Four-Layer PCB Stackup
The printed circuit board stackup (PCB stackup) arrangement is the basis of the entire PCB design system. Any deficiencies in the stackup design will ultimately affect the overall EMC performance.
In general, the design of the PCB stackup mainly comply with two rules:
1.Each alignment must have an adjacent reference layer (power or ground);
2.Keep the minimum spacing between adjacent main power and ground so as to provide a larger coupling capacitance.
Recommended PCB stackup way: one is SIG- GND(PWR)- PWR(GND)- SIG; another is GND- SIG(PWR)- SIG(PWR)- GND.
A potential problem with both stackup design is caused by the traditional 1.6 mm (62 mil) board thickness. Layer spacing will become very large which is not conducive to the control of impedance, coupling and shielding between layers; in particular, a large distance between the power ground layer can reduce the plate capacitance which is not conducive to filter out noise.
For the first option, it usually applies to the situation where there are many chips on the board. This method mainly governed by alignment and other details can get better SI characteristic, but not good to EMI characteristic. It is noteworthy that: stratum placed on the signal layer of the signal layer of the most connected, is conducive to absorbing and inhibiting radiation; increase the board area, reflecting 20H rules.
For the second option, it is usually used where the chip density is low enough and there is enough area around the chip where the required power supply copper layer is placed. The outer layer of PCB here are all ground layer, the middle two layers are signal/power layer. The power supply on the signal layer is routed with a wide line, which lowers the path impedance of the supply current and lowers the impedance of the signal microstrip path and can also shield the inner layer signal radiation through the outer layer. From the EMI control point of view, this is the best available 4-layer PCB structure. It is noteworthy that: spacing between the middle two layers of signals and power mixed layer should be large, the perpendicular alignment direction can avoid crosstalk; control the PCB board area appropriate so as to reflect 20H rules; if you want to control the alignment impedance, you should carefully route the layout under the power and ground copper island. Besides, copper placed on the ground layer and the power should be interconnected as much as possible to ensure the connectivity of DC and low frequency.