PCB Tech - Manufacturing technology of microwave multilayer PCB

Microwave printed circuit boards refer to microwave electronic components produced on specific microwave substrate copper clad laminates using common rigid PCB manufacturing methods.

The current high-speed signal transmission lines for printed circuit boards can be divided into two categories: one is high-frequency signal transmission, which is related to radio electromagnetic waves and transmits signals with sine waves, such as radar, radio and television, and communications (mobile phones, microwaves). Communication, optical fiber communication, etc.); the other is high-speed logic signal transmission. This type of product uses digital signal transmission and is related to the square wave transmission of electromagnetic waves. This type of product began to be mainly used in computers and computers, and has now been used. To home appliances and communication electronic products.

In order to achieve high-speed transmission, there are clear requirements for the electrical characteristics of the microwave printed circuit board substrate material. To achieve low loss and low delay of transmission signals, substrate materials with small dielectric constant and dielectric loss tangent must be selected, generally ceramic materials, glass fiber cloth, polytetrafluoroethylene and other thermosetting resins.

Among all resins, PTFE has the smallest dielectric constant (εr) and dielectric loss tangent (tanδ), and has good high and low temperature resistance and aging resistance. It is most suitable as a high-frequency substrate material and is currently the largest amount. Microwave printed circuit board substrate material.

This article will briefly introduce the manufacturing process of the two ceramic powder-filled microwave multilayer printed circuit boards, and discuss the laminated manufacturing technology used in more detail.

2 Microwave multilayer printed circuit board materials

Mainly research the following two high-frequency dielectric materials, microwave multilayer printed circuit board laminate manufacturing technology. The first 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

In order to use the high-frequency dielectric plate RT/duroid6002 to manufacture microwave multilayer PCB circuit boards, the supplier has developed an adhesive sheet 3001 suitable for RT/duroid low-dielectric constant high-frequency dielectric boards. It is a thermoplastic chlorofluorocopolymer with 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. In order to ensure the accuracy of the overlap between the layers of the multilayer printed circuit board, four-slot positioning pins are used to arrange the boards. The method of placing the thermocouple probe into the non-patterned area of the inner layer of the plate to be pressed is adopted to control the lamination temperature and time.

2) Close

When the press is in a cold state (usually the temperature of the press is lower than 120°C), place the above-mentioned arranged and moulded plates in the center of the press, close the press, and adjust the hydraulic system to obtain the required pressure in the area to be pressed. Under normal circumstances, an initial pressure of 100 psi is sufficient, and then the total pressure is increased to 200 psi to ensure proper fluidity of the adhesive sheet.

3) Warm up

Start the laminator and heat to 220°C. Generally, the maximum heating rate is controlled so that the temperature difference between the upper and lower furnace plates is 1 degree Celsius～5 degree Celsius.

4) Keep warm

Normally, keep the temperature at 220°C for 15 minutes to make the bonding sheet in a molten state, and have enough time to flow and wet the surface to be bonded. For thicker platoon structure, the holding time can be extended to 30 minutes to 45 minutes.

5) Cold pressing

Turn off the heating system and cool down the laminated furnace plate while maintaining the pressure until the temperature of the furnace plate drops to 120°C. Release the pressure and take out the template containing the laminate from the laminator.

2.2.3 Problems and countermeasures

1) Bonding failure

The reason is that mechanical treatment methods are used on the surface of the plate to be pressed, such as volcanic ash sandblasting, mechanical brushing, etc., and surface chemical treatment processes should be used. If the holding temperature and holding time are not enough, a thermocouple should be used to measure the lamination temperature curve again. Another reason is that the surface of the object to be pressed is stained with release agent, moisture, dirt, etc., and the cleaning of the mold, the plate arrangement procedure and the environmental conditions should be reassessed.

2) Spots or blisters on the surface of the laminate

The reason is uneven pressure, improper temperature control, and insufficient cleaning and drying of the inner layer sheet before lamination. The countermeasures taken are to select clean templates or other smooth materials and check the flatness or pressure. A thermocouple was used to detect the lamination temperature curve again. Review the cleaning and drying procedures of the single sheet to be pressed, and review the storage conditions and time of the single sheet during preparation and bonding.

3) Deformation

The reason is that the temperature is too high or the pressure is uneven, and the temperature and pressure should be accurately controlled.

2.3 The lamination process of RO4350

2.3.1 prepreg RO4403

In order to achieve effective bonding, prepreg RO4403 is selected for RO4350 material.

2.3.2 Laminating process

1) Main process parameters

Temperature: 175 degree Celsius;

Pressure: 40kg/cm2;

Time: 2 hours;

Buffer mode: 24 pieces of kraft paper on top and bottom;

Entry mode: use lower temperature (100°C) to enter the mold, and start to calculate the lamination time at 175°C;

Relief method: adopt the stepwise pressure release method.

After using the above conditions for lamination, the interlayer bonding force can still meet the requirements, but the flatness of the laminate is poor. After many tests and referring to the lamination characteristics of the prepreg RO4403 used, it was decided to use the following process parameters for lamination.

2) Arrangement method

From bottom to top, the bottom plate of the stainless steel mold/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.

24 sheets of kraft paper for cushioning on each side. The heating temperature is 175°C. The pressure is 40 kg/cm2 (for the selected mold of 30.48 cm * 25.4 cm (12 inches * 10 inches), the pressure is 31 tons). Enter the mold at room temperature and gradually heat up. The heat preservation and pressure retention time is 2 hours, and the pressure release method is to lower the temperature and release the pressure in sections.

During actual lamination, the temperature inside the plate to be pressed is monitored and measured.

In order to control the dielectric thickness of the microwave multilayer printed circuit board, the thickness of each single sheet before and after lamination and the flatness of the finished board were measured.

It can be seen that the thickness uniformity of the 8-layer microwave multilayer printed circuit board is better, which proves that the control of the relevant parameters is better.

The above-mentioned whole lamination process is relatively long. In order to shorten the manufacturing cycle and make the process easier to control, another prepreg RO4450B may be used. The lamination heating rate can be significantly increased, and the heating time can be shortened from 2 hours to 50 minutes.

3 Conclusion

Microwave printed circuit boards are developing in the direction of substrate diversification, design high precision, computer control, manufacturing specialization, surface coating diversification, shape processing numerical control and production inspection automation. Through the research on the laminated manufacturing process of two kinds of ceramic powder filled microwave multilayer printed circuit boards, certain experience has been obtained, which has laid a solid foundation for further in-depth research in the future.