Definition
A redistribution layer, or RDL, is one or more layers of patterned copper traces and dielectric film built directly onto the surface of a die or wafer. Its job is to electrically reroute the chip's native input and output pads, which are usually packed at a very fine pitch, out to a looser, more manufacturable array of connection points. RDLs are a core building block of wafer-level and fan-out packaging.
How it works
The RDL is formed by depositing alternating layers of organic dielectric, such as polyimide or BCB, and metal, almost always copper. Lithography defines the routing traces, which fan the signals from the dense on-die pads outward to a relaxed footprint where solder balls or bumps can be placed. This is what makes wafer-level chip-scale packages and fan-out packages possible: in fan-out, dies are reconstituted into a synthetic molded wafer and the RDL extends connections beyond the original die boundary to gain more I/O area.
Relevance to advanced packaging
RDLs underpin much of modern heterogeneous integration. They route power and signals across a package, connect multiple dies, and in some architectures replace a silicon interposer entirely at lower cost. As mining and AI accelerator silicon pushes toward higher pin counts and tighter pitches, RDL-based fan-out and chip-scale packaging become increasingly important to delivering all those connections reliably.
See Silicon Interposer for the higher-density routing alternative and Flip Chip for the bump attachment RDLs feed.
In Simple Terms
A redistribution layer, or RDL, is one or more layers of patterned copper traces and dielectric film built directly onto the surface of a die…