Editor: Explain how to improve the laminating quality of multilayer boards?
The rapid development of electronIC technology has promoted the continuous development of circuit board making technology. The printed circuit board has developed from the original single layer board to the double layer board, and then to the Multilayer board with more than ten layers now. Therefore, the production process of printed boards is getting higher and higher. Multilayer board lamination is one of the most important processes. The quality of multilayer board lamination directly affects the overall quality of printed circuit board. How to improve the laminated quality of multilayer boards is a problem that PCB manufacturers can not ignore.
How can we improve the laminate quality of multilayer boards? We suggest starting from the following aspects:
1、 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. Be able to provide the necessary medium layer thickness for the multilayer plate.
4. It can ensure the bonding strength and smooth appearance.
5. CU foil is mainly configured with different models according to PCB user requirements, and the quality of CU foil meets IPC standards.
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 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.
2、 The inner core plate 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:
The thickness of core plate shall be selected according to the total thickness requirements of multilayer plates. The thickness of core plate shall be consistent, the deviation shall be SMAll, and the cutting direction shall be consistent in longitude and latitude. Especially for multilayer 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 to prevent unnecessary plate bending.
2. There should be a certain distance between the overall dimension of the core plate and the effective unit, that is, the distance from the effective unit to the edge of the plate should be as large as possible without wasting materials. Generally, the distance between four layers of plate should be greater than 10mm, and the distance between six layers of plate should 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 boards, attention should be paid to the design of locating holes for multilayer boards: for 4-layer boards, only more than 3 locating holes should 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 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 plate shall be free of open circuit, short circuit, open circuit, oxidation, clean surface and residual film.
3、 Treatment process of inner core plate
When multilayer 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. Increase the specific surface of the inner copper foil in contact with the resin, so as to enhance the adhesion between the two.
2. Increase the effective wettability of the molten 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 the curing agent dicyandiamide in the liquid resin at high temperature - the influence of water on the copper surface.
4. Improve the acid resistance of the multilayer 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".
4.1 Temperature and several temperature parameters are important during 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 ℃.
4.2 The pressure and multilayer laminate pressure 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 plates. The pressure is generally determined according to the pressure parameters provided by P P suppliers, generally 15-35kg/cm2.
4.3 Time and time parameters mainly refer to the control of lamination pressurization time, temperature rise time and gel time. 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.
Therefore, how to determine the laminating temperature, pressure, and time software parameters is the key technology for multilayer laminate processing. According to years of practical experience in laminating, it is believed that the laminating software parameters "temperature, pressure, and time" can be matched organically. Only on the basis of successful pressure test can the most ideal "temperature, pressure, and time" software parameters be determined. However, the "temperature, pressure and time" parameters can be determined according to different PP combination structures, different PP suppliers, different PP models, and different PP characteristics. After lamination, it is inspected by Qc to meet the design requirements. In the process of producing printed circuit boards, as long as the production is strictly in accordance with the above requirements and relevant process technical indicators, the laminated quality of multilayer boards can be effectively improved. Circuit board manufacturing, circuit board design and PCBA processing manufacturers explain to you how to improve the laminated quality of multilayer boards?