The first technology uses ABF (Ajinomoto build-up film) and laser ablation to generate the RDL interconnect pattern. ABF, which was developed for build-up multi-layer substrate fabrication allows direct laser drilling of micro vias and Cu patterning by electroless or electroplating. (after permanganate etch). ABF has good thickness uniformity and it is suitable for impedance control for high-speed signal transmission. ABF was chosen to make the metal and via pattern instead of BCB or PI and photolithography process which are more expensive.
The process flow is shown below. In a similar fashion to more standard fan out processes chips are first attached face down to a laminating tape and compression molded into wafer form. At this point ABF film is laminated on the reconstructed wafer and vias are laser drilled. Thin film processing is used for interconnect and UBM before the solder balls are attached. The parts are then singulated.
Amkor reports that a second process based on a buried-pattern PCB substrate can also be used to generate the FO WLP. By using buried-pattern PCB substrate, laser ablation process in laser WLFO is not necessary. The 2-layer buried-pattern substrate is delivered from a PCB manufacturer with required surface flatness and stiffness. The process flow of PCB WLFO is described below. Chips are attached face down to a pre patterned PCB containing unfilled via holes and is molded. Seed layer and plating from the backside then fills the via contacts and subsequent RDL and ball placement finishes the structures.
Amkor has indicated that these processes, while very promising, are not yet fully ready for production.