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

offers the optimum flux release characteristics. The aperture pattern of the stencil
was designed to match the pattern on the bumped wafer, the same as Figure 3.8.
- Printer setup: The printer parameters must be appropriately defined to achieve
consistent prints during stencil printing. The flux must be able to roll evenly across
the stencil surface. The driving force plus speed of the squeegee are the main factors
that determine the kinematics of this rolling motion. An MPM TF100 Printer with
vision system was used for the flux printing process in this research. Parameters
optimized for the printer are listed in Table 3.4. Once all the printing parameters were
set, the vision alignment system of the printer checked the precise alignment of the
stencil aperture openings to the bumping openings on the wafer. This step was
required to be carefully handled; otherwise the resulting misalignment could cause
the partial wetting or non-wetting issue of the solder ball. After the flux was printed
onto the wafer, further inspection under a microscope was needed to eliminate the
printing defects.
57