Laser drilling method capable of machining ceramic micro holes

Abstract

The invention relates to the technical field of laser drilling, in particular to a laser drilling method capable of machining ceramic micro holes. The method comprises the following steps that a machinable ceramic substrate is prepared, and the substrate is used for forming a needle dial of the test fixture; a plurality of micropores are drilled in the substrate through laser, the hole diameter of each micropore is 15 + / -1.5 microns, and the distance between the hole walls of any two adjacent micropores is 2-5 microns; probes are installed in the micropores to form a dial structure used for contacting the circuit board to be tested; a needle disc structure is connected with an electrode surface assembly, the electrode surface assembly is provided with a gold-plated conductive layer and a connector interface and is used for conducting a probe and electrical property testing equipment, monopulse energy and frequency are accurately controlled through a layered scanning strategy of ultraviolet laser, non-thermally-dominated material removal is achieved through the photochemical ablation effect, and the electrical property testing efficiency is improved. The technical problems that 15-micron micropores cannot be processed on the ceramic substrate and the hole wall distance is reduced to be smaller than or equal to 5 microns are solved, lossless transmission of micro-current is guaranteed through the gold-plated conductive layer and the high-density connector, and finally micron-sized accurate electrical property testing is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of laser drilling, in particular to a laser drilling method capable of processing tiny holes in ceramics. Background Art

[0002] The technology of drilling tiny holes has developed along with the continuous advancement of semiconductor packaging technology. In the mid-1990s, a new type of high-density IC packaging represented by ball grid array packaging and chip size packaging came into being, and high-order substrates emerged as a new packaging carrier.

[0003] As electronic products become shorter, smaller, lighter, and thinner, coupled with the rapid growth of surface mount technology (SMT), printed circuit boards (PCBs) are rapidly developing towards higher density and finer circuitry. However, substandard test yields are a common problem in mass production. Conventional drilling techniques for high-end ABF substrates are currently unable to meet the required drilling precision.

[0004] Currently, drilling technology for this type of engineering plastics mostly remains at a 30μm aperture, with an accuracy of approximately 5μm. This micro-hole drilling technology not only achieves high-density testing results, but also, after testing, when inspected with a 20-40x magnifying glass, the test point holes are smooth and burr-free, ensuring drilling accuracy and providing a strong technical guarantee for the continued production of high-precision, high-end substrate fixtures. The main application area is drilling technology for high-end ABF substrate products, which cannot be met by drilling with ordinary materials. This is currently a focus of the entire semiconductor packaging substrate testing industry. Micro-aperture drilling can greatly improve test accuracy and enable four-terminal testing of smaller test pads, making a significant contribution to the testing of high-end substrates.

[0005] However, the current drilling technology for ceramic substrates or engineering plastics still does not have the ability to drill holes with a minimum test probe diameter of 15um, and does not have the technical capability to drill a minimum spacing of 5um between two holes, resulting in the existing drilling technology accuracy failing to meet the requirements. Summary of the Invention

[0006] The present invention provides a laser drilling method for machining micro-holes in ceramics. This method uses a layered scanning strategy of an ultraviolet laser to precisely control the energy and frequency of a single pulse and utilizes the photochemical ablation effect to achieve non-thermal material removal. This method solves the technical problem of being unable to machine 15±1.5μm micro-holes in ceramic substrates and reducing the hole wall spacing to 2-5μm, thereby resolving the problems raised in the above-mentioned background technology, namely: In the prior art, the drilling accuracy in ceramic substrates or engineering plastics cannot meet the required level.

[0007] To achieve the above-mentioned purpose, the laser drilling method for processing micro-holes in ceramics comprises the following steps: S1. Prepare a machinable ceramic substrate, which is used to form a needle plate of a test fixture; S2. Drilling a plurality of micropores on the substrate using laser, wherein the diameter of the micropores is 15±1.5 μm, and the distance between the pore walls of any two adjacent micropores is 2-5 μm; S3, installing a probe in the microhole to form a needle-disc structure for contacting the circuit board to be tested; S4. Connect the needle disk structure to the electrode surface assembly, wherein the electrode surface assembly is provided with a gold-plated conductive layer and a connector interface for connecting the probe and the electrical testing equipment.

[0008] In the above technical solution, a ceramic substrate is first prepared as a needle disk carrier; then, an ultraviolet laser is used to drill a microhole array according to the PAD coordinates of the circuit board, with a hole wall spacing of 2-5μm. Then, a three-layer stacked structure is assembled, with the bottom layer microhole positioning probe tip contacting the circuit board; the middle layer has a hollow card slot, which is 3-8μm larger than the probe clamp ring to clamp the probe and prevent it from falling off; the upper layer has a guide hole to guide the probe insertion, and finally the needle disk is connected to the electrode surface assembly. The gold-plated layer on the electrode surface conducts the probe electrical signal and is connected to the test equipment through a 1024-, 2048-, or 4096-point high-density connector. The pillars fix the multi-layer ceramic board to ensure that the coaxial deviation of the microholes in each layer is ≤3μm, forming a complete test path from probe contact to gold-plated conductivity and final connector output. The above scheme utilizes the interaction mechanism between the high photon energy of ultraviolet laser and ceramic materials. When the ultraviolet laser is focused on the surface of aluminum oxide or aluminum nitride ceramics, its short wavelength characteristics use the photochemical effect to directly destroy the molecular bonds of the material by the photon energy, rather than the thermal melting mechanism of traditional thermal processing. Through a layered scanning strategy, with each layer 1-5μm deep, laser pulses peel off the material layer by layer at microsecond intervals. Precise control of the single pulse energy can avoid the accumulation of thermal stress, thereby forming micropores with a pore size of 15±1.5μm while ensuring the pore wall roughness Ra≤0.5μm and no thermal cracks. The key to reducing the spacing between adjacent pore walls to 2-5μm lies in the diffraction limit control of the laser focus spot and the nanometer-level positioning accuracy of the displacement platform, so that the position deviation of the micropore array is ≤3μm.

[0009] On this basis, in the S1, the needle disk structure is composed of multiple layers of stacked ceramic plates, including a bottom layer: a microhole array corresponding to the PAD position of the circuit board to be tested, with a pore size of 15±1.5μm; a middle layer: a hollow card slot structure for fixing the middle clamping ring of the probe; an upper layer: a guide hole with a pore size 10%-20% larger than the bottom layer pore size for guiding the probe insertion; in the S1, the multilayer ceramic plates are fixed and stacked by pillars, and the coaxiality deviation of the microholes between each layer is ≤3μm. This solution is based on the low thermal expansion coefficient of alumina or aluminum nitride ceramics (≤7×10-6 / K) and ultra-high hardness (HV≥1500). By precisely stacking three layers of ceramic plates and controlling the coaxial deviation to ≤3μm, precise positioning and dynamic stability of the probe are achieved. The 15±1.5μm micropores on the bottom layer rigidly constrain the radial displacement of the probe. The 3-8μm gap groove in the middle layer forms an elastic tolerance space to lock the probe retaining ring to prevent it from falling off. The guide hole in the upper layer with a 10%-20% increased diameter uses a tapered entrance to compensate for insertion deviation and reduce friction and wear. During the probe pressure test, this structure cooperates to maintain the vertical motion trajectory of the probe. At the same time, with the help of the gold-plated conductive layer, the square resistance is ≤20mΩ / □, ensuring lossless transmission of micro-current signals.

[0010] In addition, in S2, the laser is an ultraviolet laser with a wavelength range of 100-400nm, a pulse frequency of 10-100kHz, and a single pulse energy of 0.1-10μJ; in S2, the drilling process adopts a layered scanning strategy, the drilling depth of each layer is 1-5μm, and the total drilling time is ≤0.5 seconds / hole; the high photon energy of the ultraviolet laser is used to induce the photochemical ablation effect of the ceramic material, and non-thermal-dominated material removal is achieved by precisely controlling the pulse frequency and single pulse energy; the layered scanning strategy is combined with dynamic tracking of the laser focus (real-time focus calibration through a closed-loop piezoelectric displacement platform) to peel off the material layer by layer at the molecular scale, compressing the heat-affected zone to ≤1μm, while controlling the hole wall taper to ≤±0.8μm; finally, under ultra-high-speed processing of ≤0.5 seconds / hole, micropores with an aperture tolerance of ±1.5μm and a side wall roughness Ra≤0.3μm are formed, meeting the precision and efficiency requirements of the 15±1.5μm micropore array of the ceramic plate, and providing a manufacturing basis for high-density test needle disks.

[0011] Compared with the prior art, the present invention has the following beneficial effects: By using a layered scanning strategy of ultraviolet laser to precisely control the energy and frequency of a single pulse, and utilizing the photochemical ablation effect to achieve non-thermal-dominated material removal, the technical difficulties of being unable to process 15±1.5μm micropores on ceramic substrates and reducing the hole wall spacing to 2-5μm are resolved. The aperture tolerance is controlled at ±1.5μm, the hole wall roughness reaches Ra≤0.3μm, and the heat-affected zone is compressed to ≤1μm. In addition, a gold-plated conductive layer and high-density connectors are used to ensure lossless transmission of microcurrents, ultimately achieving precise electrical testing of sub-micron PADs on high-end substrates. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 Schematic diagram of the drilling method of the present invention; Figure 2 Schematic diagram of an actual drilling experiment of the drilling method of the present invention; Figure 3 This is a schematic diagram showing the drilling accuracy of the drilling method of the present invention. DETAILED DESCRIPTION

[0013] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0014] At present, in order to solve the problem that the drilling precision in ceramic substrates or engineering plastics in the prior art cannot meet the requirements, the present invention provides a laser drilling method that can process tiny holes in ceramics.

[0015] Example 1 Figure 1 As shown, the steps include: S1. Select 96% alumina ceramic plate (thickness 0.8mm, HV1600) and cut it into 100×100mm substrates; S2. Drilling the ceramic substrate, UV laser parameters: wavelength 250nm, pulse frequency 50kHz, single pulse energy 5μJ; layer scanning, each layer depth 3μm, total drilling time 0.4 seconds / hole; S3. Assemble the needle plate. The needle plate includes the following structures: Bottom layer: Drill 15μm microholes (position accuracy ±2.5μm); Middle layer: laser-cut hollow card slot (diameter 5μm larger than the probe card ring); Upper layer: drilling 18μm pilot holes; The pillars fix three layers of ceramic plates, and the coaxiality deviation is ≤2.8μm; S4. Connect the needle disk to the electrode surface, where the electrode surface is sputtered with a 1.2μm gold layer (square resistance 18mΩ / □); connect a 4096-point high-density connector.

[0016] Example 2 is different from Example 1 in that the substrate material is aluminum nitride ceramic (thermal conductivity 180 W / mK, thickness 0.5 mm), the UV laser uses a wavelength of 100 nm, a pulse frequency of 10 kHz, and a single pulse energy of 0.1 μJ. The remaining step parameters are the same as Example 1.

[0017] Example 3 is different from Example 1 in that the ultraviolet laser uses a wavelength of 400 nm, a pulse frequency of 100 kHz, and a single pulse energy of 10 μJ. The remaining step parameters are the same as those of Example 1.

[0018] Comparative Example 1 is different from Example 1 in that the ultraviolet laser uses a wavelength of 550 nm, a pulse frequency of 150 kHz, and a single pulse energy of 15 μJ. The other step parameters are the same as those of Example 1.

[0019] Comparative Example 2 is different from Example 1 in that, in S2, the layered scanning is cancelled and only single penetration drilling is adopted. The pulse energy uses the total energy accumulation value of Example 1. The other step parameters are the same as Example 1.

[0020] Comparative Example 3 is different from Example 1 in that the gold plating layer is removed in S4, and only the alumina ceramic substrate is retained on the electrode surface. The remaining step parameters are the same as Example 1, and are used to verify the necessity of the gold plating layer for high-frequency signal transmission.

[0021] Comparative Example 4: A conventional CO2 laser with a wavelength of 10600 nm was used.

[0022] Experimental Example 1: Effect of laser wavelength on processing quality Table 1 Experimental comparison of different wavelengths on processing quality According to the experimental data in Table 1, the UV laser wavelength ≤ 400nm is the decisive factor in inhibiting thermal damage to ceramics. When the wavelength is controlled between 100-400nm, the thermal crack density approaches zero (≤ 0.2 / mm 2 ), the aperture tolerance is compressed to within ±1.1μm; and when the wavelength increases to 500nm, the thermal crack density increases sharply to 8.5±1.2 / mm 2 , the aperture tolerance is expanded by 375%. This critical effect verifies the irreplaceable wavelength range in this solution, and 250nm is the optimal solution for comprehensive performance.

[0023] Experimental Example 2: Thermal Stress Control Using Layered Scanning Strategy Table 2 Comparison of experiments with and without layered scanning According to the experimental data in Table 2, the layered scanning strategy adopted in Example 1 reduces the substrate fragmentation rate from 22% in a single penetration process to 0% by releasing thermal stress layer by layer. It also compresses the heat-affected zone by 82% and reduces the hole wall roughness by 86%. This technology increases the yield of the entire board of 4096 microhole arrays from near zero to 99.98%, and the processing time per hole is only 0.4 seconds, breaking through the industry shackles that high-density ceramic microholes are inevitably accompanied by high fragmentation. Even if the total energy remains unchanged when the layered structure is eliminated, the mechanical collapse of the substrate due to instantaneous thermal shock can still occur, verifying the effectiveness of layered scanning.

[0024] Experimental Example 3: Verification of electrical performance of gold-plated conductive layer Conclusion: The 1.2μm gold plating employed in Example 1 significantly reduced the signal attenuation of the ceramic microvia at 10GHz from -12.4dB without plating to -0.41dB, while simultaneously increasing the current flow capacity per pore by 20 times (0.5mA → 10mA). This structure overcomes the dielectric loss and high resistance bottlenecks of ceramic materials, enabling the microvia to carry millimeter-wave signal transmission. The extreme electrical failure of Comparative Example 3 (without plating) demonstrates the indispensable role of the gold conductive layer in this solution.

[0025] Experimental Example 4: Mechanical Life Verification of a Three-Layer Structure (Test Conditions: 20Hz Probe Compression, 15μm Probe Diameter, Coaxiality Calibrated Online Using a Laser Interferometer) Conclusion: The three-layer stacked structure outperforms the single-layer and traditional engineering plastic needle disk in terms of 100,000-cycle dropout rate, substrate wear depth, and coaxiality deviation, making it the preferred needle disk structure for this solution.

[0026] Summary: In this solution, ultraviolet laser layered scanning technology (wavelength 100-400nm, single pulse energy 0.1-10μJ, layer depth 1-5μm) is used to achieve ultra-precision microhole array processing with a pore diameter of 15μm and a hole wall spacing of 2-5μm on an alumina or aluminum nitride ceramic substrate. This method uses the photochemical ablation effect of short-wavelength laser to compress the heat-affected zone to ≤0.9μm (Experimental Example 2). Combined with nanometer-scale displacement positioning, it completely solves the problems of thermal cracking and loss of precision control in ceramic microhole processing. The single-hole processing time is ≤0.5 seconds, laying the foundation for the manufacturing of high-density test pin disks. The three-layer stacked needle-on-disk structure constructed based on this drilling process achieves precise probe positioning with a coaxial deviation of ≤3μm: 15μm micro-holes in the bottom layer rigidly constrain the probe tip to contact the circuit board PAD; 3-8μm gap slots in the middle layer create an elastic tolerance space, ensuring a probe shedding rate of ≤0.008% after 100,000 compression cycles (Experimental Example 4); and guide holes in the upper layer with a 10%-20% increased diameter control the substrate wear depth to less than 0.2μm. These three factors work together to ensure the long-term stability of the probe's vertical motion trajectory, significantly improving the 12.3% shedding rate of a single-layer structure (Experimental Example 4). In addition, this solution constructs an ultra-low-loss electrical channel by sputtering a 0.5-2μm gold-plated layer (square resistance ≤ 20mΩ / □) on the inner wall of the microhole. This reduces the attenuation of 10GHz high-frequency signals from -12.4dB of uncoated ceramic to -0.41dB (power loss is reduced from 94% to 9%), and increases the current load capacity of a single hole by 20 times to 10mA (Experimental Example 3). Combined with a 4096-point high-density connector, the yield of the microholes on the entire board reaches 99.98%, and the cost is reduced by 96% compared to traditional copper filling solutions, providing a mass production solution for submicron PAD testing of millimeter-wave chips.

[0027] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely preferred examples of the present invention and are not intended to limit the present invention. Various changes and improvements may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and improvements fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A laser drilling method for processing micro-holes in ceramics, characterized in that: The following steps are involved: S1. Prepare a machinable ceramic substrate, which is used to form a needle plate of a test fixture; S2. Drilling a plurality of micropores on the substrate using laser, wherein the diameter of the micropores is 15±1.5 μm, and the distance between the pore walls of any two adjacent micropores is 2-5 μm; S3, installing a probe in the microhole to form a needle-disc structure for contacting the circuit board to be tested; S4. Connect the needle disk structure to the electrode surface assembly, wherein the electrode surface assembly is provided with a gold-plated conductive layer and a connector interface for connecting the probe and the electrical testing equipment.

2. The laser drilling method for machining micro-holes in ceramics according to claim 1, characterized in that: In S1, the needle disk structure is composed of multiple layers of stacked ceramic plates, including: Bottom layer: equipped with a microhole array corresponding to the PAD position of the circuit board to be tested, with a pore size of 15±1.5μm; Middle layer: equipped with a hollow card slot structure for fixing the middle clamping ring of the probe; Upper layer: equipped with a guide hole, the aperture of which is 10%-20% larger than that of the bottom layer, for guiding the insertion of the probe.

3. The laser drilling method for machining micro-holes in ceramics according to claim 2, characterized in that: In S1, the multilayer ceramic plates are fixed and stacked by pillars, and the coaxiality deviation of the micropores between each layer is ≤3 μm.

4. The laser drilling method for machining micro-holes in ceramics according to claim 1, characterized in that: In the S2, the laser is an ultraviolet laser with a wavelength range of 100-400 nm, a pulse frequency of 10-100 kHz, and a single pulse energy of 0.1-10 μJ.

5. The laser drilling method for machining micro-holes in ceramics according to claim 4, characterized in that: In S2, the drilling process adopts a layered scanning strategy, the drilling depth of each layer is 1-5 μm, and the total drilling time is ≤0.5 seconds per hole.

6. The laser drilling method for machining micro-holes in ceramics according to claim 1, characterized in that: In the above-mentioned S4, the thickness of the gold-plated conductive layer is 0.5-2 μm, and the square resistance is ≤20 mΩ / □.

7. The laser drilling method for machining micro-holes in ceramics according to claim 1, characterized in that: In the S4, the connector interface is a 1024-point, 2048-point or 4096-point high-density connector.

8. The laser drilling method for machining micro-holes in ceramics according to claim 2, characterized in that: The size of the middle-layer hollow card slot structure is 3-8 μm larger than the diameter of the probe card ring, which is used to prevent the probe from falling off.

9. The laser drilling method for machining micro-holes in ceramics according to claim 1, characterized in that: In the above S1, the material of the machinable ceramic substrate is alumina ceramic or aluminum nitride ceramic.

10. The laser drilling method for machining micro-holes in ceramics according to claim 1, characterized in that: A pin-on-disk substrate is prepared by the laser drilling method according to any one of claims 1 to 9, wherein the pin-on-disk substrate satisfies: Micropore diameter 15±1.5μm, pore position accuracy ≤3μm; The minimum distance between adjacent micropore walls is ≥2μm; Contains a middle-layer hollow card slot structure, the card slot size is 3-8μm larger than the probe card ring diameter; There are no burrs or thermal cracks on the hole wall after drilling.

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