Structural layer PCB board decision system EMC performance is a very important factor. A good PCB board PCB layer structure for suppressing radiation play a good effect. In high-speed circuit system now common in most of the multi-layer PCB board rather than one-sided and double-sided PCB board. In the design of multi-faceted PCB board when needed attention to the following aspects .
1 a signal layer and a copper layer should be adjacent ;
2 signal layers should be close to the copper layer and tightly coupled ( ie, signal layers and near a small copper deposited dielectric thickness between layers ) ;
3 power copper and copper should be tightly coupled ;
4 system, the inner layer and the high-speed signal between the two should be copper , copper may be provided so that the two shielding effect for these high-speed signals and the signals of the radiation is limited to the two copper area ;
More than 5 to copper layer can effectively reduce the impedance of the PCB , reducing common mode EMI.
PCB board layers structural arrangements , mostly not fully comply with five points above . This requires selecting the appropriate PCB board layer structure according to the actual system requirements. Here now commonly used 6-layer PCB construction make an explanation .
A: Section 2 and Section 5 power and ground layers of copper , copper impedance is high because the power , the control of the common mode EMI radiation is very unfavorable . However, from the point of impedance control signal point of view, this approach is very correct . Because this layer PCB board design , Layer1 signal trace layers and Layer3, Layer4 and Layer6 constitute two of the more reasonable alignment combinations.
B: The power and ground were placed 3rd and 4th floor , this design solves the problem of power supply impedance copper , due to the poor performance of the electromagnetic shielding layer 1 and 6 layers , differential mode EMI increased. If the signal line on the minimum number of two outer layers , the trace length is very short ( shorter than the wavelength of the harmonic signal of the highest 1/20 ) , then this design differential mode EMI problems can be solved . No element on the outer area and no trace copper and copper- filled areas Ground ( every 1 /20 of the wavelength interval ) , the differential mode EMI suppression particularly good.
C: From the perspective of signal quality considerations, it is clear that C example PCB board layers most reasonable arrangement . Because of this structure on the return of the high-frequency signal path is ideal . But this arrangement has a more prominent drawback: less alignment layer signal. So this system is suitable for high-performance requirements.
D: It can achieve the desired signal integrity design environment . Adjacent to the signal layer and the ground layer , the power supply layer and the ground layer pair. Obviously, the downside is unbalanced layer structure ( copper imbalance may cause the PCB warpage ) . Solution to the problem is all the blank area of the copper layer 3 , the copper layer 3 if the first copper layer or power density close to the ground floor , this PCB board can not strictly be regarded as a structural balance of the circuit PCB board. Copper area must be connected to power or ground .
