1. Microwave printed circuit boards are specialized for microwave electronic components and are made using specific microwave substrate copper-clad laminates through conventional rigid PCB manufacturing techniques.
2. High-speed signal transmission lines on printed circuit boards fall into two main categories: the first involves high-frequency signal transmission, which relates to radio electromagnetic waves and transmits signals as sine waves, including applications in radar, radio, television, and communications (mobile phones, microwaves, optical fiber communication, etc.); the second category is high-speed logic signal transmission, which involves digital signal transmission and is associated with square wave transmission of electromagnetic waves. This type began in computers and has now expanded to home appliances and communication electronic products.
3. Achieving high-speed transmission requires specific electrical characteristics from the microwave printed circuit board substrate material. To ensure low loss and minimal delay in signal transmission, substrate materials with a low dielectric constant and dielectric loss tangent are essential. Typically, ceramic materials, glass fiber cloth, polytetrafluoroethylene, and other thermosetting resins are used.
4. Among these, PTFE stands out due to its lowest dielectric constant (εr) and dielectric loss tangent (tanδ), as well as its excellent high and low-temperature resistance and aging resistance. Consequently, it is the most suitable material for high-frequency substrates and is currently the most widely used microwave printed circuit board substrate material.
This article will briefly introduce the manufacturing process of two types of ceramic powder-filled microwave multilayer printed circuit boards and delve into the laminated manufacturing technology used in more detail.
2 Microwave multilayer printed circuit board materials
This section focuses on two high-frequency dielectric materials and their microwave multilayer printed circuit board laminate manufacturing technologies. The first material is ceramic powder-filled, short glass fiber reinforced polytetrafluoroethylene (PTFE) high-frequency dielectric material (RT/duroid6002 sheet). The second is ceramic powder-filled thermosetting resin copper clad laminate (RO4350 sheet).
2.1 Ceramic powder-filled microwave multilayer printed circuit board manufacturing process
2.2 Laminating process of RT/duroid6002
2.2.1 Bonding sheet 3001
To manufacture microwave multilayer PCB circuit boards using the high-frequency dielectric plate RT/duroid6002, an adhesive sheet 3001 has been developed. This adhesive is a thermoplastic chlorofluorocopolymer with a low dielectric constant and low loss tangent in the microwave frequency range.
2.2.2 Laminating process
1) Arrangement
Lay the RT/duroid6002 board and the bonding sheet alternately. To ensure precise overlap between layers, use four-slot positioning pins. Place a thermocouple probe into the non-patterned area of the inner layer to control lamination temperature and time.
2) Close
In a cold press state (typically below 120°C), place the arranged plates in the center of the press, close it, and adjust the hydraulic system to achieve the necessary pressure. An initial pressure of 100 psi is usually sufficient, increasing to 200 psi to ensure proper adhesive sheet flow.
3) Warm up
Start the laminator and heat to 220°C. Control the maximum heating rate so that the temperature difference between the upper and lower furnace plates is between 1°C and 5°C.
4) Keep warm
Maintain the temperature at 220°C for 15 minutes to ensure the bonding sheet melts and flows to bond effectively. For thicker structures, extend the holding time to 30-45 minutes.
5) Cold pressing
Turn off the heating system and cool the laminated furnace plate while maintaining pressure until it reaches 120°C. Release the pressure and remove the template from the laminator.
2.2.3 Problems and countermeasures
1) Bonding failure
This may result from inadequate surface treatment of the plate, such as volcanic ash sandblasting or mechanical brushing. Surface chemical treatments should be used. Check if the holding temperature and time were sufficient using a thermocouple. Additionally, clean the mold and reassess plate arrangement procedures and environmental conditions.
2) Spots or blisters on the surface
These issues may be caused by uneven pressure, improper temperature control, or insufficient cleaning and drying of the inner layer sheets. Use clean templates or smooth materials, check flatness or pressure, and revisit cleaning and drying procedures.
3) Deformation
Deformation is often due to excessive temperature or uneven pressure. Ensure precise control of temperature and pressure.
2.3 Laminating process of RO4350
2.3.1 Prepreg RO4403
For effective bonding, use prepreg RO4403 with RO4350 material.
2.3.2 Laminating process
1) Main process parameters
Temperature: 175°C;
Pressure: 40 kg/cm²;
Time: 2 hours;
Buffer mode: 24 sheets of kraft paper top and bottom;
Entry mode: Enter the mold at a lower temperature (100°C), starting lamination time at 175°C;
Relief method: Stepwise pressure release.
Using these parameters, the interlayer bonding force meets requirements, though laminate flatness is poor. Adjusted process parameters are recommended for better results.
2) Arrangement method
Layer from bottom to top: stainless steel mold bottom plate/polyester sheet/4 RO4350 monolithic/one prepreg RO4403/3 RO4350 monolithic/2 prepreg RO4403/2 RO4350 monolithic/1 prepreg RO4403/1 RO4350 monolithic/polyester flake/stainless steel mold upper cover. Use 24 sheets of kraft paper for cushioning on each side. Heat to 175°C with 40 kg/cm² pressure (31 tons for a 30.48 cm × 25.4 cm mold). Enter the mold at room temperature and gradually heat up. Heat and pressure retention is 2 hours, with stepwise pressure release.
Monitor and measure the temperature inside the plate during lamination. Assess the dielectric thickness and flatness of the microwave multilayer printed circuit board. Thickness uniformity of the 8-layer board indicates better parameter control.
To shorten the manufacturing cycle, consider using prepreg RO4450B, which allows a higher heating rate and reduces heating time to 50 minutes.
3 Conclusion
Microwave printed circuit boards are advancing towards substrate diversification, high precision design, computer control, specialized manufacturing, diverse surface coatings, numerical control for shape processing, and automated production inspection. Research on the laminated manufacturing process of these ceramic powder-filled microwave multilayer printed circuit boards provides valuable insights and lays a foundation for further in-depth research.
2. High-speed signal transmission lines on printed circuit boards fall into two main categories: the first involves high-frequency signal transmission, which relates to radio electromagnetic waves and transmits signals as sine waves, including applications in radar, radio, television, and communications (mobile phones, microwaves, optical fiber communication, etc.); the second category is high-speed logic signal transmission, which involves digital signal transmission and is associated with square wave transmission of electromagnetic waves. This type began in computers and has now expanded to home appliances and communication electronic products.
3. Achieving high-speed transmission requires specific electrical characteristics from the microwave printed circuit board substrate material. To ensure low loss and minimal delay in signal transmission, substrate materials with a low dielectric constant and dielectric loss tangent are essential. Typically, ceramic materials, glass fiber cloth, polytetrafluoroethylene, and other thermosetting resins are used.
4. Among these, PTFE stands out due to its lowest dielectric constant (εr) and dielectric loss tangent (tanδ), as well as its excellent high and low-temperature resistance and aging resistance. Consequently, it is the most suitable material for high-frequency substrates and is currently the most widely used microwave printed circuit board substrate material.
This article will briefly introduce the manufacturing process of two types of ceramic powder-filled microwave multilayer printed circuit boards and delve into the laminated manufacturing technology used in more detail.
2 Microwave multilayer printed circuit board materials
This section focuses on two high-frequency dielectric materials and their microwave multilayer printed circuit board laminate manufacturing technologies. The first material is ceramic powder-filled, short glass fiber reinforced polytetrafluoroethylene (PTFE) high-frequency dielectric material (RT/duroid6002 sheet). The second is ceramic powder-filled thermosetting resin copper clad laminate (RO4350 sheet).
2.1 Ceramic powder-filled microwave multilayer printed circuit board manufacturing process
2.2 Laminating process of RT/duroid6002
2.2.1 Bonding sheet 3001
To manufacture microwave multilayer PCB circuit boards using the high-frequency dielectric plate RT/duroid6002, an adhesive sheet 3001 has been developed. This adhesive is a thermoplastic chlorofluorocopolymer with a low dielectric constant and low loss tangent in the microwave frequency range.
2.2.2 Laminating process
1) Arrangement
Lay the RT/duroid6002 board and the bonding sheet alternately. To ensure precise overlap between layers, use four-slot positioning pins. Place a thermocouple probe into the non-patterned area of the inner layer to control lamination temperature and time.
2) Close
In a cold press state (typically below 120°C), place the arranged plates in the center of the press, close it, and adjust the hydraulic system to achieve the necessary pressure. An initial pressure of 100 psi is usually sufficient, increasing to 200 psi to ensure proper adhesive sheet flow.
3) Warm up
Start the laminator and heat to 220°C. Control the maximum heating rate so that the temperature difference between the upper and lower furnace plates is between 1°C and 5°C.
4) Keep warm
Maintain the temperature at 220°C for 15 minutes to ensure the bonding sheet melts and flows to bond effectively. For thicker structures, extend the holding time to 30-45 minutes.
5) Cold pressing
Turn off the heating system and cool the laminated furnace plate while maintaining pressure until it reaches 120°C. Release the pressure and remove the template from the laminator.
2.2.3 Problems and countermeasures
1) Bonding failure
This may result from inadequate surface treatment of the plate, such as volcanic ash sandblasting or mechanical brushing. Surface chemical treatments should be used. Check if the holding temperature and time were sufficient using a thermocouple. Additionally, clean the mold and reassess plate arrangement procedures and environmental conditions.
2) Spots or blisters on the surface
These issues may be caused by uneven pressure, improper temperature control, or insufficient cleaning and drying of the inner layer sheets. Use clean templates or smooth materials, check flatness or pressure, and revisit cleaning and drying procedures.
3) Deformation
Deformation is often due to excessive temperature or uneven pressure. Ensure precise control of temperature and pressure.
2.3 Laminating process of RO4350
2.3.1 Prepreg RO4403
For effective bonding, use prepreg RO4403 with RO4350 material.
2.3.2 Laminating process
1) Main process parameters
Temperature: 175°C;
Pressure: 40 kg/cm²;
Time: 2 hours;
Buffer mode: 24 sheets of kraft paper top and bottom;
Entry mode: Enter the mold at a lower temperature (100°C), starting lamination time at 175°C;
Relief method: Stepwise pressure release.
Using these parameters, the interlayer bonding force meets requirements, though laminate flatness is poor. Adjusted process parameters are recommended for better results.
2) Arrangement method
Layer from bottom to top: stainless steel mold bottom plate/polyester sheet/4 RO4350 monolithic/one prepreg RO4403/3 RO4350 monolithic/2 prepreg RO4403/2 RO4350 monolithic/1 prepreg RO4403/1 RO4350 monolithic/polyester flake/stainless steel mold upper cover. Use 24 sheets of kraft paper for cushioning on each side. Heat to 175°C with 40 kg/cm² pressure (31 tons for a 30.48 cm × 25.4 cm mold). Enter the mold at room temperature and gradually heat up. Heat and pressure retention is 2 hours, with stepwise pressure release.
Monitor and measure the temperature inside the plate during lamination. Assess the dielectric thickness and flatness of the microwave multilayer printed circuit board. Thickness uniformity of the 8-layer board indicates better parameter control.
To shorten the manufacturing cycle, consider using prepreg RO4450B, which allows a higher heating rate and reduces heating time to 50 minutes.
3 Conclusion
Microwave printed circuit boards are advancing towards substrate diversification, high precision design, computer control, specialized manufacturing, diverse surface coatings, numerical control for shape processing, and automated production inspection. Research on the laminated manufacturing process of these ceramic powder-filled microwave multilayer printed circuit boards provides valuable insights and lays a foundation for further in-depth research.
