Research About Signal Integrity Problem Of High-speed PCB Based On HFSS

With the rapid development of electronic technology, high-speed system’s clock frequencies is higher and higher, and the switching time of gate decrease rapidly, the transmission line in PCB has a more important influence on electronic system, signal integrity problems from transmission line become critical. In low speed system, the interconnects are “transparent” and only for conducting, but in high speed system, it must be taken as transmission. In this case, researching SI problems based on simulator is an effective method. The electromagnetic field effects of 3D structure in high-speed PCB can be simulated by accurate and reliable 3D full-wave electromagnetic fields simulator. HFSS software is 3-D full-wave electromagnetic field solver, which offers multiple state-of the-art solver technologies based on finite element, integral equation or advanced hybrid methods, it can solve a wide range of SI problems.This paper focus on the impact of material which constitute transmission line on signal. Roughness of copper, which is easy to be ignored in PCB manufacture, has a high impact on signal integrity, larger roughness is a significant source of signal integrity problems, we use the copper with different roughness to study the impact for SI. In order to reduce the loss of signal. We usually use those material with low Dk and low Df, meanwhile, the layout of PCB must take the fiberglass weaver effect into consideration, some special action should be taken to keep the instantaneous impedance the signal sees as constant as possible. Moreover, one important point shouldn’t be ignored, it is that the Dk and Df will change with frequency.the passive structure in PCB will affect the signal integrity. Corners and Vias in PCB is unavoidable, corner’s capacitive discontinuity causes a reflection for the transmitted signal. Therefore, in order to keep the characteristic impedance of the line constant, the smaller the corner’s angle, the more stable the characteristic impedance. If possible, the extra copper should be cut. Via’s characteristic impedance has many influencing factors, such as pad, anti-pad, return-via and stub, etc. Via’s design must consider many associated factors to make its characteristic impedance keep the same with transmission line.At last, manufacturing process of transmission line will make it defect and then signal integrity problems come into being. The width and dielectric thickness of transmission line is hard to control, especially when their values are very small, smaller values is more sensitive. The offset between layers will affect signal quality to some extent, it should be repair in next process. Solder mask have a great influence on characteristic impedance within small thickness, we must think over it in designing, we can reduce suitable line width if necessary.