Can flexible PCBs carry high density DDR4 signals?

摘要

Embedded systems such as Single Board Computers (SBCs), ADAS and Infotainment adopt memory-down as the sole architecture for memory channels such as DDR3/4 & LPDDR4. This has the advantage of improved signal integrity and a compact design compared to using DIMM sockets, which can often not be used due to mechanical constraints. However, such memory-down architecture consumes large amounts of PCB real-estate, especially when high-density memory is required. In some examples for 36 SDRAMs more than 30% of the PCB surface area has been used for memory. In this paper we present the concept for a new architecture based on rigid-flex PCB technology: Instead of connecting directly to the main PCB, 36 SDRAMs are attached to a high density DIMM which connects to the main PCB through a flexible interconnect and a PCB interposer. This represents a new initiative for those systems that cannot tolerate the use of DIMM sockets due to mechanical restrictions. We brand this new concept "Flex-DIMM". It offers numerous advantages over the traditional memory-down approach besides freeing up a huge amount of real estate on the main PCB while maintaining the benefits of a memory-down approach. Such a Flex-DIMM can also provide a plug-n-play approach for memory channels, which would dramatically reduce the design cycle of embedded systems as routing the memory channel can be omitted. While the above advantages make this approach very attractive, the impact of a flexible PCB in the high-speed memory channel on signal integrity, power integrity and EMC needs to be investigated. In this paper, we will present a feasibility study of DDR4-2667 operation using this new Rigid-Flex-PCB architecture. We will investigate the effect of different flexible PCB configurations on the eye-opening and compare the performance to that of the traditional memory-down approach using a high fidelity full 3D EM simulation model that includes Interposer, main PCB, Flex PCB and the high density DIMM. We will also study the effect of bending angle and radius as well as substrate choice and stitching vias.