Process Development of Via Formation by Laser Drilling on Insulating Resin

Abstract

To realize continuous miniaturization, improved performance and efficiency while minimizing cost of emerging new and highly functional electronic devices, the semiconductor industry continues to develop packaging solutions and rely on the advancement and methods of interconnecting and integrating several parts into one highly mobile and robust device [1]. FOWLP and WLCSP packaging plays a role in a shift of getting more mobility from handheld smartphones devices to autonomous driven cars or air drone utility vehicles of the future. This packaging technology requires sensitive devices to be encapsulated to protect its components from the environment such as injection molding or with a laminated resin applied on front and backside. With the device encapsulation, there is a need for the devices to be connected between packages or be stacked from another device to be functional. Thru mold via technology had emerged where laser drilling on mold surface is carried out for via formation on underlying thick Cu interconnects from FOWLP packaging standpoint which usually uses high filler size materials to overcome interlayer's mechanical characteristics mismatch [2]. Other chip scale packaging on the other hand requires further compacting such that fine filler resins are employed acting as dielectric between interconnects and land on thinner aluminum pads. This fine formation of vias however is where the challenge enters primarily on how to create smaller via size features without damaging the thin aluminum so as not to compromise reliability of under bump metallization. This paper will show an optimization results of laser drilling process parameters for various filler size insulating resin, the effects of UV laser process on Aluminum pad damage or metal penetration, and dimension of the formed vias. Laser drilling is based on a 365nm UV laser source focused into the surface of the resin material on top of the underlying aluminum pad to ablate and turn it into gaseous by-products with minimal heat conversion to achieve highly efficient via formation process. A DOE was performed on the etching rate of the encapsulating material and determine the energy density requirement to efficiently remove the bulk of resin and leave minimal or no residue without damage or puncture on the underlying aluminum pad surface. The experiments also encompass the optimization of laser beam size, selection of drilling method and sequential steps that is required to form the desired via diameter. Ultimately, the desired no damage with minimal residue on underlying aluminum pad to form a contact can be achieve which is important for excellent electrical connectivity, efficient current flow, and reliability of the package. This forms the laser via machining process can integrate well on fan-out panel based embedded packaging technology and provide solution to enable laser via machine to create via for traces routing as it is still not common in the market.