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Fan-Outs

A way of including more features that normally would be on a printed circuit board inside a package.
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Description

Fan-outs fit somewhere between system-in-packaging approaches and printed circuit boards. They are a way of extending what is inside a package by including more elements of what normally resides on a PCB.

The approach began taking hold after 2010 as a baby-step toward 2.5D and 3D-ICs. While not nearly as fast or power-efficient as a stacked die, it does allow components to be pre-integrated and tested to improve time to market as well as to incorporate some value-added approaches, such as thicker wires and better memory placement to improve overall performance, reduce RC delay and improve energy efficiency.

In one example of fan-out, a DRAM die is stacked on a logic chip in a package. This brings the memory closer to the logic, enabling more bandwidth.

Fan-out packages consist of dies and redistribution layers (RDLs). RDLs are the copper metal interconnects that electrically connect one part of the package to another. RDLs are measured by line and space, which refer to the width and pitch of a metal trace.

To make fan-out packages, dies are placed in a wafer-like structure using an epoxy mold compound. The RDLs are formed. The individual dies are cut, forming a package.

Fan-out has some challenges. When the dies are placed in the compound, they can move during the process. This effect, called die shift, can impact yield.

Fan-out is split into two segments — standard and high density. Targeted for consumer and mobile applications, standard-density fan-out is defined as a package with fewer than 500 I/Os and RDLs greater than 8μm line and space. Geared for high-end apps, high-density fan-out has more than 500 I/Os with RDLs less than 8μm line and space.

At the high-end, vendors are continually developing fan-out with increasingly smaller RDL line and space, exceeding 2μm.

At one time, fan-out was limited in I/O count. Now, high-density fan-out is moving toward higher I/O counts and invading the high-end territory held by 2.5D. Fan-out is unlikely to displace 2.5D, but it is less expensive as it does not require an interposer.


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