PCB Blog - Principle Analysis of Glass Fiber Effect in PCB board

In the actual design of PCB board, we often see that some engineers pursue the art of wiring, and the wiring on the circuit board is horizontal and vertical, which looks very beautiful, while some engineers' wiring is zigzag, which likes to be messy and easy to be neat, and some engineers always pursue short distance wiring, and those who can walk diagonally are never horizontal and vertical. Is there any difference behind these various wiring styles? This article will introduce the technology and art of PCB wiring together.

1. Horizontal and vertical problems

The basic material of PCB is copper clad plate. At present, the common FR-4 is made of electronic glass fiber cloth as reinforcement, dipped in epoxy resin, covered with copper foil of a certain thickness, and then hot pressed. At low frequency, the glass fiber cloth has little influence on the electrical properties of PCB, so it can be considered that the medium is uniform, but at high frequency, the local characteristics of the dielectric layer will have a great influence on the electrical properties of PCB. The width and spacing of warp and weft yarns are different for different glass fiber fabrics. If the horizontal and vertical wiring strategy is adopted, the PCB routing is always 0 ° or 90 ° to the edge of the substrate, which will cause the transmission line direction to be parallel to the warp and weft of the glass fiber. At this time, several situations will occur: the transmission line is directly above the warp glass fiber bundle, the transmission line is directly above the weft glass fiber bundle, and the transmission line is between two warp glass fiber bundles, The transmission line is between two latitudinal glass fiber bundles. The relative dielectric constant of the glass fiber cloth is quite different from that of the epoxy resin (about 3 for epoxy resin and 6 for glass fiber cloth). Therefore, the dielectric constant at different positions on the board is different, which leads to the difference of impedance at different positions on the board. At the same time, the dielectric constant of the same impedance line is uneven due to different positions. The effect on differential pair is more obvious, which may lead to eye map collapse.

Principle Analysis of Glass Fiber Effect in PCB

According to the above explanation, although the horizontal and vertical wiring is beautiful, it is not necessarily practical. In the actual design, functional performance must be given priority to aesthetic art. Of course, it is easy to use and beautiful. There is no problem in the actual design, which may depend on the following factors: the differential wiring is just one on the glass fiber bundle and one on the epoxy resin; Width and length of wiring; The rate of the bus, etc.

2. Solutions

In view of the above phenomenon, some possible solutions are as follows: routing should avoid the weaving spacing of glass fiber bundles; The differential wiring spacing just avoids the weaving spacing of the glass fiber bundle; Zigzag routing; Routing with a certain angle; Rotary design by designers; Rotating base plate of PCB manufacturer; Use high-end substrate materials; Use more compact glass fiber materials (small weaving spacing of glass fiber bundles); Electrical upward offset (deskew); Increase electrical margin, etc.

The above is a brief introduction to the glass fiber effect in PCB board. In the actual design process, short physical distance must be very important, so there will be time delay, but at the same time, it is necessary to choose the most reasonable routing method according to the wiring difficulty, transmission line characteristics, etc., and try to make the design beautiful and artistic.