Four-Layer High-Speed PCB Design May 29, 2007
When minimum cost and high-speed are required, a four-layer board may be the answer. High-Speed PCB design generally requires controlled impedance traces and good power and ground planes in order to meet signal integrity and power integrity requirements. Four-Layer PCBs are essentially made of 2 two-layer hardboards â€śgluedâ€ť together with a pre-preg spacer in between them. The thickness of the pre-preg is not as well controlled as that of the hardboards. So, for good controlled impedance traces, it is recommended to use the layers separated by hardboard as signal and ground. The layers separated by pre-preg can be used as power and ground.
Thus, a recommended stack up for a high-speed four-layer board is; the first layer is a signal layer, the second layer is power, the third layer is ground and the fourth is a signal layer. This results in placing most of the routing on the fourth layer closest to the ground layer, and allowing a higher component density on the first layer.
Next, the spacing between signal and ground layers and trace widths can be planned using . To provide the lowest possible power distribution impedance, the spacing between power and ground planes should be as close as practical.
- Route high-speed signals on the bottom (adjacent to ground) whenever possible
- Route high-speed signals using a minimum of vias and corners
- Maintain the proper trace width at the corners by rounding or beveling
- Avoid stubs on traces whenever possible
- Route all traces over continuous planes (VCC or GND), with no interruptions
- Decouple power and ground planes with good ceramic capacitors
- Minimize the trace length and loop area seen by decoupling capacitor currents
- Posted in : PCB
- Author : Brad