Enhanced Polymer Passivation Layer for Wafer Level Chip Scale ...

widespread use in high yield and high volume applications such as the practical lower
limit to the size of sphere that can be dropped, and the yields are statistically low [71].
Over the last 10 years, several methods for the mass transfer of solder balls to wafers
have been investigated and developed. Figure 3.12 shows the ball placement method
developed by DEK [72]. It utilized screen printing. The process begins with loading
the wafer in a conveyorized aluminum pallet. This pallet is then transported into the
flux printing machine, which can visually align the wafer with an emulsion mesh
fluxing screen. After flux printing onto all bumping pads, the wafer and pallet are
transported to the ball placement machine. Following accurate visual alignment with
the metal ball placement stencil, the wafer is brought into contact with the underside
of the stencil before the ball transfer head traverses the topside of the stencil. This
deposits a single solder ball into each of the stencil’s apertures. The alignment
process accurately ensures that the apertures coincide with the fluxed solder bump
pads.
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