PCB Technical - Multi-layer high frequency mixed pressure PCB material selection

Generally speaking, the combination of FR4 and hydrocarbon materials has only a few process problems. Mainly reflected in the hole transfer and lamination. To turn holes on this laminated structure, it is usually necessary to adopt experimental design to establish a suitable feed/speed model. The problem of lamination is mainly caused by the great difference in the pressing curve of FR4 prepreg and high-frequency material prepreg. In order to ensure the reliability of the board, when using FR4 and hydrocarbon prepregs, some methods are available for consideration.

One of the methods is to replace FR4 prepregs with high frequency prepregs and choose a suitable compression curve. The price of high-frequency prepregs is cheaper than that of high-frequency substrates, and if all prepregs use the same material, the multi-layer high-frequency hybrid PCB cycle will be relatively simple. If FR4 prepreg cannot be replaced, it must be laminated sequentially. Put the lamination cycle curve of the FR4 prepreg at the first place, and the lamination cycle curve of the high-frequency material at the back.

The use of FR4 and high-frequency PTFE circuit materials to form a multilayer high-frequency hybrid PCB usually faces more challenges. However, there will be some exceptions. Because there are some types of PTFE-based materials that have simpler circuit manufacturing processes than other PTFE materials. Although ceramic-added PTFE substrate materials have less consideration in circuit manufacturing processes than pure PTFE substrate materials, hole transfer, PTH treatment and dimensional stability are several things that must be considered.

The main consideration when PTH turns holes is PTFE, which is softer than FR4. When the hole drilling tool passes through the joint surface of soft and hard materials, the soft material will be stretched to a certain length on the hole wall of the PTH. This may cause a very serious reliability problem. Usually, through experimental design and research on the life of the drilling tool, the correct feed and drilling speed can be obtained. In many cases, this will not happen when the drilling tool is first used. Therefore, by controlling the life of the drilling tool, the impact of this problem can be minimized.

Attention should be paid to the electroplating treatment of PTH holes of two types of materials. The Plasma cycle may require two different cycles or a cycle with different stages. FR4 material is processed in the first Plasma cycle, and PTFE material is processed in the second Plasma cycle. Usually, 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 treat the PTFE material. If the wet process is to be used in PTH treatment, please treat FR4 material with potassium permanganate first, and then treat PTFE material with sodium naphthalene.

Dimensional stability, or scaling, is also a problem faced by PTFE and FR4 mixed materials (multi-layer high frequency mixed PCB). By reducing the mechanical pressure on the PTFE material as much as possible, its occurrence can be reduced. It is not recommended to scrub the material vigorously because it will increase the random mechanical pressure of the material. It is recommended to use a chemical cleaning process to prepare for the subsequent copper treatment process. The thicker the PTFE material, the less the problem of dimensional stability. The PTFE material added with glass woven cloth will have better dimensional stability.

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