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

composites via the cationic crosslinking reaction because silica fillers with micrometer size can
scatter the UV light, impede the photo reaction, and hence are incompatible with the photo-
polymerization process. Based upon their results, nanosilica composites have displayed desirable
optical properties and were chosen as the filler to reduce the thermal expansion of photo-
definable epoxy material. A photo-sensitive initiator was added into the formulation, releasing
the cations after UV exposure and initiating the epoxy crosslinking reaction. This photo-
crosslink reaction makes the epoxy a negative photoresist that can be utilized in the fabrication
process, and also make it a good wafer level protective material [22].
1.6 Research Objective and Outlines
The objective of this research was to develop a new WLCSP fabrication process with the
use of a wafer level SolderBrace coating to achieve low cost reliability improvement. This
SolderBrace material is a kind of photo-definable epoxy resin which not only replaces the
capillary underfills (the process that employs capillary action to underfill the die to substrate
undergap), but also replaces the final passivation layer of the silicon device fabrication process,
making it an attractive low cost solution for future WLCSP designs. The package proposed here
has a single bump solder joint geometry with a thick coating of SolderBrace material deposited
on the die. Processing of this coating layer can be achieved by two methods. The first is done at
the wafer level using the standard semiconductor lithography processing method. The second
application process involves printing the material on the pre-balled wafers followed by the solder
cleaning and cure.
Figure 1.6 outlines the steps for the first method. The coating is spin applied, UV defined,
and bumped. Figure 1.7 outlines the processing steps for the second method with printing over
8