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PCB Tech

PCB Tech

Summary of technology to improve the quality of multilayer PCB laminates
2021-10-12
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 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 PCBs has increased year by year. The performance of multi-layer PCB boards is developing towards the extremes of high*precision*dense*fine*large and small. An important process in multilayer PCB manufacturing is lamination. The control of lamination quality becomes more and more important in multilayer PCB manufacturing. Therefore, to ensure the quality of multilayer PCB lamination, it is necessary to have a better understanding of the multilayer PCB lamination process. For this reason, based on many years of lamination practice, how to improve the quality of multi-layer PCB 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 pressing process is in a closed system, which is 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 multi-layer PCB board. The thickness of the core board is consistent, the deviation is small, and the latitude and longitude directions of the blanking are the same, especially for multi-layer PCB 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.
 

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3. The design of positioning holes, in order to reduce the deviation between the layers of the multi-layer PCB board, so it is necessary to pay attention to the design of the positioning holes of the multi-layer PCB board: the multilayer PCB manufacturing only needs to design more than 3 positioning holes for drilling. . For multi-layer PCB boards with more than 6 layers, in addition to positioning holes for drilling, it is also necessary to design more than 5 layer-to-layer overlapping positioning rivet holes and more than 5 positioning holes for the tool board 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 users, select the appropriate PP and CU foil configuration.
 
  Customers’ requirements for PP are mainly expressed in terms 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 fully eliminate the air and volatile matter between the laminations during lamination.
 
  3. It can provide the necessary dielectric layer thickness for multi-layer PCB boards.
 
  4. It can ensure the 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 4-layer laminates are laminated. The choice of PP for multi-layer PCB 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 users, and the quality of CU foil conforms to IPC standards.
 
   Three, inner core board processing technology
 
   When multi-layer PCB 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 temperatures—the effect of moisture on the copper surface.
 
  4. The multi-layer PCB board can improve the acid resistance and prevent the pink circle in the wet process. 4. Organic matching of lamination parameters The control of lamination parameters of multilayer PCB mainly refers to the organic matching of lamination "temperature, pressure, and time".
 
   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℃, the resin is easy to fluid. It is because of the fluidity of the resin that the filling and wetting of the resin can be ensured. As the temperature gradually rises, 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 PCB 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.