Reprofiling may be the best containment method for head-on-pillow.

• The solder paste deposit on the board melts and fuses in the reflow oven.
• The solder sphere on the BGA also melts in the reflow oven.
• The sphere material and solder deposit do not fuse to become one contiguous mass.

• Higher surface area:volume ratios of both the solder deposits and the spheres. This ratio is an important factor in fine feature soldering because solder material surfaces exposed to air atmospheres oxidize readily, while the solder masses’ interior matters are more protected. As the size of a solder sphere decreases, the surface area:volume ratio increases. The smaller the solder sphere or paste deposit, the higher the surface area:volume ratio, and the greater the ratio of oxidizable-to-protected solder. As the solder oxidizes under reflow, it consumes flux, flux that will be needed to help aid in wetting when the metals reach liquidus.
  
  Although the assembler cannot change the BGA sphere size, they can change the solder deposit size. Increasing the stencil aperture diameter by as little as 0.001" will result in more solder on the board and a lower surface area:volume ratio for the paste deposit. At first glance, increasing aperture size may appear to be a risky endeavor that invites poor print quality and increased solder bridge defects, but it’s not as scary as it seems. Experience with SAC 305 solder pastes indicates they are less likely than SnPb pastes to form bridges and mid-chip balls. Our investigations into this phenomenon indicate it is a function of the alloy; the oxide film that forms on the surfaces of molten SAC 305 solder is more tenacious than its SnPb counterpart, which, unfortunately, works against us in the formation of HOP defects.
  
  
• The oxide film that forms on Pb-free solders during reflow is tougher to break through than similar films on SnPb solders. We have to take the good with the bad in Pb-free; that same property that gives us fewer solder bridges and mid-chip solder balls also gives us a higher propensity for HOP.
  
  There are two ways to systematically address the oxide film, but neither is a quick fix to an immediate assembly problem. Approach no. 1 is to turn on the nitrogen. If one wants to prevent oxides from forming in the reflow process, it’s a no-brainer to remove the oxygen from the process. That’s not an inexpensive option, however, and for some assemblers, it’s not an option at all. Approach no. 2 is to use a stronger flux – one that has higher activity or better thermal endurance. For most assemblers, this means qualifying a new material, which can be time-consuming and expensive. Couple that with the increased flux activity, which usually means decreased electrochemical reliability, and that thermal endurance is sometimes achieved by using halogenated materials in the flux, and both system-level fixes may rapidly become complicated or even totally impractical.
  
  
• Pb-free reflow processes provide more opportunity for package warpage. Thermal excursions run longer and hotter than they do in SnPb. There’s no way around it: The melting point of SAC 305 is nearly 35°C higher. If the same ramp rates were used in comparing SnPb and Pb-free profiles, the time to reach liquidus would be longer in the Pb-free process simply because of the alloy’s higher melting point. Additionally, Pb-free soak temperatures usually are 20-30°C hotter, and boards with poor thermal balance require longer soak times in Pb-free than they do in SnPb processes.
  
  All this additional thermal exposure can induce more warpage in BGA packages. As the package warps, the solder sphere can get lifted out of the paste deposit. When it is lifted out, more surfaces of both sphere and paste deposit are exposed to air, and more opportunity for oxidation occurs.