Redistribution Layer (RDL)
The redistribution layer (RDL) is the interface between chip and package for flip-chip assembly (shown in fig below). An RDL is an extra metal layer consisting of wiring on top of core metals that makes the I/O pads of the die available for bonding out other locations such as bump pads. Bumps are usually placed in a grid pattern and each one is molded with two pads (one on the top and one on the bottom) that are then attached to the RDL and package substrate respectively
Engineers use a redistribution layer (RDL) in flip-chip designs to redistribute I/O pads to bump pads without changing the I/O pad placement. The RDL, therefore, serves as the layer connecting I/O pads and bump pads. However, traditional routing capacity may be insufficient to handle sizable designs, in which the RDL may be very congested and especially when there is a less-than-optimal I/O-bump assignment. As a result, routing may not be completed within a single layer even with manual routing.
As demand for more input/output (I/O) increases, traditional wire-bond packaging may not effectively support thousands of I/Os. Flip-chip assembly is commonly used in place of wire bond because it reduces chip area while supporting many more I/Os. It also greatly reduces inductance, allows high-speed signals, and possess better heat conductivity properties. The flip-chip ball grid array (FCBGA) is also growing in popularity as an alternative methodology for high I/O count chips.
