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

2.2.5 Current approaches to improve reliability
An extensive amount of work, focused on improving the reliability of WLCSP, has been
developed to reduce shear stresses in the solder joints, especially for the large die application
with WLCSP technology.
One of the improvements is the application of the soft stress buffer layer (SBL) structure
which is formed under the solder bumps to reduce the shear stress in the solder joints [44-47].
This thick SBL coated on the die side can enhance packaging reliability, but the manufacturing
process is difficult and the production cost is high. The silicone material introduced by IMEC
research center is one example that is able to absorb deformations and reduce stresses created in
the device by the CTE mismatches of different materials. These patternable silicones were
integrated into a silicone under bump (SUB) design which improved the solder joint reliability
through reduction of the strain experienced by the solder. Figure 2.7 shows a schematic view of
SUB configuration. The process protocols for building a silicone under the bump wafer level
package using both photo-patternable silicones and printable silicones (see Figure 2.8) [48].
SUB
Figure 2.7 Illustration of SUB build-up
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