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

have generally been the modification of existing materials such as the adaptation of
redistribution layer materials and solder types, but these solutions still have the limitation of
increased performance.
In addition to the assembly adhesives that fill the gap between the die and board, solder
bracing is another WLCSP protection scheme to delocalize the stress of solder under the die.
Unlike adhesives, there is no adhesive bond between the die and board which makes solder
bracing a reworkable process. Since the packaging is done at the wafer level, relative low cost
becomes the biggest advantage of solder bracing.
The future trend of die protection may combine the functions of both technologies, and it
follows the needs for new chemical technologies that are capable of minimizing any impact on
assembly cost and improvement of reliability [53-54].
2.3.2 PolymerCollar WLP
Conceptually, solder bracing is defined for any material that mechanically supports the
solder and the front side of the die. Few commercial wafer level solder bracing materials are
available to date. PolymerCollar WLP is the most notable example of solder bracing. Figure 2.12
shows some photos of PolymerCollar WLP built on an Ultra CSP 50 daisy chain test wafer. It is
a WLP having a wafer-applied polymer reinforcement structure that surrounds the solder joints
to transfer the stress away from the interconnection joint. This PolymerCollar material performs
two functions: One is a fluxing action during solder ball attachment and the other is to form the
polymer reinforcement structure itself. The main process flow consists of:
• Application of Polymer Collar material instead of solder flux on chip side UBM pad
• Placement of preformed solder balls onto the Polymer Collar material
29