Research On Signal Integrity In Designs And Fabrication Of High-speed Transmission Lines For Printed Circuits

With the continuous development of mobile communications technology,the signal integrity(SI)of the printed circuit board(PCB)in high-speed transmission becomes more prominent.And the designs and production requirements for high-speed PCB also have become more important.Therefore,the purpose of this paper is to investigate the effects of the circuit design,coupling degree,solder resist ink and heat treatment of transmission lines on the signal integrity.Based on this,the paper will improve the circuit design and fabrication technology of PCB in high-speed transmission to provide theoretical basis and technical guidance for SI engineers.In this article,a 14-layer PCB was fabricated using the Megtron 6G materials and the reverse copper foil(RTF).According to the scatter parameters of the transmission line measured by the vector network analyzer,the effects of reference layer' continuity on signal integrity were analyzed.The results showed that the impedance in a continuous reference was stable.In addition,the return loss and insertion loss were better.In contrast,the impedance in a discontinuous reference suddenly increased in the discontinuous area,resulting in terrible reflection,enormous return loss and insertion loss.When the impedance in the discontinuous area was adjusted,the return loss and insertion loss of the signal were significantly improved.The ratio of line width and line spacing is used to express the coupling degree of the transmission line.Among them,the coupling degree increases as the ratio increasing.In order to study the influence of the coupling degree of transmission line on SI,the microstrip lines,strip lines and coplanar waveguides with different coupling values were designed and fabricated.The results showed that the insertion loss and usable area of PCB at a same test frequency increased as the ratio increasing.Therefore,under the condition of same impedance of transmission line,the usable areas of PCB and the transmission quality of the signal should be considered in the circuit designs.Solder resist ink is the cover layer to protect transmission line against the environmental destruction.In this article,the effects of a traditional solder resist ink and the novel solder resist ink of low loss factor(Low-Df)in PCB fabrication on SI were investigated.The results showed that the insertion loss of novel Low-Df solder resist ink decreased,compared with traditional solder resist ink.In addition,the PCB fabricated by Low-Df solder resist ink conformed to the IPC-TM-650 standard.So the Low-Df solder resist ink could be widely used in the production of high-speed PCB.At the same time,the influence of the thickness of the solder resist ink on SI was analyzed.The results indicated that the insertion loss effectively improved at a lower thickness of solder resist.The reliability of signal transmission of PCB under different heat treatment methods was studied through heat treatment experiments.The results showed that the influence degree of the baking plate and reflow on the transmission line was: microstrip line > coplanar waveguide > strip line.The re-circulation of hot and cold had little effect on transmission lines.These demonstrated that the heat treatments had a greater effect on the external transmission line(microstrip line and coplanar waveguide),and had little effect on the inner transmission line(strip line).