Semi-curable sheet is a major category of products sold as commodities in the PCB FR-4 copper clad plate industry. When making multilayer printed boards, PCB factories use it to bond the prepared inner boards. Previously, the production of multilayer printed circuit boards required at least two semi-cured sheets for each layer, so that the resin could fill the gap between the inner layers and make up for the possible defects of each semi-cured sheet. However, with the increasing thinness of multilayer pcb boards and the need to reduce manufacturing costs, it is not uncommon to use only one prepreg per layer when laminating multilayer pcb boards. Therefore, the quality requirements and appearance requirements of the semi-cured film are also higher and higher.
The surface smoothness of the semi-curing sheet shall be good, and the rubber particles shall be less, and there shall be no fish eye, lack of glue, exposed cloth, impurities, black lines, colored lines, and the warp and weft shall not be skewed. During the production process, the tension of the winding device should be constant, because the winding speed will decrease linearly with the increase of the coil diameter. The constant tension can ensure the winding quality of the semi-cured film, and avoid the defects such as over-tight winding and creases. The levelness and perpendicularity of the guide rollers of the gluing machine should be adjusted properly, and the winding core should have high accuracy. The operator shall track and observe the winding process and timely eliminate the nonconforming products. For PCB factories with good conditions, the CCD semi-solidified film appearance tester can be used. For semi-solidified film with defects in appearance, it will automatically record and alarm the operator for prompt treatment.
The quality of glass cloth used in the production of semi-cured sheet is higher than that of glass cloth used in the production of FR-4 copper clad plate, and the production process conditions are also different from that of adhesive sheet, so that the semi-cured sheet with higher quality factors can be produced.
For the problem of fish eye, it can be considered to add an appropriate amount of fluorocarbon surfactant into the glue. Because the fluorocarbon chain structure is far more stable than the hydrocarbon structure, it has extremely high thermal stability and chemical resistance; On the other hand, it has extremely high surface activity, which can significantly reduce the surface tension of aqueous solution at very low application concentration (for example, adding only a few parts per 10000 in the solution can reduce the surface tension of aqueous solution to below 20dyn/cm). Therefore, adding an appropriate amount of fluorocarbon surfactant into the glue can improve the permeability of the glue, prevent fish eye formation, and significantly improve the heat resistance and chemical resistance of the substrate, which is very suitable for application on lead-free and high-temperature plates. For thin semi-cured sheets, such as products with thickness less than 1080, they should be produced on the gluing machine with low oven height and low tension system.
In addition, for PCB factories with high requirements for use, it is required that there should be no bubbles on the semi-curing sheet and micro-bubbles should be eliminated as much as possible. The solution is to adjust the proportion of solvent with different boiling points in the glue solution, so that the solvent on the semi-cured sheet soaked in the glue solution will gradually volatilize. At the same time, adjust the temperature of each temperature zone of the drying oven of the gluing machine, especially the temperature of the upper section of the drying oven. At the lower part of the upper section of the drying oven, the temperature should be appropriately reduced to allow the low boiling point solvent to volatilize first. If necessary, it is necessary to adjust the production speed, and the combination of several measures is an important means to eliminate bubbles and micro-pores on the semi-curing sheet.