I have recieved a number of inquiries over the years regarding the Pack-on-Package (PoP) Manufacturing Processes. One of the most frequent questions that I have recieved is whether to use a PoP flux or a PoP solder paste.
Most people use a PoP flux because it is a much easier process to set-up and maintain. However, there are advantages to using PoP solder paste as well. When using components that exhibit high warpage levels, PoP solder paste will allow you to bridge the gap between the top and the bottom component while the warpage is occuring. Flux may be able to bridge the gap; however, there is no powder alloy present to create an alloy gap filler if the solder bump on the top package reaches the solidus point before the component warps back to the original level and makes contact with the bottom package. his would result in a "Snowman" Defect because there is not alloy present to bridge the gap. The Snowman Defect is another warpage-induced defect similar to Head-in-Pillow (HiP) or Non-Wet-Open (NWO), but there is no paste printed on the bottom substrate.
The PoP solder paste dipping process takes far more time and effort to set-up and maintain. For example, with paste, you have to make sure the dipping dwell time and dip depth is optimized perfectly for the component that you are dipping. If too much PoP solder paste is applied to the bottom of the component (component bumps), bridging can occur even before the component is placed on the bottom package. If too little PoP solder paste is present, there may not be enough flux there to create a proper solder joint.
The PoP Flux process is far more forgiving and presents a wider process window, as long as component warpage isn’t a concern.
I will talk more about the PoP dipping process set-up, optimization, and maintenance in future blog posts.