions, the mounting technologies for compact and high-density packages have advanced dramatically. Recently, Package on Package (PoP) technology has increased in use in the industry.
In the conventional PoP manufacturing process, the solder bumps of the top package are immersed into a layer of tacky flux. Once the flux immersion is completed, the top package is mounted on the bottom package and soldered together. One of the challenges of PoP manufacturing is component warpage during the reflow process that led to the incomplete merger of the solder bump from the top package with the solder land on the bottom package. Once the solder bumps of the top PoP component separate from the land of the bottom component, even if they come in contact again, the tacky flux activity is reduced, leading to an incomplete merger.
Tests were conducted on lead-free PoP solder pastes with adjustments in the flux thixotropic agents, activators and solvents, in addition to the actual flux content used in the solder paste. Various paste formulations were investigated to achieve good paste transfer onto the PoP solder spheres while ensuring that the stability of the paste over time during the paste application operations was maintained. Solder paste powder size and reflow atmosphere also were investigated.
