How to select backup when drilling circuit board?

　　The requirement of upper pad for PCB drilling is: certain surface hardness to prevent the surface burr of drilling. But it can't be too hard and wear off the bit. The resin composition of the upper and lower pad itself should not be too high, otherwise the drilling will form a molten resin ball adhering to the hole wall. The greater the thermal conductivity, the better, so that the heat generated from the drill hole can take away quickly, reduce the temperature of the drill hole, and prevent the drill bit from annealing. To have a certain rigidity to prevent the PCB plate vibration, and to have a certain elasticity when the drill bit drill contact moment immediately deformation, so that the drill bit accurately aligned at the drilling position, to ensure the accuracy of the drilling position. Material should be uniform impurities can not produce uneven soft and hard nodes, otherwise it is easy to break the bit. If the surface of the upper pad is hard and slippery, the small diameter bit may slip away from the original hole position and drill an elliptical inclined hole on the PCB.

　　The domestic pad mainly 0.2~0.5mm thick epoxy glass cloth plate and aluminum foil such as 0.3mm thickness LF2Y2 (No.2 aluminum semi-cold hardening state or LF21Y (no. 21 aluminum cold hardening state) as ordinary double panel drilling pad effect is better, achieve hardness can prevent the drilling surface burr. Because of the good thermal conductivity of aluminum and rigid elasticity, it has a certain heat dissipation effect on the bit. The material of aluminum foil is more uniform than the phenolic plate, and the partial hole cut is much less than the phenolic plate. Can reduce the drilling temperature and is an environmentally friendly material, many factories have used aluminum foil as the pad, compared with phenolic plate, epoxy plate, may not make the hole resin pollution, the commonly used aluminum foil thickness is 0.15,0.20,0.30 mm in the actual use of contact with PCB plate surface is the best but in cutting and transportation process and process control, 0.30 price is higher, generally use 0.20 mm aluminum foil, the actual thickness is generally 0.18 mm.

　　There is a composite upper pad abroad, the upper and lower two layers are 0.06mm aluminum alloy foil, the middle layer is pure fiber core, the total thickness is 0.35mm.It is not difficult to see that this structure and material can meet the requirements of the printed board drilling pad, for high quality multilayer pad, compared with aluminum foil its advantages are: high drilling quality, hole precision, due to wear small bit life increase, at the same time PCB plate back after the original shape is much better than aluminum foil, weight is much lighter, especially suitable for drilling holes.

　　The lower pad board used in China has phenolic paper board, cardboard board, wood chip board. Cardboard is softer and easy to produce burr, but uniform texture is not easy to break and bite the drill bit, but cheap, can be used in thin copper foil or single panel. The uniformity of wood board is poor, the hardness is better than the paper board, but if the drilled PCB copper foil greater than 35 micron will produce burr, I tried to use the double panel of 70 micron copper foil, the results all can not pass. The hardness of phenolic paper board is best uniform between the first two, the use effect is best but more expensive and not environmentally friendly.

　　Similarly, there is a composite lower pad, the upper and lower two layers are 0.06mm aluminum alloy foil, the middle layer is pure fiber core, the total thickness is 1.50mm.Of course, the performance is very outstanding and environmental protection, much more than the phenolic paper board, especially when drilling multi-layer board and small diameter holes can fully reflect its advantages, of course, the disadvantage is expensive.

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