1. Introduction
With the rapid advancement of electronic technology, modern electronic devices have become smaller and more complex. Printed Circuit Boards (PCBs), which serve as the backbone for interconnecting electronic components, have evolved from single-layer to multi-layer, high-density, and high-precision structures. As a result, the size of holes required on PCBs has decreased significantly, while the number and density of holes have increased. This necessitates advanced micro-hole processing technologies to ensure high-quality manufacturing.
PCB materials are highly complex, with various types and specifications. This paper focuses on the processing of tiny pores—those with diameters less than 0.6 mm and micropores below 0.3 mm. These are primarily made from epoxy resin composites, which are known for their high brittleness, hardness, fiber strength, and low interlaminar shear strength. The anisotropic nature of these materials, along with differences in thermal expansion between fibers and resin, makes them challenging to process. High cutting temperatures can lead to thermal stress, resin melting, and chip adhesion, all of which complicate the drilling process. Additionally, the abrasive nature of the material accelerates tool wear, further impacting drilling quality and efficiency.
Currently, two main techniques are used for micro-hole processing: mechanical drilling and laser drilling. While mechanical drilling offers high precision and efficiency, it faces challenges such as drill bit breakage, delamination, and burr formation. Laser drilling, on the other hand, provides non-contact processing with high accuracy and no tool wear, making it ideal for ultra-fine hole creation. However, each method comes with its own set of limitations and challenges that must be addressed for optimal performance.
2. Mechanical Drilling
Mechanical drilling is widely used due to its high efficiency and precise hole positioning. However, when dealing with micro-holes, the small diameter of the drill bit increases the risk of breakage. During the drilling process, issues such as delamination, wall damage, and burrs can occur, affecting the overall quality of the PCB.
2.1 Cutting Force
The axial force and torque during drilling play a critical role in determining the success of the process. Factors like feed rate, cutting speed, fiber orientation, and the presence of pre-drilled holes significantly influence these forces. As feed rate and cutting speed increase, so do the axial force and torque, leading to greater tool wear and heat generation.
The axial force can be divided into static and dynamic components. The static component affects the chisel edge, while the dynamic component impacts the main cutting edge. The dynamic component has a greater effect on surface roughness. Studies show that the presence of a pre-drilled hole can reduce the static component force, especially for smaller apertures.
2.2 Bit Wear and Breakage
Drill bit wear in PCB composites is caused by both chemical and frictional factors. At higher temperatures, the resin can chemically attack the carbide tool, leading to accelerated wear. Friction becomes the dominant factor at lower speeds. The ratio of drill radius to fiber bundle width also influences tool life. Research indicates that larger ratios lead to more severe wear and shorter tool life.
During micro-hole drilling, increased feed rate and depth lead to higher axial force and torque. Poor chip removal causes resin melting and adhesion, resulting in chip clogging and eventual breakage of the drill bit. Common failure modes include buckling, torsion, and combined failures, often due to chip blockage and increased torque.
2.3 Drilling Damage Forms
(1) Delamination
Delamination is one of the most serious defects in PCB drilling, especially in glass fiber-reinforced composites. It occurs when the axial force exceeds the bonding strength between layers, causing separation. Techniques like variable feed control, pre-drilled guide holes, and vibration dampers help reduce this issue.
(2) Hole Wall Damage
Hole wall damage can affect insulation and copper layer integrity after metallization. The angle between the cutting direction and fiber orientation, as well as the thickness of the fiber bundle, significantly influences the extent of damage. Experiments have shown that fiber wrinkles vary across different layers, with some showing up to 30 μm protrusions.
(3) Stains
Stains occur when heat from the drilling process causes resin to melt and adhere to the hole walls or interlayers. Proper tool sharpening and cutting parameters can minimize these stains and improve surface quality.
(4) Burrs
Burr formation is common in composite drilling, especially when the drill bit does not fully penetrate the material. Residual fibers can accumulate at the hole edges, forming burrs. Techniques like vibration drilling and optimized tool geometry can reduce this problem.
3. Vibration Drilling
Vibration drilling is a novel technique that introduces controlled vibrations during the drilling process, allowing for intermittent cutting. This method improves drilling accuracy, reduces surface roughness, and minimizes burrs. Compared to conventional drilling, vibration drilling produces lower axial forces and better tool life.
Studies have shown that optimizing vibration parameters, such as frequency and amplitude, can significantly enhance drilling performance. For example, using a vibration amplitude of 6 μm and a frequency of 300 Hz can reduce axial force to as low as 1.5 N. This method is particularly effective for multi-layer composites, where stepwise parameter adjustments improve drilling quality and efficiency.
4. Laser Drilling
Laser drilling is increasingly preferred for micro-hole processing due to its non-contact nature, high speed, and precision. It is especially suitable for materials that are difficult to machine with traditional methods. CO2, KrF excimer, and Nd:YAG lasers are commonly used, each with distinct advantages depending on the material being processed.
CO2 lasers operate in the infrared range and are effective for thermal processing. They can produce high-quality holes but may cause thermal damage if not properly controlled. KrF excimer lasers, operating in the ultraviolet range, use photochemical mechanisms to minimize thermal effects, making them ideal for sensitive materials. Nd:YAG lasers offer flexibility with both thermal and photochemical processes, providing excellent control over hole quality and depth.
Each laser type has its own challenges, such as beam diffraction, energy distribution, and thermal expansion. Proper selection of laser parameters ensures minimal damage and accurate hole formation, especially in multi-layer composites.
5. Conclusion
In summary, the choice of drilling method depends on the specific requirements of the PCB design and material properties. Mechanical drilling remains a viable option for many applications, especially when combined with techniques like vibration drilling to enhance performance. Laser drilling, although more expensive, offers superior precision and is ideal for ultra-fine holes. Future research should focus on optimizing both methods to achieve higher efficiency, better quality, and longer tool life in PCB manufacturing.
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