Temporary Bonding and Debonding Technologies for Fan-Out Wafer-Level Packaging

Handheld consumer electronics are requiring more complex packaging designs to accommodate higher component densities and reduce form factor. Fan-out wafer-level packaging (FOWLP) has garnered much attention lately as a cost-effective way to achieve high interconnect density and manage larger I/O counts within an affordable package. Two principal approaches to manufacturing FOWLP components have evolved: chip-first and chip-last, which refer to the point in the process flow where the chips are attached and over-molded in the package. Many variants of these two main schemes are now starting to appear. Chip-first processes are more mature and have been in high-volume production for some time, while chip-last schemes are still largely in development. Both processing flows present unique substrate handling challenges that can be addressed by the use of a carrier support system. For example, reconstituted substrates formed in chip-first processes are prone to severe warping and bending, especially at high processing temperatures. Brewer Science is developing thermally stable polymeric adhesive systems and release agents for temporarily attaching reconstituted wafers and panels to a rigid carrier that mechanically stabilizes the substrates through processing and then allows the carrier to be separated with low force when processing is finished. Chip-first FOWLP processing also has two variants known as face-down and face-up. These designations refer to the position of the active face of the die with respect to the carrier when the dies are over-molded at the start of the process flow. While both variants have advantages and disadvantages, either flow can require that the substrate be "flipped" at some point so that processing on the opposing face of the die or package can be done. Transferring the reconstituted substrate from the original carrier to a temporarily bonded second carrier is often the most reliable way to mitigate downstream handling problems and preserve yield. Chip-last FOWLP processing usually starts with the build-up of a redistribution layer interconnect (RDL) on a carrier. The dies are flip chip-attached to the RDL and then over-molded near the very end of the process flow, which ensures that known good die (KGD) will not be lost in a defective package. In most cases, a temporary adhesive is not utilized to attach the build-up RDL to the carrier. Instead, the RDL is fabricated on a thin, laser-sensitive release layer that can be activated by exposure through the carrier at the point when separation is desired. On the other hand, some flows require the RDL to be flipped prior to die-attach, which again creates the need for a temporarily bonded carrier system to provide support. This paper will highlight how temporary bonding materials and processes can enable all of the prevailing FOWLP flows and explain how adhesives and release layer systems can be selected to achieve reliable performance.