PCB Tech - PCB circuit board CNC Drilling Machine: Use of Backing Plate

PCB printed circuit board drilling, the use of upper and lower pads is to prevent the surface and bottom of the circuit board from blooming and producing burrs, so that the drilling surface of the is smooth, improving the quality of the PCB printed circuit board and increasing the yield rate. Because the use of this auxiliary material has a certain cost, it is actually necessary for the above reasons, which can greatly increase the qualification rate of the product and reduce the cost.

The requirement of the upper backing plate for PCB drilling is: the PCB surface has a certain hardness to prevent burrs on the upper surface of the drilling. But it should 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 plates used in China are mainly 0.2～0.5mm thick phenolic paper rubber sheets, epoxy glass cloth sheets and aluminum foils such as LF2Y2 (No. 2 anti-rust aluminum semi-cold hardened state or LF21Y (No. 21 anti-rust aluminum) with a thickness of 0.3 mm. 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. Because of the good thermal conductivity of aluminum, it has rigidity and elasticity, and has a certain heat dissipation effect on the drill. The material of aluminum foil Compared with phenolic board, the probability of breaking drills and partial holes due to no impurities is much less than that of phenolic board. It can reduce the drilling temperature and is an environmentally friendly material. Compared with the oxygen plate, the hole will not be contaminated by the resin because of the resin contained. The thickness of the commonly used aluminum foil during use is 0.15, 0.20, and 0.30 mm. The contact between 0.15 and the surface of the plate is the best in actual use. However, it is not easy to control the process 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 PCB printed circuit 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, holes High positioning accuracy, longer life of the drill bit due to wear, and at the same time, the sheet 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 its hardness is better than that of paper board. However, if the PCB copper foil drilled is larger than 35 microns, burrs will occur. 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.