The requirement of the upper PCB drilling entry sheet is to have a certain surface hardness to prevent burrs on the upper surface of the drilling. But it must not be too hard and wear the drill bit. It is required that the resin composition of the upper and lower backing plates should not be too high, otherwise molten resin balls will be formed and adhered to the hole wall during drilling. The larger the thermal conductivity, the better, so as to quickly take away the heat generated during drilling, reduce the temperature of the drill when drilling the hole, and prevent the drill from annealing. There must be a certain rigidity to prevent the plate from shaking when the drill is lifted, and a certain elasticity to deform immediately when the drill bit is in contact with the drill bit, so that the drill bit can be accurately aligned with the position to be drilled to ensure the accuracy of the drilling position. The material should be uniform and there should be no nodes with uneven softness and hardness caused by impurities, otherwise the drill bit will be easily broken. If the surface of the upper backing board is hard and slippery, the small-diameter drill bit may slip and deviate from the original hole to drill an oblique elliptical hole on the circuit board.
The upper backing board used in China is mainly 0.2～0.5mm thick phenolic paper rubber sheet epoxy glass cloth board and aluminum foil such as LF2Y2 (No. 2 anti-rust aluminum semi-cold hardening state or LF21Y (21 No. rust-proof aluminum cold work hardening state) As the upper backing plate for ordinary double-sided drilling, it has a good hardness and can prevent burrs on the upper surface of the drilling. Due to the good thermal conductivity of aluminum, it has rigidity and elasticity, which has a certain heat dissipation effect on the drill. The material of aluminum foil is more uniform than that of phenolic plate. There is no impurity. The probability of breaking drills and partial holes is much less than that of phenolic plate. It can reduce the drilling temperature and is an environmentally friendly material. Compared with phenolic board and epoxy board, the holes will not be contaminated by resin because of the resin contained. The thickness of aluminum foil is usually selected as 0.15, 0.20, and 0.30 mm in the process of use. In actual use, the thickness of the aluminum foil is 0.15 and the surface of the board. The contact is the best, but the process is not easy to control during cutting, transportation and use. The price of 0.30 is a bit higher. Generally, 0.20 mm aluminum foil is used as a compromise, and the actual thickness is generally 0.18 mm.
There is a composite upper backing board abroad, the upper and lower layers are 0.06mm aluminum alloy foil, the middle layer is a pure fiber core, and the total thickness is 0.35mm. It is not difficult to see that this structure and material can meet the requirements of the upper backing plate for printed board drilling. It is used for the upper backing plate of high-quality multilayer boards. Compared with aluminum foil, its advantages are: high drilling quality and hole position accuracy High, due to wear and tear, the life of the small drill bit is increased. At the same time, the plate returns to its original shape after being subjected to external force. It is much better than aluminum foil, and the weight is much lighter. It is especially suitable for drilling small holes.
The underlay boards used in China are phenolic paper board, cardboard and wood chip board. The cardboard is soft and easy to produce burrs, but the texture is uniform and it is not easy to break the drill bit and bite the bit, but the price is cheap, and it can be used in thin copper foil or single-sided board. Sawdust board has poor texture uniformity and better hardness than paper board. However, if the copper foil of the drilled circuit board is larger than 35 microns, burrs will be generated. I have tried to use this board to drill a double-sided board with a 70-micron copper foil, but all failed. Phenolic paper board has the best hardness uniformity between the first two, the best use effect, but it is more expensive and not environmentally friendly.
There is also a composite underlay plate abroad. The upper and lower layers are 0.06mm aluminum alloy foil, the middle layer is a pure fiber core, and the total thickness is 1.50mm. Of course, the performance is very outstanding and environmentally friendly, which greatly exceeds the phenolic paper board, especially when drilling multi-layer boards and small diameter holes can fully reflect its advantages, of course, the disadvantage is that it is expensive. Although the use of this auxiliary material has a certain cost, it is actually necessary for the above reasons, because it greatly improves the product qualification rate and reduces the cost.
