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.
