The article is quoted from Shenzhen Honglijie Electronics!
The current PCB high-speed signal transmission line can be divided into two categories: one is a high-frequency signal transmission class, which is related to the electromagnetic waves of radio, with sinusoidal transmission of signals, such as radar, radio and television and communications (cell phones, microwave communications, fiber-optic communications, etc.); the other is a high-speed logic signal transmission class, this type of product to digital signal transmission, with the transmission of electromagnetic waves of the square wave transmission, this type of Products began to be mainly used in computers, computers, and now has been applied to home appliances and communications electronics.
In order to achieve high-speed transmission, the electrical characteristics of microwave PCB substrate materials have clear requirements. To achieve low loss and low delay of the transmission signal, it is necessary to choose the dielectric constant and dielectric loss angle tangent small substrate materials, generally ceramic materials, glass fiber cloth, PTFE and other thermosetting resins.
Among all the resins, PTFE has the smallest dielectric constant (εr) and dielectric loss angle tangent (tanδ), and good resistance to high and low temperatures and aging, which is most suitable for high-frequency substrate materials, and it is the largest microwave PCB substrate material at present.
This article will be on the basis of a brief introduction to the two ceramic powder filled microwave multilayer PCB manufacturing process flow, the lamination manufacturing process technology used in a more detailed discussion.
2 microwave multilayer PCB materials
The main study of the following two high-frequency dielectric materials microwave multilayer PCB lamination manufacturing process technology. The first is ceramic powder-filled, glass short fiber reinforced polytetrafluoroethylene (PTFE) high-frequency dielectric materials (RT/duroid6002 sheet); the second is ceramic powder-filled thermosetting resin copper-clad laminate (RO4350 sheet).
2.1 Ceramic Powder Filled Microwave Multilayer PCB Manufacturing Process
The following describes the two high frequency dielectric board lamination process technology.
2.2 Lamination Process for RT/duroid6002
2.2.1 Bonding Sheet 3001
In order to fabricate microwave multilayer PCBs using the HF dielectric sheet RT/duroid6002, the supplier has developed Bonding Sheet 3001, which is suitable for high frequency dielectric boards with low dielectric constants in the RT/duroid. It is a thermoplastic chlorofluoro*** polymer with low dielectric constant and low loss angular tangent in the microwave frequency range.
2.2.2 Laminating Process
1) Layup
Stack the RT/duroid6002 boards alternately with the bonding sheets. In order to ensure the precision of overlap between layers of multi-layer PCB, four groove positioning pins are used to arrange the board. The method of placing the thermocouple probe into the non-graphic area of the inner layer of the board to be pressed is used for the control of lamination temperature and time.
2) Closure
When the press is in a cooler state (usually press temperature is below 120 ℃), the above lined up and loaded with molded boards placed in the center of the press, close the press, adjust the hydraulic system so that the area to be pressed to obtain the required pressure. Typically, an initial pressure of 100 psi is sufficient, followed by a full pressure ramp up to 200 psi to ensure proper flow of the bonded sheet.
3) Heating
Start the laminator and heat to 220°C. The maximum heating rate is controlled. In general, control the maximum heating rate so that the temperature difference between the upper / lower furnace plate 1 ℃ ~ 5 ℃.
4) Holding
Typically, hold at 220°C for 15 minutes to allow the bonding sheet to be in a molten state and to have enough time to flow and wet the surface to be bonded. For thicker lined structures, the holding time can be extended to 30 minutes to 45 minutes.
5) Cold Pressing
Turn off the heating system and cool the laminator plate while maintaining pressure until the plate temperature drops to 120 °C. Relieve the pressure and remove the template containing the laminate from the laminator.
2.2.3 Problems and countermeasures
1) Bonding failure
The reason for this is that the surface of the plate to be pressed is treated mechanically, such as volcanic ash sandblasting, mechanical brushing of the plate, etc., and the surface chemical treatment process should be used. On the insulation temperature and insulation time is not enough, should use thermocouples on the lamination temperature curve again for determination. Another reason is that the surface of the object to be pressed is stained with mold release agent, moisture, dirt, etc., should be re-assessed on the mold cleaning, plate lining procedures and environmental conditions.
2) laminate surface spots or blisters
The reason is that the pressure is not uniform, improper temperature control, lamination before the inner layer of the single piece of cleaning and drying is not sufficient. The countermeasures taken are to choose a clean template or other polished materials, check the flatness or pressure. Use thermocouples to check the lamination temperature curve again. Review the cleaning and drying procedures for the sheets to be pressed, and also review the storage conditions and timing of the sheets during preparation and bonding.
3) Deformation
The reason is that the temperature is too high or the pressure is uneven, the temperature and pressure should be precisely controlled.
2.3 Lamination process of RO4350
2.3.1 Semi-curing sheet RO4403
In order to realize effective bonding, semi-curing sheet RO4403 is selected for RO4350 material.
2.3.2 Lamination process
1) Main process parameters
Temperature: 175℃;
Pressure: 40kg/cm2;
Time: 2 hours;
Buffer: 24 pieces of kraft paper for each upper and lower pad;
Mode of entering the mold: lower temperature (100℃) is used to enter the mold, and the laminating time starts to be calculated at 175℃;
Pressure release method: the method of releasing the pressure is adopted by segments.
After laminating with the above conditions, the interlayer bonding force can still meet the requirements, but the flatness of the laminate is poor. After several tests and with reference to the lamination characteristics of the semi-cured sheet RO4403, it was decided to change to the following process parameters for lamination.
2) row plate way
From bottom to top of the stainless steel mold under the bottom plate / polyester sheet / 4 RO4350 single / a half-cured sheet RO4403 / 3 RO4350 single / 2 half-cured sheet RO4403 / 2 RO4350 single / a half-cured sheet RO4403 / 1 single / polyester sheet / RO4350 single. Stainless steel mold top cover plate.
24 sheets of kraft paper for cushioning on each side. Heating temperature is 175°C. Pressure is 40kg/cm2 (31 tons for the chosen 30.48cm x 25.4cm (12" x 10") mold). The mold was introduced at room temperature and gradually heated up. The holding time for holding pressure is 2 hours, and the way to release the pressure is to lower the temperature and release the pressure in sections.
The actual lamination is done by monitoring and measuring the temperature inside the plate to be pressed.
In order to control the dielectric thickness of the microwave multilayer PCB, the thickness of each individual piece and the flatness of the finished board were measured before and after lamination, and the measurement results are shown in Tables 1 and 2 below.
It can be seen that the thickness uniformity of the 8-layer microwave multilayer PCB is better, which proves that the control of the relevant parameters is relatively good.
The whole lamination process is longer, in order to shorten the manufacturing cycle, more convenient to control the process, may wish to use another semi-cured sheet RO4450B, lamination heating rate can be significantly improved, the heating time from 2 hours shortened to 50 minutes. 3 Conclusion
Microwave PCB is diversifying the substrate, the design of high-precision, computer-controlled manufacturing specialization, the surface of the surface of the plating diversification, shape processing CNC. Plating diversification, shape processing CNC and production inspection automation direction of development. Through the study of two kinds of ceramic powder filled microwave multilayer PCB lamination manufacturing process, a certain amount of experience, for the future further in-depth study laid a solid foundation.