Back drilling processing method of circuit board

Abstract

The invention discloses a back drilling processing method of a circuit board, which comprises the following steps of: calibrating a drilling coordinate through an optical positioning point and a CCD (Charge Coupled Device) visual system, and marking an annular groove around a to-be-back-drilled hole by laser; a hard alloy drill bit is adopted for pre-drilling till the distance between the hard alloy drill bit and a target layer is 5-15 micrometers; replacing a diamond coating drill bit for secondary fine drilling, and monitoring the impedance change of the drill bit in real time to control the drilling depth; and residual glue on the hole wall is removed through low-temperature plasma treatment, and back drilling machining is completed through supercritical CO2 cleaning. The invention provides the high-precision and low-damage back drilling processing method, the technical problems of depth control, hole wall quality and multi-stage back drilling positioning are solved, and the dependence on chemical etching is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of printed circuit board processing, and particularly to a back drilling method for a printed circuit board. Background Art

[0002] In high-frequency / high-speed PCBs, residual columnar conductors in overly long metallized holes (such as vias) are prone to causing signal reflection and electromagnetic interference.

[0003] The existing back drilling process has the following defects:

[0004] 1. The control accuracy of drilling depth is low, and it is easy to damage the inner layer circuit;

[0005] 2. The hole wall is rough after drilling, and there are residual copper burrs;

[0006] 3. During multi-stage back drilling, positioning errors accumulate, affecting the yield;

[0007] 4. It relies on chemical etching to remove residues, resulting in large environmental pollution. Summary of the Invention

[0008] In view of the above problems, the present invention provides a back drilling method for a printed circuit board with high precision and low damage, which solves the problems of depth control, hole wall quality, and multi-stage back drilling positioning, and reduces the dependence on chemical etching.

[0009] The technical solution adopted by the present invention is as follows:

[0010] A back drilling method for a printed circuit board, comprising the following steps:

[0011] S1. Calibrate the drilling coordinates through optical positioning points and a CCD vision system, and laser mark an annular groove around the hole to be back drilled;

[0012] S2. Use a cemented carbide drill bit to perform pre-drilling until it reaches 5 - 15 μm from the target layer;

[0013] S3. Replace the diamond-coated drill bit for secondary fine drilling, and monitor the impedance change of the drill bit in real time to control the drilling depth;

[0014] S4. Remove the residual glue on the hole wall through low-temperature plasma treatment, and complete the back drilling process by using supercritical CO2 cleaning.

[0015] In a further technical solution, in step S1, the diameter of the optical positioning points is 0.1 - 0.3 mm, and they are distributed in the four diagonal regions of the PCB board.

[0016] In a further technical solution, in step S1, the depth of the laser-marked annular groove is 5 - 10 μm, and the width is 20 - 50 μm.

[0017] In a further technical solution, in step S2, the tip angle of the pre-drilling bit is 25°-35°, the rotational speed of the pre-drilling is 10,000-15,000 rpm, and the feed rate is 1.0-1.5 m / min.

[0018] In a further technical solution, in step S3, the rotational speed of the secondary fine-drilling bit is 20,000-30,000 rpm, and the feed rate decreases stepwise with the increase of the drilling depth. The initial feed rate is 0.8-1.2 m / min, and the final feed rate is 0.3-0.5 m / min.

[0019] In a further technical solution, in step S3, the coating thickness of the diamond-coated bit is 2-5 μm, and the cutting edge of the bit has a chip groove with a helix angle of 15°-25°.

[0020] In a further technical solution, in step S3, the specific method for real-time monitoring of the bit impedance change is as follows:

[0021] Judge whether the residual copper layer is drilled through by the current feedback signal between the bit and the PCB board, and trigger a stop signal when the impedance value drops to the set threshold.

[0022] In a further technical solution, in step S4, the gas for low-temperature plasma treatment is a mixed gas of argon and oxygen, and the mixing ratio is 3:1-5:1, and the treatment temperature is 40-60 °C.

[0023] In a further technical solution, in step S4, the pressure of the supercritical CO2 cleaning is 8-12 MPa, the temperature is 35-45 °C, and the cleaning time is 10-20 min.

[0024] In a further technical solution, the present invention is applicable to multi-order back-drilling processing. The positioning coordinates are updated by laser marking before each layer of back-drilling, and the cumulative positioning error between layers is ≤±5 μm.

[0025] The beneficial effects of the present invention are as follows:

[0026] 1. The two-stage drilling (pre-drilling + fine-drilling) combined with impedance monitoring, the depth error is ≤±8 μm;

[0027] 2. The laser ring marking is used to assist the bit alignment, and the positioning accuracy of the multi-order back-drilling is improved by 40%;

[0028] 3. The supercritical cleaning technology reduces environmental pollution. Detailed implementation mode

[0029] The embodiments of the present invention will be described in detail below.

[0030] Embodiment:

[0031] A back-drilling processing method for a circuit board, comprising the following steps:

[0032] S1. Calibrate the drilling coordinates through optical positioning points and a CCD vision system, and laser-mark an annular groove around the hole to be back-drilled;

[0033] S2. Use a cemented carbide drill bit to perform a pre-drill to a depth of 5 - 15 μm from the target layer;

[0034] S3. Replace the drill bit with a diamond-coated drill bit for secondary fine drilling, and monitor the change in drill bit impedance in real time to control the drilling depth;

[0035] S4. Remove the residual glue on the hole wall through low-temperature plasma treatment, and complete the back-drilling process by supercritical CO2 cleaning.

[0036] In another embodiment, in step S1, the diameter of the optical positioning points is 0.1 - 0.3 mm, and they are distributed in the four diagonal regions of the PCB board.

[0037] In another embodiment, in step S1, the depth of the laser-marked annular groove is 5 - 10 μm, and the width is 20 - 50 μm.

[0038] In another embodiment, in step S2, the tip angle of the pre-drill bit is 25° - 35°, the rotation speed of the pre-drill is 10000 - 15000 rpm, and the feed rate is 1.0 - 1.5 m / min.

[0039] In another embodiment, in step S3, the rotation speed of the secondary fine drill bit is 20000 - 30000 rpm, the feed rate decreases stepwise as the drilling depth increases, the initial feed rate is 0.8 - 1.2 m / min, and the final feed rate is 0.3 - 0.5 m / min.

[0040] In another embodiment, in step S3, the coating thickness of the diamond-coated drill bit is 2 - 5 μm, and the drill bit edge has a chip flute with a helix angle of 15° - 25°.

[0041] In another embodiment, in step S3, the specific method for real-time monitoring of the change in drill bit impedance is:

[0042] Judge whether the residual copper layer is drilled through through the current feedback signal between the drill bit and the PCB board, and trigger a stop signal when the impedance value drops to the set threshold.

[0043] In another embodiment, in step S4, the gas for low-temperature plasma treatment is a mixed gas of argon and oxygen, the mixing ratio is 3:1 - 5:1, and the treatment temperature is 40 - 60 °C.

[0044] In another embodiment, in step S4, the pressure of supercritical CO2 cleaning is 8 - 12 MPa, the temperature is 35 - 45 °C, and the cleaning time is 10 - 20 min.

[0045] In another embodiment, the present invention is applicable to multi-stage back drilling processing. Before each layer of back drilling, the positioning coordinates are updated through laser marking, and the cumulative positioning error between layers is ≤ ±5 μm.

[0046] The above-described embodiments merely represent specific embodiments of the present invention, and the description thereof is relatively specific and detailed. However, it should not be construed as limiting the scope of the patent of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention.

Claims

1. A back drilling method for a circuit board, characterized in that, It includes the following steps: S1. Calibrate the drilling coordinates through the optical positioning points and the CCD vision system, and laser mark an annular groove around the to-be-back-drilled hole; S2. Use a cemented carbide drill bit to perform a pre-drilling until reaching 5 - 15 μm away from the target layer; S3. Replace the diamond-coated drill bit to perform a secondary fine drilling, and monitor the change of the drill bit impedance in real time to control the drilling depth; S4. Remove the residual glue on the hole wall through low-temperature plasma treatment, and complete the back drilling process by supercritical CO2 cleaning.

2. The back drilling method for a circuit board according to claim 1, wherein In step S1, the diameter of the optical positioning points is 0.1 - 0.3 mm, and they are distributed in the four diagonal areas of the PCB board.

3. The back drilling method for a circuit board according to claim 1, wherein In step S1, the depth of the laser-marked annular groove is 5 - 10 μm, and the width is 20 - 50 μm.

4. The back drilling method for a circuit board according to claim 1, characterized in that, In step S2, the tip angle of the pre-drilling drill bit is 25° - 35°, the rotation speed of the pre-drilling is 10000 - 15000 rpm, and the feed rate is 1.0 - 1.5 m / min.

5. The back drilling method for a circuit board according to claim 1, characterized in that, In step S3, the rotation speed of the secondary fine drilling drill bit is 20000 - 30000 rpm, and the feed rate decreases step by step as the drilling depth increases. The initial feed rate is 0.8 - 1.2 m / min, and the final feed rate is 0.3 - 0.5 m / min.

6. The back drilling method for a circuit board according to claim 1, characterized in that, In step S3, the coating thickness of the diamond-coated drill bit is 2 - 5 μm, and the drill bit cutting edge has a chip flute with a helix angle of 15° - 25°.

7. The back drilling method for a circuit board according to claim 1, characterized in that, In step S3, the specific method for real-time monitoring of the change of the drill bit impedance is: Judge whether the residual copper layer is drilled through through the current feedback signal between the drill bit and the PCB board, and trigger a stop signal when the impedance value drops to the set threshold.

8. The back drilling method for a circuit board according to claim 1, wherein In step S4, the gas for low-temperature plasma treatment is a mixed gas of argon and oxygen, and the mixing ratio is 3:1 - 5:1, and the treatment temperature is 40 - 60 °C.

9. The back drilling method for a circuit board according to claim 1, characterized in that In step S4, the pressure of supercritical CO2 cleaning is 8 - 12 MPa, the temperature is 35 - 45 °C, and the cleaning time is 10 - 20 min.