PCB design multilayer circuit board lamination technology
With the rapid development of electronic technology, the development of printed circuit board technology has been promoted. PCB boards are gradually evolving through the development of single-sided, double-sided and multi-layer, and the proportion of PCB multi-layer boards has increased year by year. PCB multi-layer boards are also moving towards two extremes: high, fine, dense, fine, large and small. Lamination is an important process in the manufacture of PCB multilayer boards. Control of laminate quality is becoming more and more important in the manufacture of PCB multilayer boards. Therefore, in order to ensure the lamination quality of the PCB multilayer board, it is necessary to better understand the lamination process of the PCB multilayer board.
To this end, PCB manufacturers have summarized how to improve the lamination quality of multilayer PCBs based on years of experience in lamination technology, as follows:
First, the inner core board design that meets the lamination requirements
Due to the gradual development of laminating machine technology, the hot press is from the original non-vacuum hot press to the current vacuum hot press. The hot pressing process is in a closed system and cannot be seen and cannot be touched. Therefore, it is necessary to properly design the laminate in the PCB before lamination, and some reference requirements are provided here.
1. The size of the core board should be kept at a certain distance from the effective components, that is, the distance between the effective component and the edge of the PCB should be as large as possible, and no material is wasted. Generally, the distance between the four-layer board and the six-layer board should be greater than 10 mm, and the more the number of layers, the larger the distance.
2. The core board of the PCB board does not need to be open, short circuit, open circuit, no oxidation, clean board surface, no residual film.
3. Select the core thickness according to the requirements of the total thickness of the PCB multilayer board. The thickness of the core plate is the same, the deviation is small, and the latitude and longitude directions are the same. Especially for PCB multilayer boards of more than six layers, the latitude and longitude directions of each inner core board must be the same, that is, the latitude and longitude directions overlap, and the latitude and latitude directions overlap to prevent unnecessary bending. Board of Directors.
4. In order to reduce the deviation between the PCB multilayer and the layer, we should pay attention to the design of the PCB multi-layer positioning hole. The design of the four-layer board requires only three or more positioning holes. In addition to designing the drilling locating holes, a multi-layer PCB board of 6 or more layers requires more than 5 overlapping layer positioning rivets and more than 5 rivet tool board locating holes. However, the design of the positioning hole, the rivet hole, and the tool hole is generally the number of layers, the more the number of holes is designed, and the position of the side is as much as possible. The main purpose is to reduce the alignment deviation between the layers and leave more room for production and manufacturing. The target shape design satisfies the requirements of the shooter to automatically recognize the shape of the target, usually designed as a complete circular or concentric circle.
2. Meet the requirements of PCB board users, choose the appropriate PP and CU foil configuration
Customer requirements for PP are mainly reflected in dielectric layer thickness, dielectric constant, characteristic impedance, withstand voltage and laminate surface smoothness. Therefore, you can choose PP according to the following aspects:
1. It can ensure the bonding strength and smooth appearance.
2. When laminating, the resin can fill the gap of the printed guide line.
3. Provides the necessary dielectric layer thickness for PCB multilayer boards.
4. The air and volatile materials between the laminates can be sufficiently removed during the lamination process.
5. The quality of the CU foil complies with the IPC standard.
3. Inner core board processing technology
When the PCB multilayer laminate is laminated, the inner core sheet needs to be processed. The processing of the inner panel includes black oxidation treatment and browning treatment. The oxidation treatment process forms a black oxide film on the inner copper foil. The black oxide film has a thickness of 0.25-4). 50 mg / cm 2 . The browning process (horizontal browning) is the formation of an organic film on the inner copper foil. The inner board processing has the following functions:
1. Increase the specific surface of the inner copper foil in contact with the resin to enhance the adhesion between the two.
2. Increase the acid resistance of the multilayer board during the wet process and prevent the pink circle.
3. Inhibit the effect of curing agent dicyandiamide on the surface decomposition of copper at high temperature.
4. The effective wettability of the molten resin to the copper foil is increased during the flow, so that the flowing resin has sufficient ability to extend into the oxide film and exhibits strong grip after curing.
4. Organic matching of lamination parameters
The control of the lamination parameters of the PCB multi-layer board mainly refers to the organic matching of the lamination temperature, pressure and time.
Several temperature parameters are important during 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 heating rate. When the melting temperature rose to 70 ° C, the resin began to melt. It is because of the further increase in temperature that the resin further melts and begins to flow. The resin is easy to flow during the time of 70-140 °C. It is because of the fluidity of the resin that the filling and wetting of the resin can be ensured. As the temperature increases, the fluidity of the resin undergoes a small to large, then small change, and finally, when the temperature reaches 160-170 ° C, the fluidity of the resin is 0, which is called the curing temperature.
In order to make the resin more filling and wetting, it is very important to control the heating rate. The heating rate is an embodiment of the lamination temperature, that is, the time and temperature at which the temperature rise is controlled. The control of heating rate is an important parameter for the quality of PCB laminate lamination. The heating rate is usually controlled at 2-4 ° C / min. The heating rate is closely related to the different types and amounts of PP.
The heating rate of 7628PP can be faster, ie 2-4C / min, 1080 and 2116PP can be controlled at 1.5-2C / MIN, and the amount of PP is very large, the heating rate can not be too fast, because the heating speed is too fast, the wetting of PP Poor properties, large resin fluidity, short time, easy to cause skateboard, affecting the quality of lamination. The temperature of the hot plate mainly depends on the heat transfer of the steel plate, steel plate, kraft paper, etc., and is generally 180 - 200 °C.
The lamination pressure of the PCB multi-layer board is based on the basic principle that the resin can fill the inter-layer voids and discharge the interlayer gas and volatile substances. Since the hot press is divided into a non-vacuum press and a vacuum pump press, there are several ways in which the pressure starts from the pressure: the first stage pressure, the two stage pressure, and the multistage pressure. General pressure and two-stage pressure are used for non-vacuum presses. The vacuum unit uses two stages of pressure and multiple stages of pressure. Multistage pressures are commonly applied to high, fine and fine multilayer boards. The pressure is usually determined by the pressure parameters provided by the PP supplier, typically 15-35 kg / cm2.
The time parameter is mainly controlled by factors such as lamination pressure time, heating time, and gel time. For two-stage and multi-stage lamination, controlling the lamination quality to control the time of the main pressure and determining the transition time from the initial pressure to the main pressure is critical. If the main pressure is applied too early, it will lead to excessive resin extrusion and glue flow, resulting in insufficient glue for laminates, sheets and even skateboards and other undesirable phenomena. If the applied main pressure is too late, the bonding interface will become weak, ineffective or bubble.