Multilayer board lamination technology

In order to completely solve the problem of bubbles generated during the pressing of multilayer boards and improve the interlayer adhesion, the use of vacuum lamination technology is an inevitable trend. In terms of positioning technology, pinless positioning technology (MASSI.AM) has been widely used for 4-6 layer boards, and X-ray positioning drilling has been applied to improve the positioning accuracy of multilayer boards. In terms of heating methods, in addition to the commonly used electric heating and hot oil heating, Deli Technology (Zhuhai) Co., Ltd. has introduced lamination technology using copper foil resistors for direct heating, which not only greatly reduces energy consumption, but also heats evenly and improves the lamination quality.

PCB vacuum lamination unit

Due to the rapid development of electronic technology, the continuous development of printed circuit technology has been promoted. PCB boards have developed from single-sided to double-sided to multi-layer, and the proportion of multilayer boards has increased year by year. Multilayer boards are showing the development towards two extremes: highprecisionfine*large and small.

An important process in the manufacture of 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 multi-layer boards, it is necessary to have a better understanding of the lamination process of multi-layer boards. For this reason, I have made the following summary on how to improve the lamination quality of multi-layer boards in terms of process technology based on many years of lamination practice:

1.Design inner core boards that meet the lamination requirements.

Due to the gradual development of lamination machine technology, the hot press machine has changed from the previous non-vacuum hot press machine to the current vacuum hot press machine. The hot pressing process is in a closed system, which cannot be seen or touched. Therefore, the inner layer board needs to be reasonably designed before lamination. Here are some reference requirements:

(1)The core board thickness should be selected according to the total thickness requirements of the multi-layer board.

The core board thickness should be consistent, the deviation should be small, and the warp and weft directions of the cutting materials should be consistent. Especially for 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 to prevent unnecessary board bending.

(2)There should be a certain distance between the outer dimensions of the core board and the effective unit

that is, the distance from the effective unit to the edge of the board should leave as much space as possible without wasting materials. Generally, the spacing of the four-layer board is required to be greater than 10mm, and the spacing of the six-layer board is required to be greater than 15mm. The higher the number of layers, the larger the spacing.

(3)The design of the positioning holes. In order to reduce the deviation between the layers of the multi-layer board, it is necessary to pay attention to the design of the positioning holes of the multi-layer board:

4-layer boards only need to design more than 3 positioning holes for drilling.

Multi-layer boards with more than 6 layers need to design more than 5 overlapping positioning rivet holes between layers and more than 5 tool plate positioning holes for rivets in addition to the positioning holes for drilling.

However, the designed positioning holes, rivet holes, and tool holes are generally designed with more holes for higher layers, and the positions are as close to the edge as possible.

The main purpose is to reduce the alignment deviation between layers and leave more space for production and manufacturing. The target shape design should meet the requirements of the target shape automatic recognition of the target shape by the target shooting machine as much as possible, and is generally designed as a complete circle or concentric circles.

1. The inner core board requires no open, short, short circuit, no oxidation, clean board surface, and no residual film.
2. Meet the requirements of PCB users and choose the appropriate PP and CU foil configuration.

Customers’ requirements for PP are mainly reflected in the requirements of dielectric layer thickness, dielectric constant, characteristic impedance, withstand voltage, and smoothness of laminate surface. Therefore, when choosing PP, you can choose according to the following aspects:

1. Resin can fill the gaps of printed conductors during lamination.
2. It can fully exclude air and volatiles between laminates during lamination.
3. It can provide the necessary dielectric layer thickness for multilayer boards.
4. It can ensure bonding strength and smooth appearance.

Based on many years of production experience, I personally think that PP can be configured with 7628, 7630 or 7628+1080, 7628+2116 when laminating 4-layer boards. The PP selection for multilayer 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 is required to be placed symmetrically to ensure the mirror effect and prevent board bending.

5. CU foil is mainly configured in different models according to the requirements of PCB users, and the quality of CU foil meets IPC standards.
6. Inner core board processing process

When laminating multilayer boards, the inner core board needs to be processed. The processing processes of the inner board include black oxidation processing and browning processing. The oxidation processing is to form a layer of black oxide film on the inner copper foil, and the thickness of the black oxide film is 0.25-4). 50mg/cm2. The browning processing (horizontal browning) is to form a layer of organic film on the inner copper foil. The functions of the inner board processing process are:

1. Increase the specific surface area of ​​the inner copper foil in contact with the resin to enhance the bonding force between the two.
2. Increase the effective wettability of the molten resin on the copper foil when it flows, so that the flowing resin has sufficient ability to extend into the oxide film and show strong grip after curing.
3. Block the decomposition of the curing agent dicyandiamide in the liquid resin at high temperature and the influence of moisture on the copper surface.
4. Improve the acid resistance of the multilayer board in the wet process and prevent pink circles. 4. Organic matching of lamination parameters The control of the lamination parameters of the multilayer board mainly refers to the organic matching of the three lamination “temperature, pressure, and time”.

1: Temperature, there are several important temperature parameters 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 change in the speed of heating. The melting temperature is when the temperature rises to 70℃ and the resin begins to melt. It is because of the further increase in temperature that the resin further melts and begins to flow. During the period of 70-140℃, the resin is a fluid, and it is because of the flowability of the resin that the filling and wetting of the resin are guaranteed.

As the temperature gradually rises, the fluidity of the resin changes from small to large, then to small, and finally when the temperature reaches 160-170℃, 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 better, it is very important to control the heating rate. The heating rate is the specific 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 multilayer boards. The heating rate is generally controlled to be 2-4℃/MIN. The heating rate is closely related to different PP models and quantities. The heating rate for 7628PP can be faster, that is, 2-4℃/min, and the heating rate for 1080 and 2116PP is controlled at 1.5-2℃/MIN. At the same time, if the number of PP is large, the heating rate cannot be too fast, because if the heating rate is too fast, the wettability of PP is poor, the resin fluidity is large, and the time is short, it is easy to cause sliding, affecting the lamination quality. The temperature of the hot plate mainly depends on the heat transfer of the steel plate, steel plate, leather kraft paper, etc., which is generally 180-200℃.

2: Pressure, the lamination pressure of the multi-layer board is based on whether the resin can fill the interlayer voids and exhaust the interlayer gas and volatiles. Since the hot press is divided into non-vacuum press and vacuum hot press, there are several ways of one-stage pressurization, two-stage pressurization and multi-stage pressurization from the pressure. Generally, non-vacuum presses use general pressurization and two-stage pressurization. Vacuum presses use two-stage pressurization and multi-stage pressurization. Multi-stage pressurization 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, which is generally 15-35kg/cm2.

3: Time, time parameters are mainly the control of the timing of lamination pressurization, the control of the timing of heating, gel time and other aspects. For two-stage lamination and multi-stage lamination, controlling the timing of the main press and determining the conversion time from the initial press to the main press are the key to controlling the quality of the lamination. If the main pressure is applied too early, it will cause the resin to be squeezed out and too much glue to flow, resulting in defects such as lack of glue, thin boards, or even slipping. If the main pressure is applied too late, it will cause defects such as loose lamination interface, voids, or bubbles.