PCB Blog - Selection of Substrate for Microwave PCB Circuit Board

For the selection of Microwave PCB Circuit Board substitute, its dielectric property should be considered first, but at the same time, the type and thickness of copper foil on its surface, environmental adaptability, processability and other factors must be considered, as well as the cost.

The types and thickness of copper foil for Microwave PCB Circuit Board substitute are 35 μ M and 18 μ M Two kinds. The thinner the copper foil is, the easier it is to obtain high graphic precision, so the microwave graphics with high precision should be no more than 18 μ M copper foil. If 35 is selected μ M copper foil, the high graphic accuracy makes the processability worse, and the rate of unqualified products will inevitably increase. IPCB's Microwave PCB manufacturing experience believes that the type of copper foil also affects the graphic accuracy. At present, there are two types of copper foil: calendered copper foil and electrolytic copper foil. The calendered copper foil is more suitable for manufacturing high-precision graphics than the electrolytic copper foil, so when selecting the Microwave PCB material, you can consider selecting the base material plate of the calendered copper foil.

Machinability selection of Microwave PCB Circuit Board substitute With the continuous improvement of design requirements, some microwave PCB substrates are equipped with aluminum liners. The appearance of such aluminum lined base materials has brought additional pressure to the manufacturing process. The graphic production process is complicated, the contour processing is complicated, and the production cycle is prolonged. Therefore, when available or not, the base materials with aluminum lining plates should not be used as much as possible.

The TMM series microwave substrate of Rogers is composed of thermosetting resin filled with ceramic powder. Among them, the TMM10 base material is filled with more ceramic powder, which is brittle in performance, bringing great difficulties to the pattern manufacturing and contour processing process, and is easy to be damaged or form internal cracks, with relatively low yield. At present, the method of laser cutting is used to process the shape of TMM10 sheet, which has high cost, low efficiency and long production cycle. Therefore, if possible, RT/Duroid series base plates of Rogers Company that meet the corresponding dielectric performance requirements can be preferred.

Environmental adaptability selection of Microwave PCB Circuit Board substitute The existing microwave substrate has no problem with the standard required ambient temperature range of - 55 ℃~+125 ℃. But there are two other things to consider,

1. The influence of porosity on the selection of substrate. For microwave plates requiring through hole metallization, the greater the Z-axis thermal expansion coefficient of substrate, which means the greater the possibility of metallized hole fracture under high and low temperature impact. Therefore, on the premise of meeting the dielectric properties, the substrate with smaller Z-axis thermal expansion coefficient should be selected as far as possible

2. The influence of humidity on the selection of base material plate: the water absorption of base material resin itself is very small, but after adding reinforcement materials, the overall water absorption will increase, which will affect the dielectric property when used in a high humidity environment. Therefore, the base material with low water absorption should be selected, or structural and technological measures should be taken to protect it.

The structure design of Microwave PCB is more and more complicated due to the appearance of microwave boards, and the requirements for dimensional accuracy are high. The production quantity of the same variety is large, so it is necessary to apply the NC milling technology. Therefore, the characteristics of numerical control processing should be fully considered when designing the microwave plate, and the internal corners of all processing places should be designed as rounded corners to facilitate one-time processing and forming.

The structural design of microwave board should not pursue too high accuracy, because the size deformation tendency of non-metallic materials is large, and the processing accuracy of metal parts cannot be used to require microwave board. The high accuracy requirement of the profile may be due to the fact that when the microstrip line is connected to the profile, the profile deviation will affect the length of the microstrip line, thus affecting the microwave performance. In fact, a gap of 0.2mm should be reserved between the end of the microstrip line and the edge of the plate, so as to avoid the influence of the profile processing deviation.

Microwave PCB manufacturing process, Microwave is different from ordinary single and double sided boards and multilayer boards. It not only plays the role of structural parts and connectors, but also acts as a signal transmission line. The manufacturing of microwave is restricted by such factors as the number of microwave manufacturing layers, the characteristics of microwave raw materials, the requirements for metalized hole manufacturing, the final surface coating method, the characteristics of circuit design, the accuracy requirements of manufacturing lines, the progressiveness of manufacturing equipment and liquid medicine, etc. The manufacturing process will be adjusted accordingly according to specific requirements. For example, the circuit nickel gold electroplating process is subdivided into graphic nickel gold electroplating positive plate process and graphic nickel gold electroplating negative plate process. Therefore, different manufacturing processes are adopted for different microwave types and processing requirements,

Microwave PCB Process Description

1. When the circuit graphics are interconnected, the negative process of graphic nickel gold electroplating can be selected

2) In order to improve the qualified rate of microwave manufacturing, the negative printing process of graphic nickel gold electroplating shall be adopted as far as possible. Because if the positive plate process of graphic nickel gold electroplating is adopted, the quality problem of nickel gold infiltration will occur if the operation is not properly controlled

3) The microwave board of ROGERS with the brand of RT/duroid 6010 substrate will be scrapped due to the long hair on the edge of the line during the pattern electroplating after etching, so the positive plate process of pattern nickel gold electroplating must be adopted

4) When the manufacturing accuracy of the circuit is required to be within ± 0.02mm, the wet film plate making process must be adopted at the corresponding points of each process

5) When the manufacturing accuracy of the circuit is required to be more than ± 0.03mm, the dry film or wet film plate making process can be used at the corresponding part of each process

6. For PTFE medium microwave plates, such as ROGERS RT/duroid 5880, RT/duroid 5870, ULTRALAM2000, RT/duroid 6010, etc., sodium naphthalene solution or plasma can be used for hole metallization. However, TMM10, TMM10i, RO4003, RO4350, etc. do not require activation pretreatment.

The manufacturing of Microwave PCB Circuit Board is developing towards the common rigid processing of FR-4, and more and more rigid manufacturing processes and technologies are applied to microwave processing. It is embodied in the multilayer of microwave manufacturing, the refinement of circuit manufacturing accuracy, the three-dimensional of CNC machining and the diversification of surface coating. In addition, with the further increase of the types of microwave PCB substrates and the continuous improvement of design requirements, iPCB is required to further optimize the existing microwave manufacturing process and keep pace with the times to meet the growing requirements for microwave PCB manufacturing.