PCB Tech - Summary of Technology for Improving Laminate Quality of Multilayer Circuit Board

Summary of Technology for Improving Laminate Quality of Multilayer Circuit Board

Due to the rapid development of electronic technology, the continuous development of printed circuit technology has been promoted. PCB boards have developed through single-sided-double-sided and multi-layered, and the proportion of multi-layer circuit boards has been increasing year by year. The performance of the multi-layer board is developing to the extremes of high-precision, large and small. An important process of multilayer board manufacturing is lamination. The control of lamination quality is becoming more and more important in multilayer board manufacturing. Therefore, to ensure the quality of multi-layer circuit board lamination, it is necessary to have a better understanding of the multi-layer circuit board lamination process. For this reason, based on many years of lamination practice, how to improve the quality of multi-layer board lamination is summarized as follows in process technology:

1. The inner core board is designed to meet 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 press process is in a closed system, invisible and intangible. Therefore, it is necessary to design the inner layer board reasonably before lamination. Here are some reference requirements:

1. The thickness of the core board should be selected according to the total thickness of the multilayer 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 multilayer circuit boards with more than 6 layers. The latitude and longitude directions of the inner core boards must be consistent, That is, the warp direction overlaps the warp direction, and the weft direction overlaps the weft direction to prevent unnecessary plate bending.

2. There must be a certain distance between the outer dimensions of the core board and the effective unit, that is, the distance between the effective unit and the edge of the board should be as large as possible without wasting materials. Generally, the distance between the four-layer board is greater than 10mm, The six-layer board requires a spacing greater than 15mm, and the higher the number of layers, the greater the spacing.

3. The design of the positioning hole is to reduce the deviation between the layers of the multilayer board, so it is necessary to pay attention to the design of the positioning hole of the multilayer circuit board: the 4-layer board only needs to design more than 3 positioning holes for drilling. For multi-layer boards with more than 6 layers, in addition to designing positioning holes for drilling, it is also necessary to design more than 5 layer-to-layer 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 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.

4. The inner core board requires no opening, short, open circuit, no oxidation, clean board surface, and no residual film.

Second, to meet the requirements of PCB board users, select the appropriate PP and 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. Resin can fill the gaps of the printed wires during lamination.

2. It can ensure the bonding strength and smooth appearance.

3. It can provide the necessary dielectric layer thickness for multilayer circuit boards.

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

Based on many years of production experience, I personally think that PP can be configured with 7628, 7630 or 7628+1080, 7628+2116 when 4-layer laminates are laminated. The choice of PP for multi-layer boards with more than 6 layers is mainly 1080 or 2116, and 7628 is mainly used as PP to increase the thickness of the dielectric layer. At the same time, PP requires symmetrical placement to ensure mirror effect and prevent plate bending.

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

Three, inner core board processing technology

When multi-layer circuit boards are 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. 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.

3. Prevent the decomposition of the curing agent dicyandiamide in the liquid resin at high temperature-the influence of moisture on the copper surface.

4. Improve the acid resistance of the multilayer circuit board in the wet process and prevent pink circles.

Fourth, the organic matching of the lamination parameters The control of the lamination parameters of the multilayer circuit board mainly refers to the organic matching of the "temperature, pressure, and time" of the lamination.

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 increases, 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 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. Time and 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.

3, pressure, multilayer circuit board lamination 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 is a period of pressurization from the pressure. There are several ways of 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.

Therefore, how to determine the lamination temperature, pressure, and time software parameters is the key technology for multi-layer laminate processing. According to many years of practical experience in lamination, it is believed that the "temperature, pressure, and time" of the lamination software parameters are organically matched, and only the pressure is tested first. On the basis of OK, the most ideal "temperature, pressure, time" software parameters can be determined. But the "temperature, pressure, time" parameters can be determined according to different PP combination structures, different PP suppliers, different PP models, and different characteristics of the PP itself to determine the corresponding lamination parameters.