PCB Technical - PCB design multilayer circuit board lamination technology

PCB design multilayer circuit board lamination technology

With the rapid development of electronic technology, it has promoted the continuous development of printed circuit board technology. PCB circuit boards have progressed through the development of single-sided PCBs, double-sided circuit boards, and multilayer circuit boards, and the proportion of PCB multilayer circuit boards has increased year by year. PCB multi-layer boards are also developing towards the extremes of high, fine, dense, fine, large and small. An important process in the manufacture of PCB multilayer circuit boards is lamination. The control of lamination quality becomes more and more important in the manufacture of multilayer boards. Therefore, to ensure the quality of PCB multi-layer board lamination, it is necessary to have a better understanding of the PCB multi-layer circuit board lamination process.

For this reason, multi-layer circuit board manufacturers, based on years of experience in lamination technology, summarize how to improve the quality of multilayer circuit board lamination in process technology as follows:

1. Design an inner core board that meets the requirements of lamination

Due to the gradual development of laminating machine technology, the heat press has changed from the previous non-vacuum heat press to the current vacuum heat press. The heat pressing process is in a closed system, which is invisible and intangible. Therefore, a reasonable design of the PCB inner layer board is required 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 between the effective unit and the edge of the PCB should be as large as possible without wasting materials. Generally, a four-layer circuit board The required spacing is greater than 10mm, and the six-layer circuit board requires a spacing greater than 15mm. The higher the number of layers, the greater 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 selected according to the total thickness of the PCB multi-layer circuit board. The thickness of the core board is consistent, the deviation is small, and the latitude and longitude directions of the blanking are consistent, especially for the PCB multi-layer boards with more than 6 layers. To be consistent, that is, the warp direction overlaps the warp direction, and the weft direction overlaps the weft direction to prevent unnecessary board bending.

4. The design of positioning holes, in order to reduce the deviation between layers of PCB multi-layer circuit boards, it is necessary to pay attention to the design of positioning holes of PCB multi-layer boards: 4-layer boards only need to design more than 3 positioning holes for drilling. Can. In addition to the positioning holes for drilling, the multi-layer PCB circuit boards with more than 6 layers need to be designed with more than 5 layer and layer overlapping positioning rivet holes and more than 5 tool board positioning holes for rivets. However, the designed positioning holes, rivet holes, and tool holes are generally higher in the number of layers, and the number of designed holes should be correspondingly larger, and the position should be as close to the side as possible. The main purpose is to reduce the layer-to-layer alignment deviation and leave a larger space for production. The target shape is designed to meet the requirements of the shooting machine to automatically identify the target shape as much as possible, and the general design is a complete circle or concentric circle.

Two, to meet the requirements of PCB circuit board users, select the appropriate PP, CU foil configuration

Customers’ requirements for PP are mainly manifested in the requirements of dielectric layer thickness, dielectric constant, characteristic impedance, withstand voltage, and 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 the PCB multilayer circuit board;

4. It can fully remove the air and volatile matter between the laminations during lamination;

5, CU foil is mainly configured with different models according to the requirements of PCB circuit board users, and the quality of CU foil conforms to IPC standards.

Three, 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 and browning treatment. The oxidation treatment process is to form a black oxide film on the inner 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 functions of the inner layer board treatment process are:

1. Increase the contact surface of the inner copper foil and the resin to enhance the bonding force between the two;

2. Improve the acid resistance of the multilayer circuit board in the wet process process and prevent the pink ring;

3. Prevent the decomposition of the curing agent dicyandiamide in the liquid resin at high temperatures-the effect of moisture on the copper surface;

4. Increase the effective wettability of the molten resin to the copper foil when the molten resin flows, so that the flowing resin has sufficient capacity to stretch into the oxide film, and show a strong grip after curing.

Fourth, organic matching of lamination parameters

The control of PCB multi-layer board lamination parameters mainly refers to the organic matching of lamination "temperature, pressure, and time".

1, temperature

Several temperature parameters are more 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, the resin starts to melt. It is because of the further increase in temperature that the resin further melts and starts to flow. During the period of temperature 70-140 degree Celsius, the resin is easy to fluid. It is precisely because of the fluidity of the resin that the filling and wetting of the resin can be ensured. As the temperature gradually rises, the fluidity of the resin goes 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 better fill 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 the temperature rises to how high. The control of the heating rate is an important parameter for the quality of PCB multilayer laminates, and the heating rate is generally controlled at 2-4°C/MIN. The heating rate is closely related to the different types and quantities of PP.

For 7628PP, the heating rate can be 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 of the resin is poor, the resin has high fluidity, and the time is short. It is easy to cause slippage and affect the quality of the laminate. The temperature of the hot plate mainly depends on the heat transfer of the steel plate, steel plate, corrugated paper, etc., generally 180-200°C.

2, pressure

PCB multilayer laminate pressure is based on whether the resin can fill the voids between layers and exhaust 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 ways to start from the pressure: one-stage pressurization, two-stage pressurization and multi-stage pressurization. Generally, non-vacuum presses use general pressurization and two-stage pressurization. The vacuum machine adopts two-stage pressurization and multi-stage pressurization. Multi-stage compression 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/cm2.

3. Time

Time parameters are mainly the control of the timing of lamination and press, the control of the timing of temperature rise, and the gel time. For two-stage lamination and multi-stage lamination, controlling the timing of the main pressure and determining the transition time from the initial pressure to the main pressure is the key to controlling the quality of the lamination. If the main pressure is applied too early, it will cause the resin to be extruded and too much glue, which will cause the laminate to lack glue, the board is thin, and even the slippery board. If the main pressure is applied too late, it will cause defects such as weak, void, or air bubbles in the lamination bonding interface.