Micro-hole Processing Technology of Circuit Board Composite（2）
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Update time : 2017-07-19 16:06:46
2 Mechanical drilling
Mechanical drilling PCB material, the processing efficiency is higher, the hole positioning accuracy, the quality of the hole is also higher. However, when drilling small holes, because the drill bit diameter is too small, easy to break, drilling process may also appear material stratification, hole wall damage, burrs and stains and other defects.
2.1 Cutting force
The various problems that occur during the machining process are directly or indirectly related to axial forces and cutting torques. The main factors affecting axial force and torque are feed rate, cutting speed, fiber bundle shape, and presence or absence of preform Force and torque also have an impact. The axial force and torque increase with the increase of cutting speed and cutting speed. As the feed rate increases, the thickness of the cutting layer increases, and the cutting speed increases, the number of cutting fibers per unit time increases, the tool wear increases rapidly, so the axial force and torque increases.
The axial force can be divided into static component force FS and dynamic component force FD. The force of the axial force has different influence on the cutting edge. The static force of the axial force FS affects the cutting of the cutting edge, while the dynamic component force FD mainly affects the cutting of the main cutting edge, the dynamic component force FD on the surface roughness The impact is greater than the static component force FS. The axial force increases with the feed, the cutting speed on the axial force is not very obvious. In addition, in the case of prefabricated holes, when the pore size is less than 0.4 mm, the static component force FS decreases abruptly with the increase of the pore diameter, and the trend of the dynamic component force FD is relatively flat.
Due to the different processing properties of the composite matrix and the reinforcing fiber, the effect of the matrix resin and the fiber on the axial force during mechanical drilling is different. Khashaba studied the effect of the type of matrix and fiber on axial force and torque, and found that the shape of the fiber bundle had a significant effect on the axial force, while the matrix resin type had little effect on the axial force.
2.2 Drill wear and break
PCB composite micro-drilling wear includes chemical wear and friction wear. Chemical wear is due to the high temperature decomposition products released from the PCB material on the micro-drilling material WC-Co cement in the Co binder caused by chemical erosion. At about 300 ℃, this erosion reaction has been more obvious. When the drilling speed is less than 150mm / min, chemical wear is no longer the main form of wear, friction and wear become the main form of wear. PCB micro-drilling wear and cutting speed, feed and drill radius of the fiber bundle width ratio of the ratio. Inoue et al. Have shown that the ratio of the radius of the drill to the width of the fiber bundle (glass fiber) has a greater effect on the tool life. The larger the ratio, the greater the tool cutting fiber bundle width and the tool wear. In the practical application, the new drill bit to drill up to 2500 holes to be ground, a grinding drill hole up to 2000 holes need to re-grinding, the second grinding bit up to 1500 holes need to re-grinding, three grinding bits up to 1000 holes scrapped.
In the process of PCB microporosity, the axial force and torque increase with the increase of feed rate and drilling depth, which is mainly related to the chip condition. With the increase of the drilling depth, the chip discharge is difficult. In this case, the cutting temperature is increased, the resin material melts and firmly bonds the glass fiber and the copper foil pieces to form a tough cutting body. This cutting body has affinity with the parent material of the PCB. Once this cutting body is produced, the discharge of the chips is stopped and the axial force and torque are drastically increased, resulting in the breakage of the microporous bit. PCB micro-bit drill broken shape of the buckling break, reverse the broken and buckling twist broken, generally more than coexist. The mechanism of breakage is mainly chip clogging, which is a key factor in the increase in drilling torque. Reducing axial force and cutting torque is the key to reducing the breakage of microporous drill bits.
2.3 Drilling hole damage form
(1) layered
Mechanical drilling GFRP (glass fiber reinforced) laminate process may be a variety of damage, the most serious is the interlayer layer, which led to the material around the hole wall of the sharp decline in performance, drill tip applied axial force is The main reason for the stratification. Layers can be divided into drill-in layers and drilled into layers. Drilling into the layer is the drill bit cutting edge and the lamellar contact, the role of the cutting force in the direction of the axial direction of the peeling force generated by the drill bit through the drill groove to remove the layer and the interlayer, the formation of layered layer on the surface layer ; Drilled stratification is when the drill bit near the bottom of the board, the thickness of the material is not cut more and more thin, the ability to resist deformation into a lower, the load exceeds the bonding force between the layers, There was a stratification, which happened before the board was drilled. The axial force is the main cause of the stratification. The cutting speed, the type of the substrate and the fiber bundle are also affected by the delamination. The drilling and drilling of the epoxy composites decrease with the increase of the drilling speed, The degree of damage to the stratification is greater than that of the drill. The main measures to reduce the stratification are the use of variable feed technology, preset guide holes, the use of pads and non-support drilling using viscous dampers and so on.
(2) hole wall damage
The micropores are drilled on the PCB of the composite material, and various forms of damage around the holes lead to the reduction of the insulating properties between the holes and the cracking of the copper walls. The relative angle between the cutting direction and the fiber direction, the thickness of the glass fiber bundle of the hole wall, the position of the drilling point on the glass cloth, etc. will have different effects on the damage of the hole wall.
Document 6 with a diameter of 1.0mm drill, speed 5000rpm, drilling fiberglass / epoxy resin composite (8 layers of 90 ° staggered, each layer 0.2mm), the test showed that: each layer of drilling around the degree of damage is not the same, in the 1, 3, 5, 7, 8 layers of fiber wrinkles protruding large, the largest protrusion of 30μm; and 2,4,6 layer fiber folds prominent smaller, less than 5μm. In the intersection area where the weft yarn and the warp overlap, the fiber bundle has the largest thickness and the hole wall width is the largest at 45 ° angle. In the central area, the maximum damage width occurs at 90 ° with the angle of the fiber.
Aoyama et al. Studied the influence of the main angle of the tool on the surface roughness of the hole and found that the surface roughness of the hole was the largest when it was 30 °.
(3) stains
In the case of mechanical drilling of the composite material, due to the squeezing between the bit and the composite material, the friction between the inverted cone and the hole wall and the small chip between the edge of the drill and the wall of the hole Heat, so that the resin melts, and adhere to the composite material in the sandwich or the mouth of the copper foil and hole wall, the formation of stains. The appropriate amount of cutting and grinding of tiny bits can reduce the generation of stains and reduce the stain index.
(4) burr
When the composite material is drilled, the reinforcing material and the substrate in front of the drill bit produce many cracks due to the stress transfer. When the drill bit does not reach the bottom of the hole, the reinforcing material is degummed from the substrate, resulting in a pull- Cut off from the root. In the case of drilling through holes, these reinforcing materials which are not cut from the roots can not be removed together with the chips, but are dumped toward the edges. The matrix is softened, flowing and re-condensed to the reinforcing material On the formation of burrs. The size of the exit burrs is mainly affected by the drilling force and the drilling temperature. Drilling with carbide drills in composite drilling, changing tool geometry and construction, and vibration drilling can reduce burrs.
Mechanical drilling PCB material, the processing efficiency is higher, the hole positioning accuracy, the quality of the hole is also higher. However, when drilling small holes, because the drill bit diameter is too small, easy to break, drilling process may also appear material stratification, hole wall damage, burrs and stains and other defects.
2.1 Cutting force
The various problems that occur during the machining process are directly or indirectly related to axial forces and cutting torques. The main factors affecting axial force and torque are feed rate, cutting speed, fiber bundle shape, and presence or absence of preform Force and torque also have an impact. The axial force and torque increase with the increase of cutting speed and cutting speed. As the feed rate increases, the thickness of the cutting layer increases, and the cutting speed increases, the number of cutting fibers per unit time increases, the tool wear increases rapidly, so the axial force and torque increases.
The axial force can be divided into static component force FS and dynamic component force FD. The force of the axial force has different influence on the cutting edge. The static force of the axial force FS affects the cutting of the cutting edge, while the dynamic component force FD mainly affects the cutting of the main cutting edge, the dynamic component force FD on the surface roughness The impact is greater than the static component force FS. The axial force increases with the feed, the cutting speed on the axial force is not very obvious. In addition, in the case of prefabricated holes, when the pore size is less than 0.4 mm, the static component force FS decreases abruptly with the increase of the pore diameter, and the trend of the dynamic component force FD is relatively flat.
Due to the different processing properties of the composite matrix and the reinforcing fiber, the effect of the matrix resin and the fiber on the axial force during mechanical drilling is different. Khashaba studied the effect of the type of matrix and fiber on axial force and torque, and found that the shape of the fiber bundle had a significant effect on the axial force, while the matrix resin type had little effect on the axial force.
2.2 Drill wear and break
PCB composite micro-drilling wear includes chemical wear and friction wear. Chemical wear is due to the high temperature decomposition products released from the PCB material on the micro-drilling material WC-Co cement in the Co binder caused by chemical erosion. At about 300 ℃, this erosion reaction has been more obvious. When the drilling speed is less than 150mm / min, chemical wear is no longer the main form of wear, friction and wear become the main form of wear. PCB micro-drilling wear and cutting speed, feed and drill radius of the fiber bundle width ratio of the ratio. Inoue et al. Have shown that the ratio of the radius of the drill to the width of the fiber bundle (glass fiber) has a greater effect on the tool life. The larger the ratio, the greater the tool cutting fiber bundle width and the tool wear. In the practical application, the new drill bit to drill up to 2500 holes to be ground, a grinding drill hole up to 2000 holes need to re-grinding, the second grinding bit up to 1500 holes need to re-grinding, three grinding bits up to 1000 holes scrapped.
In the process of PCB microporosity, the axial force and torque increase with the increase of feed rate and drilling depth, which is mainly related to the chip condition. With the increase of the drilling depth, the chip discharge is difficult. In this case, the cutting temperature is increased, the resin material melts and firmly bonds the glass fiber and the copper foil pieces to form a tough cutting body. This cutting body has affinity with the parent material of the PCB. Once this cutting body is produced, the discharge of the chips is stopped and the axial force and torque are drastically increased, resulting in the breakage of the microporous bit. PCB micro-bit drill broken shape of the buckling break, reverse the broken and buckling twist broken, generally more than coexist. The mechanism of breakage is mainly chip clogging, which is a key factor in the increase in drilling torque. Reducing axial force and cutting torque is the key to reducing the breakage of microporous drill bits.
2.3 Drilling hole damage form
(1) layered
Mechanical drilling GFRP (glass fiber reinforced) laminate process may be a variety of damage, the most serious is the interlayer layer, which led to the material around the hole wall of the sharp decline in performance, drill tip applied axial force is The main reason for the stratification. Layers can be divided into drill-in layers and drilled into layers. Drilling into the layer is the drill bit cutting edge and the lamellar contact, the role of the cutting force in the direction of the axial direction of the peeling force generated by the drill bit through the drill groove to remove the layer and the interlayer, the formation of layered layer on the surface layer ; Drilled stratification is when the drill bit near the bottom of the board, the thickness of the material is not cut more and more thin, the ability to resist deformation into a lower, the load exceeds the bonding force between the layers, There was a stratification, which happened before the board was drilled. The axial force is the main cause of the stratification. The cutting speed, the type of the substrate and the fiber bundle are also affected by the delamination. The drilling and drilling of the epoxy composites decrease with the increase of the drilling speed, The degree of damage to the stratification is greater than that of the drill. The main measures to reduce the stratification are the use of variable feed technology, preset guide holes, the use of pads and non-support drilling using viscous dampers and so on.
(2) hole wall damage
The micropores are drilled on the PCB of the composite material, and various forms of damage around the holes lead to the reduction of the insulating properties between the holes and the cracking of the copper walls. The relative angle between the cutting direction and the fiber direction, the thickness of the glass fiber bundle of the hole wall, the position of the drilling point on the glass cloth, etc. will have different effects on the damage of the hole wall.
Document 6 with a diameter of 1.0mm drill, speed 5000rpm, drilling fiberglass / epoxy resin composite (8 layers of 90 ° staggered, each layer 0.2mm), the test showed that: each layer of drilling around the degree of damage is not the same, in the 1, 3, 5, 7, 8 layers of fiber wrinkles protruding large, the largest protrusion of 30μm; and 2,4,6 layer fiber folds prominent smaller, less than 5μm. In the intersection area where the weft yarn and the warp overlap, the fiber bundle has the largest thickness and the hole wall width is the largest at 45 ° angle. In the central area, the maximum damage width occurs at 90 ° with the angle of the fiber.
Aoyama et al. Studied the influence of the main angle of the tool on the surface roughness of the hole and found that the surface roughness of the hole was the largest when it was 30 °.
(3) stains
In the case of mechanical drilling of the composite material, due to the squeezing between the bit and the composite material, the friction between the inverted cone and the hole wall and the small chip between the edge of the drill and the wall of the hole Heat, so that the resin melts, and adhere to the composite material in the sandwich or the mouth of the copper foil and hole wall, the formation of stains. The appropriate amount of cutting and grinding of tiny bits can reduce the generation of stains and reduce the stain index.
(4) burr
When the composite material is drilled, the reinforcing material and the substrate in front of the drill bit produce many cracks due to the stress transfer. When the drill bit does not reach the bottom of the hole, the reinforcing material is degummed from the substrate, resulting in a pull- Cut off from the root. In the case of drilling through holes, these reinforcing materials which are not cut from the roots can not be removed together with the chips, but are dumped toward the edges. The matrix is softened, flowing and re-condensed to the reinforcing material On the formation of burrs. The size of the exit burrs is mainly affected by the drilling force and the drilling temperature. Drilling with carbide drills in composite drilling, changing tool geometry and construction, and vibration drilling can reduce burrs.