PCB additive manufacturing process designed for PCB

Some STM32 application customers who use external SDRAM reflect that in EMC test, the radiation interference caused by SDRAM signal exceeds the standard. If the chassis can’t shield the radiation interference in the terminal products, then this kind of problem often needs to be solved by modifying the PCB design of SDRAM signal.
Here, how to improve the radiation interference in the application design of SDRAM PCB is summarized, which can be used as a reference.
Countermeasures of SDRAM Radiation Interference in PCB Design
Because SDRAM works at a high frequency and has steep rising and falling edges, it is necessary to treat its signal wiring as a high-speed signal transmission line in PCB design. Usually, the following basic principles should be noted:
1. keep the integrity of SDRAM signal.
The distortion of SDRAM signal will further broaden the radiation spectrum of the signal, which will bring more serious radiation problems. Therefore, attention must be paid to the integrity design of SDRAM signal.
It is recommended to use four or more layers of boards to control the characteristic impedance of SDRAM signals to 50 ohms, reduce the use of vias on SDRAM bus as much as possible, keep the continuity of impedance, and reduce signal reflection caused by impedance discontinuity;
SDRAM signal trace spacing should follow the principle of 3W, and the center spacing between two traces should be kept at least 3 times the line width as much as possible, which can reduce signal distortion caused by interference between signals.
Make SDRAM as close to MCU as possible, and shorten the length of signal wiring from MCU to SDRAM (usually no more than 120mm);
2. Keep the shortest SDRAM signal return path
For multilayer PCB, the return path of high-speed signal is the projection of its trace on the reference plane. In PCB design, attention should be paid to keeping the reference plane intact and continuous. If the signal return path is cut off due to signal layer change or power layer division, the SDRAM signal must have the shortest return path by increasing layer change capacitance/layer change ground via hole and power plane jumper capacitance.
3. Put SDRAM signals (especially clock signals) on the inner layer of PCB.
Among SDRAM signals, the clock signal has the strongest radiation level, and its radiation can be reduced by placing it on the inner layer of PCB and shielding it with copper foil on the outer layer.
As for the design that SDRAM and FLASH are externally connected to the FMC interface of STM32, because SDRAM and FLASH share some MCU pins, their complex routing topology further enhances the radiation interference of SDRAM signals. It is suggested that the routing of SDRAM and FLASH should be laid on the inner layer of PCB as much as possible, and at the same time, these signals should be shielded by the ground plane on the outer layer of PCB.
4. Keep the SDRAM routing area as far away as possible from other signals/cables.
Other long wires or cables can be used as antennas to send out the coupled SDRAM radiation signals, so they should be arranged in the area away from SDRAM signals in PCB. If necessary, magnetic beads or filters can be placed at the connection ends of these wires or cables to attenuate SDRAM radiation signals.