How to improve the quality of multilayer board lamination?

In order to thoroughly solve the problem of bubbles generated in the process of multilayer PCB board pressing and improve the adhesion between layers, it is an inevitable trend to adopt vacuum lamination technology. In terms of positioning technology, the dowel free positioning technology (MASSI. AM) has been widely used for 4-6 layers of plates, and X-ray positioning drilling has been applied to improve the positioning accuracy of multi-layer plates. In terms of heating mode, apart from the commonly used electric heating and hot oil heating, Deli Technology (Zhuhai) Co., Ltd. has introduced the laminating technology using copper foil resistance for direct heating, which not only greatly reduces energy consumption, but also uniformly improves the laminating quality.

Due to the rapid development of electronic technology, the printed circuit technology has been developing continuously. PCB boards have developed from single side to double side and multi-layer, and the proportion of multilayer PCB boards is increasing year by year. multilayer PCB boards are characterized by the extreme development of high precision density fine size and small size. Lamination is an important process in multilayer PCB board manufacturing, and the control of laminate quality is becoming increasingly important in multilayer PCB board manufacturing. Therefore, to ensure the laminating quality of multilayer PCB boards, it is necessary to have a good understanding of the laminating process of multilayer PCB boards. Therefore, based on years of laminating practice, I make the following summary on how to improve the laminating quality of multilayer PCB boards in terms of technology:

1、 The inner core plate shall be designed to meet 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 can not be seen or touched. Therefore, it is necessary to reasonably design the inner laminates before laminating. Here are some reference requirements:

1. The core plate thickness shall be selected according to the total thickness requirements of multilayer PCB plates. The core plate thickness shall be consistent, with small deviation, and the cutting direction shall be consistent in longitude and latitude. Especially for multilayer PCB plates with more than 6 layers, the longitude and latitude directions of each inner layer core plate must be consistent, that is, the longitude direction overlaps with the longitude direction, and the latitude direction overlaps with the latitude direction, so as to prevent unnecessary plate bending.

2. There should be a certain distance between the overall dimensions of the core plate and the effective units, that is, the distance between the effective units and the edge of the plate should be as large as possible without wasting materials. Generally, the distance between four layers of plate is required to be greater than 10mm, and the distance between six layers of plate is required to be greater than 15mm. The higher the number of layers, the greater the distance.

3. For the design of locating holes, in order to reduce the deviation between layers of multilayer PCB boards, it is necessary to pay attention to the design of locating holes for multilayer PCB boards: for 4-layer boards, only more than 3 locating holes need to be designed for drilling. In addition to the design of locating holes for drilling, more than 5 overlapping locating rivet holes for layers and more than 5 locating holes for tool plates for rivets shall be designed for multilayer PCB plates with more than 6 layers. However, the higher the number of layers of the designed locating holes, rivet holes, and tool holes are, the more the number of designed holes should be, and the location should be 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 shall be designed to meet the requirements of automatic recognition of target shape by the shooting machine as much as possible, and generally designed as a complete circle or concentric circle.

4. The inner core board shall be free of open, short, open circuit, oxidation, and the board surface shall be clean and free of residual film.

2、 Meet PCB user requirements and select appropriate PP and CU foil configurations.

The customer's requirements for PP are mainly shown in the thickness of the dielectric layer, dielectric constant, characteristic impedance, withstand voltage, and the smoothness of the laminate surface. Therefore, when selecting PP, you can choose according to the following aspects:

1. Resin can fill the gap of printed wire during lamination.

2. The air and volatile matter between laminations can be fully eliminated during lamination.

3. It can provide the necessary medium layer thickness for the multilayer PCB plate.

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

Based on years of production experience, I personally believe that the PP can be configured with 7628, 7630, or 7628+1080, 7628+2116, etc. for 4-layer laminate. 1080 or 2116 are the main PPs for multilayer PCB boards with more than 6 layers, and 7628 is mainly used to increase the thickness of the medium layer. At the same time, PP shall be placed symmetrically to ensure mirror effect and prevent bending.

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

3、 Treatment process of inner core plate

When multilayer PCB boards are laminated, the inner core boards need to be treated. The treatment process of inner layer plate 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, and the thickness of the black oxide film is 0.25-4). 50 mg/cm2. The browning process (horizontal browning) is to form an organic film on the inner copper foil. The role of the inner laminate treatment process is:

1. The specific contact surface between the inner copper foil and the resin is increased to enhance the adhesion between them.

2. Increase the effective wettability of the melt resin on the copper foil when flowing, so that the flowing resin has sufficient ability to penetrate into the oxide film, and show strong grip after curing.

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

4. It can improve the acid resistance of the multilayer PCB board in the wet process and prevent the pink circle. 4、 The control of laminate parameters mainly refers to the organic matching of "temperature, pressure and time".

1: There are several important temperature parameters in the process of temperature and lamination. That is, the melting temperature of the resin, the curing temperature of the resin, the set temperature of the hot disk, the actual temperature of the material and the change of the heating rate. Melting temperature means that the resin begins to melt when the temperature rises to 70 ℃. It is precisely because of the further increase in temperature that the resin further melts and starts to flow. During the period of 70-140 ℃, the resin is easy to flow. It is precisely because of the fluidity of the resin that the resin can be filled with glue and wetted. As the temperature gradually increases, the fluidity of the resin changes from small to large and then to small. 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 better filled and wetted, it is important to control the heating rate. The heating rate is the embodiment of the laminating temperature, that is, to control when and how high the temperature rises. The control of the heating rate is an important parameter of the laminate quality. The heating rate is generally controlled at 2-4 ℃/MIN. The heating rate is closely related to different types and quantities of PP. For 7628PP, the heating rate can be faster, that is, 2-4 ℃/min; for 1080 and 2116PP, the heating rate can be controlled at 1.5-2 ℃/min. At the same time, there is a large amount of PP, and the heating rate cannot be too fast, because the heating rate is too fast, the wettability of PP is poor, the resin fluidity is large, and the time is short, which is easy to cause sliding plates and affect the laminating quality. The temperature of hot plate mainly depends on the heat transfer of steel plate, steel plate and kraft paper, which is generally 180-200 ℃.

2: The pressure and the laminated pressure of the multilayer PCB board are based on the basic principle that whether the resin can fill the interlayer cavity and exhaust the interlayer gas and volatile matter. As the hot press is divided into non vacuum press and vacuum hot press, there is a section of pressurization starting from the pressure. Two stage pressurization and multi stage pressurization. General pressurization and two-stage pressurization are adopted for general non vacuum press. The vacuum pumping unit adopts two-stage pressurization and multistage pressurization. Multi section pressurization is usually used for high, fine and thin multilayer PCB plates. The pressure is generally determined according to the pressure parameters provided by P P suppliers, generally 15-35kg/cm2.

3: The time and time parameters are mainly the control of lamination pressure time, temperature rise time, gel time, etc. For two-stage lamination and multi-stage lamination, the key to control the quality of lamination is to control the time of main pressure and determine the time of conversion from initial pressure to main pressure. If the main pressure is applied too early, it will lead to too much resin extrusion and glue flow, resulting in insufficient glue in the laminate, thin plate, or even sliding plate. If the main pressure is applied too late, it will cause defects such as insecure bonding interface, cavities, or bubbles.