Head-in-pillow defects related to solder paste or flux performance are classified as Materials Issues. These include poor transfer efficiency on standard apertures, insufficient wetting (fluxing) capacity, low oxidation barrier and low activity. The key to over coming head-in-pillow defects is to get each component sphere to contact, and stay in contact, with the soldering material, mainly the solder paste. If the solder paste itself has poor or inconsistent transfer efficiency, then how do we know that there is even going to be contact between the sphere and the paste? Low area ratios can account for lot of the transfer issues, especially if the stencils are not electro-polished or Electro-formed (e-fab), but with that said, you must match the material set to the process and stencil design. The solder paste can cover a lot of overlap. The second half of the solder paste equation is the fluxing action. There are three parts to this; activation, oxidation barrier and stencil / tack life. High activation is an obvious choice because this is the working part of the flux, which removes the oxides from the solder and the spheres. Oxidation barriers, such as a higher rosin content of the paste's flux, are useful because it will protect the alloy from forming new oxide, which means there's more activation for the component's oxide. Also, it usually adds tack, which is a huge benefit. Because if the paste stays tacky, and the package does warp, the paste will stretch to provide a continuum, so the solder and component will be a single alloy mass upon reflow. There are artificial ways to add an oxidation barrier and additional activation, such as nitrogen reflow or a flux / paste dipping process. Nitrogen reflow PREVENTS the formation of additional oxides during the reflow process, but does not REMOVE oxides and hydroxides already formed on the components. Flux or paste dipping are viable options because this adds activation directly on the component, rather than leaving it to chance on the board. Plus, this flux or paste can be used for rework on the back-end as well. Of course, Material solutions can overcome both the Supply and Process Issues. More information may be found at Online Help: Indium Knowledge Base.