PCB Technical - Multi-layer high frequency mixing board

Compared with the data of the traditional multi-layer PCB board, the key characteristics of the multi-layer high-frequency hybrid board are very different. It can not only use multi-layer high-frequency PCB materials mixed with high-frequency materials and FR4, but also multi-layer PCB materials mixed with high-frequency materials with different dielectric constants. With the development of technology, the hybrid structure of high frequency board + FR4 is also understood by more and more people. At the same time, it also brings more benefits and challenges to designers and manufacturers.

The selection of high-frequency mixed multi-layer PCB materials mainly considers the following three factors: price, reliability, and electrical characteristics. The price of high-frequency circuit boards is generally higher than FR4. Sometimes a combination of two different materials is used to solve the cost problem. In most cases, in the multilayer PCB (multi-layer high-frequency hybrid board), those layers related to the circuit are more important, while the other layers are less critical. In this situation, low-priced FR4 materials can be used in those layers that are not related to the circuit, and high-frequency plates with high prices can be used in those layers that are related to the circuit.

When there is a material with high CTE characteristics in the hybrid multi-layer board (multi-layer high-frequency hybrid board), in order to improve the reliability, it is necessary to consider the hybrid multi-layer PCB (multi-layer high-frequency hybrid board). Some high-frequency PTFE materials have very good CTE characteristics, but their reliability is the local area that needs to be emphasized. When FR4 with low CTE characteristics and high CTE materials form a multi-layer PCB together, the composition CTE demand is within an acceptable scale.

In order to achieve better electrical characteristics, some mixed multi-layer PCB materials (multi-layer high-frequency mixed-pressure boards) will also include materials with different dielectric constants. For example, for some couplers and filters, the use of materials with different dielectric constants often has greater advantages.

Although there are some compatibility issues when using RF4 and high-frequency circuit boards (multi-layer high-frequency hybrid boards) together, this usage is becoming more and more. At the same time, some production-related issues also require more attention

The high-frequency data used in the hybrid multi-layer data structure and the data used in circuit fabrication are very different in manufacturing process. If the high-frequency data of PTFE base material is used in the process of circuit production, such as the processing of hole drilling and PTH electroplating, it will bring many problems. There is not much problem with the data of hydrocarbon base material when using the standard FR4 circuit board manufacturing process.

The combination of FR4 and hydrocarbon data generally has only a few process problems. Mainly reflected in the hole transfer and lamination. To turn holes on this laminated structure, it is generally necessary to choose experimental design to establish a suitable feed/speed model. The problem of lamination is mainly caused by the big difference in the pressing curve of FR4 prepreg and high-frequency data prepreg. In order to ensure the reliability of the board, when using FR4 and hydrocarbon prepregs, there are some options for consideration. One of the methods is to replace FR4 prepreg with high-frequency prepreg and choose a suitable compression curve. The price of high-frequency prepregs is relatively cheap compared to high-frequency substrates, and if all prepregs use the same materials, the lamination cycle will be relatively simple. If the FR4 prepreg cannot be replaced, the sequential lamination method must be used. Put the lamination cycle curve of FR4 prepreg at the first place, and the lamination cycle curve of high-frequency materials at the back.

The use of FR4 and high-frequency PTFE circuit materials to form a hybrid multi-layer PCB (multi-layer high-frequency hybrid board) generally faces more challenges. However, there will be some exceptions. Since there are some varieties of materials using PTFE as the base material, the circuit manufacturing process is simpler than other PTFE materials. Although the material of the ceramic-added PTFE substrate has less consideration in the circuit manufacturing process than the material of the pure PTFE substrate, the hole transfer, PTH treatment and the stability of the scale are several things that must be considered.

PTFE is the main consideration when turning PTH into holes, which is softer than FR4. When the turning hole passes through the joint surface of the soft and hard materials, the soft materials will be stretched to a certain length on the hole wall of the PTH. This may lead to a very serious reliability problem. Generally, after experimental design and the research on the life of the rotating hole, the correct feed and rotating speed can be obtained. In many situations, this situation does not occur when the hole-removing tool is first used. Therefore, by controlling the life span of the revolving device, the impact of this problem can be minimized.

The electroplating treatment of the PTH holes of the two types of materials should be paid attention to. The Plasma cycle may require two different cycles or one cycle including different stages. FR4 data is processed in the first Plasma cycle, and PTFE data is processed in the second Plasma cycle. Generally, FR4 Plasma process uses CF4-N2-O2 gas, and PTFE uses helium or hydrazine gas. In order to improve the moisture solubility of the through-hole wall, it is recommended to use helium to process PTFE materials. If the wet process is to be used in PTH processing, first use potassium permanganate to process FR4 data, and then use sodium naphthalene to process PTFE data.

Scale stability, or scaling, is also a problem faced by PTFE and FR4 hybrid materials (multi-layer high-frequency hybrid plates). After the greatest possible reduction of the mechanical pressure on the PTFE material, its production can be reduced. It is not recommended to wipe the data forcefully, as it will increase the random mechanical pressure on the data. Advocating the use of chemical finishing processes can prepare for the subsequent copper treatment process. Thicker PTFE materials have fewer problems with dimensional stability. The PTFE material added with glass woven cloth will have better dimensional stability.

In short, there will be a few compatibility issues in the production of hybrid multilayer PCBs (multi-layer high-frequency hybrid boards) composed of FR4 and high-frequency materials. However, some key points in the circuit manufacturing process require special treatment