Introducing the micro hole processing technology of the most widely used epoxy resin matrix composite material in PCB

1 Introduction

With the rapid development of electronic technology, modern electronic products have become smaller and smaller, and functions have become more and more complex. PCBs that support and interconnect electronic components have evolved from single-sided to double-sided, multi-layer, high-precision The development of high density and high reliability, the volume is shrinking, the density is exponentially increasing, the aperture required for processing on the PCB is getting smaller and smaller, the number of holes is increasing, and the distance between holes is getting smaller and smaller. Therefore, high quality micro hole processing technology is required.

PCB specifications are complex and there are many types of products. This paper introduces the processing technology of tiny pores (small pores with a diameter of 0.6mm or less and micropores of 0.3mm or less), which are the most widely used epoxy resin composite materials in PCB. Composite PCB has high brittleness, high hardness, high fiber strength, high toughness, low interlaminar shear strength, anisotropy, poor thermal conductivity and large difference in thermal expansion coefficient between fiber and resin. When cutting temperature is high, it is easy to cut. The fiber around the zone creates thermal stress at the interface with the substrate; when the temperature is too high, the resin melts and sticks to the cutting edge, making processing and chip removal difficult. The cutting force of the drilled composite material is very uneven, and it is easy to cause defects such as delamination, burrs and splitting, and the processing quality is difficult to ensure. This material is extremely abrasive to the processing tool, and the tool wear is quite serious. The wear of the tool in turn leads 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 stripping between the composite layer and the layer. Therefore, PCB composite materials are difficult to process non-metal composite materials, and the processing mechanism is completely different from that of metal materials. At present, the micro hole processing methods mainly include mechanical drilling and laser drilling.

2 Mechanical drilling

When mechanically drilling PCB materials, the processing efficiency is high, the hole positioning is accurate, and the quality of the holes is also high. However, when drilling micro holes, the diameter of the drill bit is too small, and it is easy to break. During the drilling process, defects such as material delamination, hole wall damage, burrs and stains may occur.

2.1 Cutting force

Various problems occurring during mechanical drilling 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 presence or absence of pre-made hole-to-shaft. The force and torque also have an effect. The axial force and torque increase with increasing feed rate 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, and the amount of tool wear increases rapidly, so the axial force and torque increase.

The axial force can be divided into static component force FS and dynamic component force FD. The axial force component has different effects on the cutting edge. The static component force FS of the axial force affects the cutting of the chisel edge, while the dynamic component force FD mainly affects the cutting of the main cutting edge. The dynamic component force FD is the surface roughness. The effect is greater than the static component force FS. The axial force increases with the feed rate, and the influence of the cutting speed on the axial force is not obvious. In addition, in the case of a prefabricated hole, when the hole diameter is less than 0.4 mm, the static component force FS sharply decreases as the aperture increases, and the dynamic component force FD decreases more flatly.

Due to the different processing properties of the composite matrix and the reinforcing fibers, the matrix resin and the fibers have different effects on the axial force during mechanical drilling. Khashaba studied the effects of the type of matrix and fiber on axial forces and torque. It was 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.

Introducing the micro hole processing technology of the most widely used epoxy resin matrix composite material in PCB

2.2 Bit wear and break

PCB composite micro-drill wear includes chemical wear and friction wear. Chemical wear is caused by chemical attack of the pyrolysis product released from the PCB material on the Co binder in the micro-drilled material WC-Co cemented carbide. At about 300 ° C, this erosion reaction has become more apparent. At drilling speeds below 150 mm/min, chemical wear is no longer the dominant form of wear and friction wear becomes the primary form of wear. The wear of the PCB micro-drill is also related to the ratio of cutting speed, feed rate and bit radius to fiber bundle width. Inoue et al.'s research shows that the ratio of the radius of the drill to the width of the fiber bundle (glass fiber) has a great influence on the tool life. The larger the ratio, the larger the fiber bundle width of the cutter and the larger the tool wear. In practical applications, the new drill bit drills up to 2,500 holes for grinding. Once the drill bit reaches 2000 holes, it needs to be ground again. The second grinding bit reaches 1500 holes and needs to be ground again. The three grinding bits reach 1000 holes and are scrapped.

During the micropore processing of the PCB, the axial force and torque increase with the increase of the feed rate and the drilling depth, and the main reason is related to the chip removal state. As the drilling depth increases, the chip discharge is difficult. In this case, the cutting temperature rises, the resin material melts and firmly bonds the glass fiber and the copper foil pieces to form a tough cutting body. The cutting body has an affinity with the PCB matrix material. Once such a cutting body is produced, the discharge of the chips is stopped, and the axial force and torque are sharply increased, thereby causing the microporous bit to break. The fracture form of the PCB micro-hole drill bit has buckling, torsion breaking and buckling and torsion breaking, and generally both coexist. The breaking mechanism is mainly chip clogging, which is a key factor causing the increase of drilling torque. Reducing the axial force and cutting torque is the key to reducing the breakage of the micro-hole drill.

2.3 Drilling damage form

(1) Layering

Various damages may occur during mechanical drilling of GFRP (glass fiber reinforced) laminates, the most serious of which is interlayer delamination, which results in a sharp drop in material properties around the hole walls, and the axial force applied by the drill tip is The main reason for stratification. Layering can be divided into drilling stratification and drilling delamination. The drilling delamination is when the cutting edge of the drill bit is in contact with the laminate, the cutting force generated in the circumferential direction by the cutting force in the circumferential direction is separated from the layer by the drill cutting groove, and a layered region is formed on the surface of the laminate. Drilling delamination is when the drill bit approaches the bottom of the laminate, the thickness of the uncut material is thinner and the resistance to deformation is further reduced. Where the load exceeds the adhesion between the laminates, A stratification occurs, which occurs before the laminate is drilled through. Axial force is the main cause of delamination. The cutting speed, the type of substrate and fiber bundle also have an effect on delamination. The driller and the delamination of the epoxy composite decrease with the increase of the drilling speed, and the drilling The degree of delamination damage is greater than that of drilling. The main measures to reduce stratification are: variable feed technology, preset guide holes, use of bolsters, and the use of viscous dampers for unsupported drilling.

(2) Hole wall damage

The micropores are drilled on the composite PCB, and various forms of damage occurring around the holes result in a decrease in insulation between the holes and a break in the copper layer of the holes after metallization of the holes. The relative angle between the cutting direction and the fiber direction, the thickness of the glass fiber bundle on the hole wall, the position of the drilled glass on the glass cloth, etc., all have different effects on the damage of the hole wall.

Document 6 drilled a glass fiber/epoxy composite (8 layers of 90° staggered, 0.2 mm per layer) with a 1.0 mm diameter drill bit and a rotational speed of 5000 rpm. The test showed that the damage around each hole was different. The 1,3,5,7,8 layers of fiber wrinkles are large, with a maximum protrusion of 30 μm; while the 2, 4, and 6 layers of fiber wrinkles are less prominent, and the minimum is less than 5 μm. In the overlapping area of ​​the weft yarn and the warp yarn, the fiber bundle thickness is the largest at the fiber angle of 45°, and the hole wall damage width is the largest; and in the center region, the maximum damage width occurs at an angle close to 90° to the fiber.

Aoyama et al. studied the influence of the tool leading angle on the surface roughness of the machined hole wall. When the lead angle was 30°, the surface roughness of the hole wall was the largest, up to 50 μm.

(3) stains

When mechanically drilling composite materials, due to the extrusion of the chisel edge and the composite material, the friction between the inverted cone and the hole wall, and the large amount of cutting generated by the fine chip between the edge of the drill bit and the wall of the hole and the rotary friction of the bit The heat causes the resin to melt and adhere to the copper foil and the pore walls at the interlayer or orifice of the composite to form stains. Proper cutting and grinding of the micro drill can reduce the generation of stains and reduce the stain index.

(4) burr

When drilling composite materials, due to the transfer of stress, when the drill bit does not reach the bottom of the hole, the reinforcing material and the matrix in front of the drill bit will generate many cracks, so that the reinforcing material is degummed from the substrate, resulting in the removal of the image, resulting in the reinforcement material not being able to Cut off from the root. When the hole is drilled, these reinforcing materials that are not cut off from the root cannot be removed together with the chips, but are poured toward the edge of the hole, and the substrate softens, flows, and re-condenses to the reinforcing material poured on the edge of the hole due to the heat of cutting. On, forming a burr. The size of the exit burr is mainly affected by the drilling force and the drilling temperature. Drilling with carbide drills, changing tool geometries and structures, and vibratory drilling techniques can reduce burrs in composite drilling.

3 Vibration drilling

Vibration drilling is a branch of vibration cutting. It is a novel drilling method based on cutting theory and vibration theory. Ordinary drilling is a continuous cutting process, while vibratory drilling is a pulsed intermittent cutting process that produces a controlled relative motion between the drill bit and the workpiece during the drilling process. During the vibration drilling process, when the main cutting edge is not separated from the workpiece (no separate vibration drilling), the parameters such as cutting speed and direction are periodically changed; when the main cutting edge is separated from the workpiece, the separation is separated (separate vibration drilling) When the cutting process becomes a pulsed intermittent cutting.

When the vibration parameters (vibration frequency and amplitude), feed rate and spindle speed are selected reasonably, the drilling accuracy, dimensional accuracy and roundness, reduction of hole surface roughness, reduction of exit burrs and extension of tool life can be significantly improved. The axial force variation trend of vibratory drilling GFRP composites is similar to that of ordinary drilling, but the axial force is smaller than ordinary drilling, and the axial force is affected by the feed rate, vibration frequency and amplitude. Wang et al.'s research shows that when the amplitude is 6μm, the vibration frequency is 300Hz, and the feed rate is 250mm/min, the axial force can reach a minimum of 1.5N. The glass fiber in the GFRP material is criss-crossed, its strength and hardness are very large, it is not easy to cut, and the matrix around it is soft, which tends to force the drill bit to make the knife, changing the direction of the bit advancement and forming a large deviation of the drill. Vibration drilling has a rigid effect. When the drill is drilled, the bending force is small due to the force of the drill bit, and the drilling positioning error is also much smaller than that of ordinary drilling.

For multi-layer composite materials, step-type multi-variable variable-parameter vibration drilling is a more optimized process, which can well solve the contradiction between the quality and efficiency of fiber composite drilling. It fully considers the structure, performance and specific processing of the multi-layer composite material, maintains the optimal processing state in the drilling process, and uses the optimal drilling parameters of the top layer material when drilling, and adopts the most The optimal drilling parameters of the lower layer material divide the drilling process into multiple sections. The vibration parameters and cutting parameters are abrupt and stepwise depending on the performance of the laminated material, which can optimize the vibration cutting parameters and process. The effect is better than the ordinary drill under the corresponding conditions. Zhao Hongwei et al. used an electronically controlled micro-hole vibratory drilling machine to perform micro-hole drilling tests on multilayer composites. The step-by-step three-parameter vibration drilling has significantly reduced the drilling positioning error r, the hole expansion ΔD, and the outlet burr height H value compared to ordinary drilling. Rumkumar et al. compared the axial force, torque and tool wear of GFRP composite vibratory drilling and ordinary drilling. It was found that ordinary drilling has a sharp increase in axial force and torque when the number of drilled holes is more than 30, and the vibration drill The number of drilled holes can be more than 60, and the value of axial drilling, torque and tool wear is smaller than that of ordinary drilling.

4 Laser drilling

PCB composites use mechanical drilling when machining micropores with a diameter of less than 0.2mm. The tool wear is accelerated, easy to break, and the cost increases. The laser beam can reduce the spot diameter to the micron level, which is an ideal tool for processing micropores. As a non-contact drilling technology, laser drilling focuses the laser beam into a very small spot. The energy of the spot melts or the gasified material forms micropores. It has high drilling speed, high efficiency, no tool loss, and machined surface. High quality, especially suitable for micro-hole drilling of composite materials. In particular, a large number of high-density group hole processing is performed on various materials such as hard, brittle, and soft.

Laser drilling of composite materials is prone to complex physical and chemical changes. There are two main mechanisms for material removal: 1 thermal processing mechanism, laser heating materials, melting and gasification of materials; 2 photochemical mechanism, laser energy is directly used Overcoming the chemical bonds between the molecules of the material, the material is broken down into tiny gaseous molecules or atoms. The key to drilling fiber reinforced composites is to choose a suitable laser source, mainly based on the characteristics of the material being processed, such as the absorption of specific wavelength light, melting and gasification temperature, thermal conductivity and so on. Commonly used laser sources are CO2 lasers, KrF excimer lasers, and Nd:YA G lasers.

4.1 CO2 laser processing

The CO2 laser has a wavelength range of 9.3 to 10.6 μm and belongs to the infrared laser. The material to be cut is a thermal processing mechanism. When CO2 laser drills resin-based fiber reinforced composite materials, the laser power and processing time have a great influence on the processing quality. Setting proper laser power and processing time can significantly improve the processing quality. Aoyama et al. used a CO2 continuous laser with a wavelength of 10.6 μm and a maximum output of 25 OW to drill micropores with a diameter of 0.3 mm on a glass fiber/epoxy composite. The laser power was 35 W and the processing time was OAS. ,auxiliary

When the gas is air, the epoxy resin on the surface of the hole wall hardly appears.

Thermal damage; when the laser power is 75W, the processing time is 0.1s, and the auxiliary gas is nitrogen, a black substance appears on the surface of the hole wall. This is because the laser energy continuously irradiates the resin, so that the temperature of the resin is not cooled, and when it accumulates to a certain extent, the resin is thermally damaged. Hirogaki et al. used a CO2 pulsed laser with a wavelength of 10.6 μm and a maximum output of 100 W to drill glass/epoxy resin and aramid fiber/epoxy composites. It was found that epoxy resin hardly appeared if the irradiation time was less than 5 ms. Thermal damage. This is because reducing the irradiation time of the laser pulse can reduce the energy absorbed by the material, and the time interval between the pulses gives the material some cooling, so the thermal damage of the resin is further reduced.

4.2 KrF excimer laser processing

The KrF excimer laser has a wavelength of 248 nm, which belongs to the ultraviolet laser, and the material is a photochemical mechanism. High-energy ultraviolet photons can split the material directly into atoms for the purpose of cutting off the material. KrF excimer laser can significantly reduce laser processing thermal damage. Zheng et al. used a KrF laser with a wavelength of 248 nm, a pulse width of 20 ns, and an energy density of 400 nd/cm2 to drill a glass fiber/epoxy composite. No black material appeared on the pore walls, and the depth of the pores could be accurately controlled. The secondary pulse drilling depth is 0.12 μm.

However, KrF excimer laser drilling may occur when tapping holes, which is due to the diffraction effect of the beam at the edge of the machined shape, which reduces the density and etch rate of the energy; another reason may be the use of uncorrected prisms. The spherical deviation caused by. As the energy density increases, the taper gradually decreases, and even a negative taper occurs. This may be due to the fact that the beam energy density is greater than the critical energy at which diffraction occurs at the boundary and the defocusing causes the beam diameter to become larger.

4.3 Nd:YAG laser processing

The commonly used wavelengths of Nd:YAG lasers are 1.06μm and 355nm, which belong to infrared laser and ultraviolet laser respectively. The two wavelengths correspond to the thermal processing mechanism and photochemical mechanism respectively. Laser power and pulse frequency have a significant impact on thermal damage during Nd:YAG laser drilling. Yang et al. used a Nd:YAG laser with a wavelength of 355 nm and an average power of 12 W to drill a 1.6 mm thick glass fiber/epoxy composite. It was found that at a given pulse frequency, the higher the power, the higher the processing temperature. The coking of the epoxy resin and the melting of the glass fiber are accelerated, and the equivalent width of the thermal damage increases as the average power of the laser increases. At a given laser power, the equivalent width of thermal damage is greatest at a pulse frequency of 7 kHz, and increases with frequency as it is less than 7 kHz. When it exceeds 7 kHz, the width of thermal damage decreases. This is because the higher the frequency, the shorter the time interval between laser pulses, and the shorter the cooling time of the machined surface. When the frequency exceeds 7 kHz, the higher the pulse frequency, the longer the pulse duration, and the peak power of the laser pulse. The smaller the temperature, the lower the temperature of the machined surface and the equivalent width of the thermal damage. Drilling with a Nd:YAG laser with a wavelength of 355 nm, a power of 0.3 W, and a pulse frequency of 1 KHz showed almost no thermal damage on the surface of the hole wall.

Due to the type of composite reinforcing fiber and the direction of each layer of fiber, the accuracy of the hole in the Nd:YAG laser drilling process, the discontinuity of the hole at the interface between the layers and the fiber expansion are caused. Rodden et al. used a Nd:YAG laser with a wavelength of 1064 nm and a pulse width of 0.1 ms to drill a 2 mm thick carbon fiber/epoxy composite laminate, and found that the shape of the hole changed from a circle to an ellipse and a hole at the interface between the layers. The shape is discontinuous. The former is because the heat transfer coefficient of carbon fiber is much larger than the heat transfer coefficient of epoxy resin. The heat is first conducted along the direction of the carbon fiber, causing the hole to be stretched along the direction of the carbon fiber; the latter is because the carbon fiber direction of each layer Different, resulting in discontinuities in the shape of the holes between the layers. When Cheng et al. used a Nd:YAG pulsed laser with a wavelength of 1.06 μm and a maximum average output energy of 135 W and a pulse duration of 0.5 to 5 ms to drill a carbon fiber/PEEK composite material of about 1 mm thick, it was found that carbon fibers around the hole appeared at the end. The radial expansion is up to 50%. The irreversible change of the partially filled structure due to the intense thermal expansion of the fiber and the rapid pressurization of the micropores in the fiber structure reinforce this effect.

5 Conclusion

Combining the research of mechanical and laser drilling technology of resin-based composite PCBs in recent years, and analyzing various factors affecting the processing quality and possible problems in processing, the following conclusions can be drawn:

(1) For mechanical drilling, low feed rates, high spindle speeds and the use of new tools can improve the quality of the drilled surface.

(2) Vibration drilling has a rigid effect. When the vibration frequency, amplitude, feed rate and spindle speed are selected properly, the drilling accuracy, dimensional accuracy and roundness can be significantly improved, the surface roughness of the hole and the burr of the outlet can be reduced. Extend tool life.

(3) Regardless of whether continuous or pulsed laser is used, the laser power has a great influence on the quality of drilling. Choosing a suitable laser power can obtain better processing quality.

(4) For pulsed lasers, the pulse frequency and peak power have a great influence on the drilling quality. Selecting the laser with short pulse time and high peak power and appropriately increasing the time interval between pulses can significantly improve the processing quality.

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