With the rapid development of electronic technology, the continuous development of printed circuit board technology has been promoted. PCB boards have progressed through the development of single-sided, double-sided, and multi-layer boards, and the proportion of PCB multi-layer boards is increasing year by year. At the same time, PCB multilayer boards are also developing towards two extremes: high, precise, dense, thin, large and small. An important process in the manufacture of PCB multilayer boards is lamination, and the control of lamination quality is becoming more and more important in the manufacture of multilayer boards. Therefore, to ensure the quality of PCB multilayer board lamination, it is necessary to have a better understanding of the PCB multilayer board lamination process.
How to improve the lamination quality of multilayer circuit boards is summarized as follows in terms of process technology:
一、 Design the inner core board that meets the lamination requirements
Due to the gradual development of lamination machine technology, the heat press has changed from the previous non-vacuum heat press to the current vacuum heat press. The heat press process is in a closed system, which cannot be seen or touched. Therefore, it is necessary to properly design the inner layer of the PCB before lamination. Here are some reference requirements:
1. There must be a certain distance between the outer dimension of the core board and the effective unit, that is, the distance from the effective unit to the edge of the PCB should leave as much space as possible without wasting materials. Generally, four-layer boards require spacing Greater than 10mm, the six-layer board requires a spacing greater than 15mm, the higher the number of layers, the larger the spacing.
2. The inner core board of the PCB circuit board requires no open, short, open circuit, no oxidation, clean board surface, and no residual film.
3. The thickness of the core board should be select according to the total thickness of the PCB multilayer board. The thickness of the core board is consistent, the deviation is small, and the latitude and weft directions of the blanking are consistent. Especially for PCB multilayer boards with more than 6 layers, the warp and weft directions of each inner core board must be Consistent, that is, the warp direction overlaps with the warp direction, and the weft direction overlaps with the weft direction to prevent unnecessary board bending.
4. The design of the positioning holes is to reduce the deviation between the layers of the PCB multilayer board. Therefore, attention should be paid to the design of the positioning holes of the PCB multilayer board: 4-layer boards only need to design more than 3 positioning holes for drilling. . In addition to designing positioning holes for drilling, multi-layer PCBs with more than 6 layers need to design more than 5 overlapping positioning rivet holes and more than 5 tool plate positioning holes for rivets. However, the designed positioning holes, rivet holes, and tool holes are generally higher in layers, and the number of designed holes is correspondingly larger, and the positions should be kept as far as possible. The main purpose is to reduce the alignment deviation between layers and leave a larger space for manufacturing. The design of the target shape should meet the requirements of the target machine for automatic recognition of the target shape as much as possible. Generally, it is designed as a complete circle or a concentric circle.
二、To meet the requirements of PCB circuit board users, choose the appropriate PP, CU foil configuration
The customer's requirements for PP are mainly reflected in the requirements of dielectric layer thickness, dielectric constant, characteristic impedance, withstand voltage, and the smoothness of the laminate surface. Therefore, when choosing PP, you can choose according to the following aspects:
1. It can ensure the bonding strength and smooth appearance;
2. Resin can fill the gaps of printed wires during lamination;
3. It can provide the necessary dielectric layer thickness for PCB multilayer boards;
4. It can be fully laminated during lamination Exclude air and volatiles between laminations;
5. CU foils are mainly configured with different models according to the requirements of PCB circuit board users, and the quality of CU foils meets IPC standards.
三、Inner layer core board treatment process
When the PCB multilayer board is laminated, the inner core board needs to be processed. The treatment process of the inner layer board includes black oxidation treatment process and browning treatment process. The oxidation treatment process is to form a black oxide film on the inner layer copper foil. The thickness of the black oxide film is 0.25-4). 50mg/cm2. The browning treatment process (horizontal browning) is to form an organic film on the inner copper foil. The effects of the inner plate treatment process are:
1. Increase the specific surface of the inner copper foil and the resin to enhance the bonding force between the two;
2. Improve the acid resistance of multi-layer circuit boards in the wet process and prevent pink circles;
3. Prevent the decomposition of the curing agent dicyandiamide in the liquid resin at high temperature and the influence of water on the copper surface;
4. Increase the effective wettability of the copper foil when the molten resin flows, so that the flowing resin has sufficient ability to penetrate into the oxide film and show a strong grip after curing.
四、Organic matching of lamination parameters
The control of PCB multilayer board lamination parameters mainly refers to the organic matching of "temperature, pressure and time" of lamination.
1. Temperature
Several temperature parameters are important during lamination. That is, the melting temperature of the resin, the curing temperature of the resin, the set temperature of the hot plate, the actual temperature of the material and the speed of heating up. The melting temperature refers to the resin starting to melt when the temperature rises to 70°C. It is due to the further increase in temperature that the resin further melts and begins to flow. During the period of temperature 70-140 ℃, the resin is easy to flow, and it is precisely because of the flowability of the resin that the filling and wetting of the resin are guaranteed. As the temperature gradually increases, the fluidity of the resin undergoes a process from small to large, then to small, and finally when the temperature reaches 160-170C, the fluidity of the resin is 0, and the temperature at this time is called the curing temperature.
In order to make the resin better filled and wet, it is very important to control the heating rate. The heating rate is the embodiment of the lamination temperature, that is, to control when and how high the temperature rises. The control of the heating rate is an important parameter for the lamination quality of PCB multilayer boards, and the heating rate is generally controlled at 2-4C /MIN. The heating rate is closely related to the different models and magnitudes of PP.
For 7628PP, the heating rate can be faster at 2-4°C/min. For 1080 and 2116PP, the heating rate can be controlled at 1.5-2°C/min. Poor performance, high fluidity of resin, short time, easy to cause slippage and affect lamination quality. The temperature of the hot plate mainly depends on the heat transfer conditions of the steel plate, steel plate, leather kraft paper, etc., generally 180-200°C.
2. Pressure
The lamination pressure of PCB multilayer boards is based on whether the resin can fill the interlayer voids and exhaust the interlayer gases and volatiles as the basic principle. Since the hot press is divided into a non-vacuum press and a vacuum hot press, there are several methods of one-stage pressurization, two-stage pressurization and multi-stage pressurization starting from the pressure. Generally, the non-vacuum press adopts general pressurization and two-stage pressurization. The vacuum machine adopts two-stage pressurization and multi-stage pressurization. Multi-stage pressure is usually used for high, fine and fine multi-layer boards. The pressure is generally determined according to the pressure parameters provided by the PP supplier, generally 15-35kg/c
