In order to process accurately, it is necessary to make a fine tolerance between the drilling machine and the punching hole. Generally, for paper substrate, the punching should be 0.002 - O. 004 in larger than the drilling machine, while for glass substrate, it is about half of this tolerance. Figure 10-2 shows an example of the required tolerance between the drilling machine and the punching. Punching holes on printed circuit boards is a mechanical operation, just like drilling holes. However, the drilling is not as good as the diameter accuracy of the hole, the lubrication of the hole wall, and the delamination of the pad and the base plate. In general, large holes are slightly punched than small holes. For example, for holes less than 0.9 mm in the reinforced paper substrate and holes less than 1.2 mm in the reinforced glass cloth substrate, punching failure is very common. Therefore, the load of active punching machine depends on the type of substrate and their cutting force. The cutting force accepted by the paper substrate is 1200psi (maximum), while the maximum cutting force accepted by the epoxy glass substrate is 20000psi. Therefore, the paper substrate can accept a pressure of about 16t. In order to improve the safety factor, the acceptance pressure of 32t is often used. The compressive strength of epoxy glass substrate is 70% higher than that of paper based phenyl ester substrate. Even simple boards need high compressive strength. When the active punching machine is used, the edge of the copper foil is always tilted. Therefore, it is not advisable to paint lines on both sides of the board, which will cause the pad to fall. In addition, if the spacing between holes is too small, cracks can appear. In this case, the operating procedure should be changed and the copper foil etching should not be carried out before punching. In this way, the copper foil can play a reinforcing role and help prevent cracks. It is used in mass production of consumer printed circuit board products, and this kind of printed circuit board uses paper based benzene ester and epoxy based substrates. Another defect of punching is that the bonding pad delamination and the substrate crack at the junction hole. In addition, punching will cause the hole to be conical and the surface to be properly thick. Therefore, printed circuit boards that do not meet professional requirements have a high demand for the surface lubrication of plated through-hole. For paper benzol base plates (XXX and similar types), in order to prevent fragmentation, it is required to heat the temperature to 50-70 ℃ in advance before punching. For example, XXXPC and FR-2 substrates can be punched at room temperature, which only needs to be higher than 20 '℃. Non woven reinforced glass substrate (epoxy and polyester) has excellent punching function. Select 50 - 100 μ The die clearance of m can obtain the lubricating hole wall with proper electroplating. The number of printed circuit boards is enough, at least 2000, and punching will be economical. Therefore, large quantities of non plated through-hole reinforced paper substrates are more suitable for punching, while others are suitable for drilling. In the process of punching, the damage rate of small punching is relatively high, because: 1) the alignment is not standardized: it can be found simply through fine inspection tools; 2) Poor description: This is generally because the punching is too small to meet the requirements The following skills can reduce many problems in punching, such as pad delamination. 1) Punching shall be carried out on the copper clad surface; 2) Etch before punching; 3) The punched pad must be large enough.
