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-layered, and the proportion of PCB multilayer boards is increasing year by year. At the same time, PCB multilayer boards are also developing towards the extremes of high, precise, dense, fine, 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 lamination quality of PCB multi-layer boards, it is necessary to have a good understanding of the lamination process of PCB multi-layer boards.
How to improve the lamination quality of multi-layer circuit boards is summarized as follows in terms of process technology:
1. Design the inner core board that meets the lamination requirements
Due to the gradual development of laminating machine technology, the hot press has changed from the previous non-vacuum hot press to the current vacuum hot press. The hot pressing process is in a closed system, which cannot be seen or touched. Therefore, a reasonable design of the inner layer of the PCB is required before lamination. Here are some reference requirements:
a. There must be a certain distance between the external dimensions of the core board and the effective unit, that is, the distance from the effective unit to the edge of the PCB should be as large as possible without wasting materials. Generally, four-layer boards require spacing. More than 10mm, the six-layer board requires a spacing greater than 15mm, and the higher the number of layers, the larger the spacing.
b. The inner core board of the PCB circuit board requires no open, short, open circuit, no oxidation, the board surface is clean, and there is no residual film.
c. The thickness of the core board should be selected according to the total thickness of the PCB multi-layer board. The thickness of the core board is consistent, the deviation is small, and the warp and weft directions of the blanking materials are consistent. Especially for PCB multi-layer 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, preventing unnecessary board bending.
d. In the design of positioning holes, in order to reduce the deviation between the layers of the PCB multilayer board, attention should be paid to the positioning hole design of the PCB multilayer board: the 4-layer board only needs to design more than 3 positioning holes for drilling. . For multi-layer PCB circuit boards with more than 6 layers, in addition to the positioning holes for drilling, it is necessary to design more than 5 positioning rivet holes for overlapping layers and more than 5 positioning holes for tool board for rivets. However, the designed positioning holes, rivet holes and tool holes are generally higher in layers, and the number of designed holes should be correspondingly larger, and the positions should be as close to the side as possible. The main purpose is to reduce the misalignment between layers and leave a larger space for manufacturing. The design of the target shape should meet the requirements of the automatic identification of the target shape of the target machine as much as possible, and it is generally designed as a complete circle or a concentric circle.
2. To meet the requirements of PCB circuit board users, select the appropriate PP, CU foil configuration
The customer's requirements for PP are mainly reflected in the requirements of the thickness of the dielectric layer, 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:
a. It can ensure bond strength and smooth appearance;
b. Resin can fill the gap of printed conductors during lamination;
c. It can provide the necessary dielectric layer thickness for the PCB multilayer board;
d. It can fully eliminate the air and volatiles between the laminations during lamination;
e. The CU foil is mainly configured with different models according to the user requirements of the PCB circuit board, and the quality of the CU foil conforms to the IPC standard.
3. Inner core board processing technology
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 brown treatment process. The oxidation treatment process is to form a black oxide film on the inner layer copper foil, and the thickness of the black oxide film is 0.25-4). 50mg/cm2. The browning process (horizontal browning) is to form an organic film on the inner copper foil. The inner layer board treatment process has the following functions:
a. Increase the specific surface of the contact between the inner layer copper foil and the resin to enhance the bonding force between the two;
b. Make the multi-layer circuit board improve the acid resistance and prevent the pink circle in the wet process;
c. Prevent the decomposition of the curing agent dicyandiamide in the liquid resin at high temperature - the influence of moisture on the copper surface;
d. Increase the effective wettability of the molten resin to the copper foil when it flows, so that the flowing resin has sufficient ability to extend into the oxide film, and shows a strong grip after curing.
4. Organic matching of lamination parameters
The control of the lamination parameters of the PCB multilayer board mainly refers to the organic matching of the "temperature, pressure and time" of the lamination.
a. Temperature
There are several temperature parameters that are important in the lamination process. 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 rate of temperature increase. The melting temperature is when the temperature rises to 70°C and the resin begins to melt. It is due to the further increase in temperature that the resin melts further and begins to flow. During the temperature of 70-140°C, the resin is easy to fluid. It is precisely because of the fluidity of the resin that the resin can be filled and wetted. As the temperature gradually increases, the fluidity of the resin undergoes a change from small to large, then to small, and finally when the temperature reaches 160-170 ° C, the fluidity of the resin is 0, and the temperature at this time is called the curing temperature.
In order to make the resin fill and wet well, it is very important to control the heating rate. The heating rate is the embodiment of the lamination temperature, that is, when the temperature rises and how high. The control of the heating rate is an important parameter for the lamination quality of the PCB multilayer board, and the heating rate is generally controlled at 2-4℃/MIN. The heating rate is closely related to the different types and quantities of PP.
For 7628PP, the heating rate can be a little faster, that is, 2-4°C/min. For 1080 and 2116PP, the heating rate can be controlled at 1.5-2°C/MIN. At the same time, the number of PP is large, and the heating rate cannot be too fast, because the heating rate is too fast, PP The wettability is poor, the resin fluidity is large, and the time is short, which is easy to cause slippage and affect the lamination quality. The temperature of the hot plate mainly depends on the heat transfer of the steel plate, steel plate, kraft paper, etc., and is generally 180-200 °C.
b. Pressure
The lamination pressure of PCB multilayer boards is based on whether the resin can fill the voids between the layers and exhaust the gas and volatiles between the layers 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 from the perspective of pressure. General non-vacuum presses use general pressurization and two-stage pressurization. The vacuum pump adopts two-stage pressurization and multi-stage pressurization. Multi-stage pressure is usually used for high, fine and fine multilayer boards. The pressure is generally determined according to the pressure parameters provided by the PP supplier, generally 15-35kg/c.
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