1.Problems caused by skin effect

With the development of communication, cloud computing and cloud storage technology, as well as the development of Higher Ethernet and Cloud Server, PCB will further develop to high speed/high frequency, pCB signal transmission performance will also restrict the development of high-speed transmission technology to a certain extent. 4G Era, PCB single channel signal transmission rate has been increased from 10 gbps to 25 GBPS, 5G era is expected to further increase to more than 50 GBPS. When the frequency reaches 1 Ghz, the signal transmission thickness on the surface of the conductor is only 2.1 m, if the conductor Surface roughness is 3-5 m, when the signal transmission frequency is increased to 10Ghz, the signal transmission thickness on the conductor surface is 0.7 m, and the signal transmission is in the roughness range. When the signal is transmitted in the roughness range, the standing wave and reflection of the transmitted signal will become more and more serious, and the transmission path will become longer and the loss will increase (the effect is shown in figure 1) .

Due to the skin effect, IF HIGH-SPEED PCB continues to use conventional (STD) copper foil, the result is: As the signal transmission frequency increases, the skin effect caused by the signal "distortion" more serious. As a result, the use of Low-roughness copper foils on high-speed materials such as Mid Loss and Low Loss materials uses RTF copper foils as standard copper foils, while Very Low Loss materials are also RTF copper foils, but customer designs are mostly made with Ultra low profile (HVLP) copper foil; for Ultra low loss materials, HVLP copper foil has become standard. The surface morphology of STD, RTF and HVLP copper foil (0.5 Oz thick) can be seen by SEM and metalloscope (see figure 2) . The surface roughness of STD copper foil is about 5 m, the surface roughness of RTF copper foil is about 3 m, the surface roughness of HVLP copper foil is less than 2 M. It is understood that copper foil suppliers are also currently developing Surface roughness NP-copper foil below 1 m, due to reliability issues have not been resolved, the actual product has not been applied

TRANSMISSION LINE LOSS IN PCB mainly includes dielectric loss and conductor loss. For Conventional FR4 MATERIALS, 1 Ghz is the watershed between dielectric loss and conductor loss (see Fig. 3) . Conductor loss is dominant below 1 Ghz, and dielectric loss is dominant above 1 Ghz.

However, for Very Low Loss material, dielectric Loss is no longer the main Loss, and conductor Loss at 10GHz frequency is about 60% of the total Loss of the transmission line. Fig. 4 is the calculation of conductor loss (conduction loss) and dielectric loss (dielectric loss) of microstrip line and Strip line based on the simulation of Metallocene IT-968 material. As can be seen from the diagram, the theoretical loss of the Strip line is basically the same as the measured loss, and the dielectric loss is much less than the conductor loss in both microstrip and strip lines.

From Mid Ultra Low Loss materials, the proportion of conductor Loss increases gradually. The measured losses of high-speed materials with different types of copper foil are also different. Figure 5 shows the test results of strip line signal loss using IT-968 with STD, RTF , and HVLP copper foil.

2.Processing of copper foil with low roughness

Although the surface of HVLP copper foil is smooth, the existing PCB process will lead to the increase of Surface roughness and affect the effect of HVLP copper foil. According to the inner circuit, the inner layer needs to be pre-treated by dry film and Brown process. After these two processes, the Surface roughness of HVLP copper foil Rz will increase from 1.5 m to about 3 M. In order to solve this problem, a corresponding low-roughness process has been introduced in the market. Compared with the traditional Brown solution, this process will not micro-etch the surface of the HVLP copper foil, but will deposit a layer of tin after cleaning the surface of the copper foil, the surface of copper foil was modified by Siloxane, which can bridge PP and improve the adhesion of copper foil to PP to some extent6

Using this process and the traditional brown process, HVLP copper foil Surface roughness comparison table 1. As can be seen from the diagram, the existing dry film pretreatment and brown process can increase the Surface roughness of the copper foil to a certain extent; after adopting the low roughness process, the Surface roughness of the copper foil is basically the same as the original copper foil.

Table 1 comparison of copper foil Surface roughness after traditional brown process and low roughness process

Fig. 7 is a Very Low Loss material with HVLP copper foil that has been improved by a Low roughness process. The test results show that the signal loss can be reduced by 0.03-0.05 db/Inch (12.5 Ghz) . Objectively speaking, this improvement is not significant for Very Low Loss material, and combined with the improvement effect and cost input, the process cost-effective is not satisfactory, so the process has not been widely used. At the same time for HVLP copper, because of its own or has a certain roughness, which leads to the process can only achieve this effect. This range of improvements may make more sense for future Ultra Low Loss materials, and the process is expected to work even better when NP copper foil is officially commercially available.

Source: The web

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