With the rapid development of electronic technology, electronic products tend to be miniaturized, complicated and more complete in function. Therefore, for PCB printed circuit boards, it has also developed from the original single-sided board to double-sided board and multi-layer board. High precision, high density, high reliability, and miniaturization have become the major trends in the development of PCB printed circuit boards. Therefore, while the corresponding circuit boards are processed with more and more apertures, the apertures are getting smaller and smaller, and the distance between holes is getting smaller and smaller.
The specifications of printed circuit boards are relatively complicated, and there are many types of products. At present, the most widely used in printed circuit boards is the micro-hole processing technology of epoxy resin-based composite materials (small holes with a diameter of 0.6 mm or less, and micro-holes with a diameter of 0.3 mm or less).
Composite material circuit board has high brittleness, high hardness, high fiber strength, high toughness, low interlayer shear strength, anisotropy, poor thermal conductivity and a large difference in thermal expansion coefficient between fiber and resin. When the cutting temperature is high, it is easy to cut The fiber-matrix interface around the cutting area generates thermal stress; when the temperature is too high, the resin melts and sticks to the cutting edge, resulting in difficulties in processing and chip removal. The cutting force of drilling composite materials is very uneven, and defects such as delamination, burrs and splitting are prone to occur, and the processing quality is difficult to guarantee. This material is extremely abrasive to processing tools, and the tool wear is quite serious. The tool wear will in turn lead to greater cutting force and heat generation. If the heat cannot be dissipated in time, it will lead to low melting point components in the PCB material. Melting and peeling between layers of composite materials. Therefore, PCB composite materials are difficult-to-process non-metallic composite materials, and their processing mechanism is completely different from that of metal materials. At present, the micro hole processing methods mainly include mechanical drilling and laser drilling. This article introduces mechanical drilling to you.
When mechanically drilling PCB materials, the processing efficiency is high, the hole positioning is accurate, and the hole quality is also high. However, when drilling tiny holes, the drill bit is easily broken due to its small diameter, and defects such as material delamination, hole wall damage, burrs and stains may also occur during the drilling process.
Various problems in the mechanical drilling process are directly or indirectly related to axial force and cutting torque. The main factors affecting axial force and torque are feed rate, cutting speed, fiber bundle shape and whether there is a prefabricated hole on the shaft. Force and torque also play a role. The axial force and torque increase with the increase of feed rate and cutting speed. As the feed rate increases, the thickness of the cutting layer increases, while the cutting speed increases, the number of cut fibers per unit time increases, and the tool wear increases rapidly, so the axial force and torque increase.
Axial force can be divided into static component force FS and dynamic component force FD. The components of the axial force have different effects on the cutting edge. The static component FS of the axial force affects the cutting of the chisel edge, while the dynamic component FD mainly affects the cutting of the main cutting edge. The dynamic component FD affects the surface roughness. The influence is greater than the static force component FS. The axial force increases with the feed rate, and the effect of cutting speed on the axial force is not obvious. In addition, in the case of prefabricated holes, when the hole diameter is less than 0.4mm, the static component force FS decreases sharply with the increase of the hole diameter, while the trend of the dynamic component force FD decreases relatively flat.
Due to the different processing properties of the composite matrix and reinforcing fibers, the matrix resin and fibers have different effects on the axial force during mechanical drilling. Khashaba studied the effect of matrix and fiber type on the axial force and torque, and found that the shape of the fiber bundle has a significant effect on the axial force, while the type of matrix resin has little effect on the axial force.
PCB composite micro-drilling wear includes chemical wear and friction wear. The chemical wear is caused by the chemical attack of the Co binder in the micro-drilling material WC-Co cemented carbide by the pyrolysis products released from the PCB material. At about 300°C, this corrosion reaction is more obvious. When the drilling speed is lower than 150mm/min, chemical wear is no longer the main form of wear, and friction wear becomes the main form of wear. The wear of PCB micro-drills is also related to the ratio of cutting speed, feed rate and drill radius to fiber bundle width. The research of Inoue et al. shows that the ratio of the drill bit radius to the fiber bundle (glass fiber) width has a great influence on the tool life. The larger the ratio is, the larger the cutting fiber bundle width is, and the tool wear increases accordingly. In practical application, up to 2,500 holes need to be ground for the new drill bit, 2,000 holes need to be ground after the first grinding, 1,500 holes need to be ground after the second grinding, and 1,000 holes need to be ground after the third grinding.
During PCB microhole machining, the axial force and torque increase with the increase of feed rate and drilling depth, and the main reason is related to the state of chip removal. As the drilling depth increases, it is difficult to discharge chips. In this case, the cutting temperature increases, and the resin material melts and firmly bonds the glass fiber and copper foil fragments to form a tough cutting body. This cutting body has affinity with the PCB matrix material. Once this cutting body is produced, the discharge of chips will stop, and the axial force and torque will increase sharply, resulting in the breakage of the micro-hole drill. The fracture forms of PCB microhole drill bits include buckling fracture, torsional fracture and buckling torsional fracture, and generally the two coexist. The fracture mechanism is mainly chip jamming, which is the key factor causing the increase of drilling torque. Reducing axial force and cutting torque is the key to reducing micro-drill bit breakage.
Drilling damage forms include: delamination, hole wall damage, stains, and burrs. The following are the countermeasures:
1. Layering
Various damages may occur during mechanical drilling of GFRP (glass fiber reinforced) laminates, the most serious of which is interlayer delamination, which leads to a sharp drop in material properties around the hole wall. The axial force exerted by the drill tip is The main reason for stratification. Stratification can be divided into drill-in stratification and drill-out stratification. Drilling into delamination means that when the cutting edge of the drill bit is in contact with the laminate, the cutting force acting in the circumferential direction generates the rotary cutting force in the axial direction through the cutting groove of the drill bit to separate the layers from the layers, forming a delamination area on the surface of the laminate ;Drilling out of delamination means that when the drill bit is approaching the bottom of the laminate, the ability to resist deformation is further reduced due to the thickness of the uncut material becoming thinner. Where the load exceeds the bonding force between the laminates, the Delamination occurred, and this occurred before the laminates were drilled through. Axial force is the main cause of delamination. Cutting speed, base material and fiber bundle type also affect delamination. The drilling-in and drilling-out delamination of epoxy composites decreases with the increase of drilling speed, and the drilling The degree of damage to the delamination is greater than that of the drilled delamination. The main measures to reduce delamination are: the use of variable feed technology, preset pilot holes, the use of backing plates and the use of viscous dampers when drilling without support.
2. Hole wall damage
Drilling micro-holes on composite PCBs, various forms of damage around the holes lead to metallization of the holes, reduced insulation performance between holes and rupture of the copper layer on the hole wall. The relative angle between the cutting direction and the fiber direction, the thickness of the glass fiber bundles on the hole wall, and the position of the drilling point on the glass cloth will all have different effects on the damage of the hole wall.
Document 6 uses a drill bit with a diameter of 1.0mm and a rotating speed of 5000rpm to drill glass fiber/epoxy resin composite materials (8 layers staggered at 90°, each layer is 0.2mm). The fiber folds of layers 1, 3, 5, 7, and 8 protrude greatly, with the maximum protrusion reaching 30 μm; while the fiber folds of layers 2, 4, and 6 protrude slightly, with the smallest point less than 5 μm. In the overlapping intersection area of weft and warp yarns, the thickness of the fiber bundle is the largest at a fiber angle of 45°, and the damage width of the hole wall is the largest; while in the central area, the maximum damage width occurs at an angle close to 90° with the fiber.
Aoyama et al. studied the effect of the cutting edge angle of the tool on the surface roughness of the machined hole wall, and found that when the cutting edge angle is 30°, the surface roughness of the hole wall is the largest, up to 50 μm.
3. Stain
When mechanically drilling composite materials, due to the extrusion of the chisel edge of the drill bit and the composite material, the friction between the inverted cone and the hole wall, and the small chips embedded between the edge of the drill bit and the hole wall, a large amount of cutting is generated The heat melts the resin and adheres to the interlayer of the composite material or the copper foil at the hole and the hole wall to form stains. Appropriate cutting amount and grinding of tiny drill bits can reduce the generation of stains and reduce the stain index.
4. Glitch
When drilling composite materials, due to stress transmission, when the drill bit does not reach the bottom of the hole, many cracks will occur in the reinforcement material and the matrix in front of the drill bit, so that the reinforcement material will be degummed from the matrix, resulting in a pull-out phenomenon, resulting in the failure of the reinforcement material. Cut off at the root. When the hole is drilled, these reinforced materials that have not been cut from the root cannot be discharged together with the chips, but poured to the edge of the hole, and the matrix softens and flows due to the cutting heat, and then re-condenses to these reinforced materials that are poured on the edge of the hole on, forming burrs. The size of the exit burr is mainly affected by the drilling force and drilling temperature. Drilling with cemented carbide drills, changing tool geometry and structure, and using vibration drilling techniques in composite material drilling can reduce burrs.
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