Principle and Application of Laser Drilling in HDI Manufacturing
在HDI（高密度互连）板的制造过程中，激光钻孔是实现“微孔径，高密度互连”的核心工艺。它利用高能激光束的热或光化学效应来创建盲孔和埋孔（有时小至10μm以下），其直径通常在 25μm到150μm之间 在印刷电路板（PCB）基板上。这取代了传统机械钻孔无法实现的微小孔径加工，为高密度互连（HDI）板的“多层和微型化”发展提供了关键支持。与机械钻孔相比，激光钻孔具有“无工具磨损、高孔径精度以及与柔性/高频基板的兼容性”等优点，使其成为HDI板（尤其是2层及以上的HDI）制造中不可或缺的工艺。
1. 激光钻孔的核心原理：能量聚焦和基板相互作用机制
激光钻孔的本质是“将高能激光束聚焦在局部区域”。14层板是指包含14个导电电路层（铜箔层）的印刷电路板。它通过交替堆叠13层绝缘基材（如FR-4基板）和14个导电层，然后在高温高压下压合在一起，形成一个整体。层间的电气连接是通过金属化孔（通孔、盲孔、埋孔）实现的。其核心结构特征反映在“逐层规划和功能分区” 的基板上，并通过能量转换破坏基板的分子结构以实现材料去除。根据HDI基板的类型（如FR-4，PI柔性基板，高频PTFE基板，它主要分为两类： 热消融机制 和 光化学消融机制。其具体原理差异如下：
(1) 热消融机制：适用于树脂基底（主流应用）
基于树脂的材料，如用于HDI板的FR-4和PI柔性基板，通常通过热气化机制进行钻孔。核心工艺分为四个阶段，本质上是“将激光能量转化为热能，快速加热基板至分解温度，并通过气化和熔化喷溅实现材料去除”。
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能量聚焦台:
由激光发生器（如CO₂激光、紫外线激光）产生的连续或脉冲激光束通过光学系统（聚焦透镜、反射镜）聚焦，形成一个直径只有几微米到几十微米的“高能光斑”，能量密度高达 10⁶-10⁹ W/cm²，精确作用于基板上需要钻孔的区域。
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基底加热和分解台:
聚焦的激光能量迅速传递到基板（树脂+玻璃纤维/PI膜）上，使局部温度急剧上升至树脂的分解温度（通常为400-600℃，FR-4树脂约为450℃，PI树脂为550℃） 微秒（μs）或甚至纳秒（ns）内。此时，树脂分子链分解成CO₂、H₂O和小分子有机物等气态产物，而玻璃纤维（在FR-4中）或PI膜在高温下部分熔化并气化。
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材料去除阶段:
基板分解产生的气态产物和熔融物质在高温和压力的作用下以“飞溅”的形式从基板表面分离，形成初步的“孔坑”。同时，激光束可以通过“多脉冲扫描”（即对同一区域重复作用多个脉冲激光）逐步加深孔坑，直到满足设计要求（例如，盲 Via 需要准确控制深度以避免穿透下层电路）。
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钻孔壁冷却和成型阶段:
After the laser pulse ends, the temperature of the hole wall area drops rapidly (the cooling rate can reach 10⁵℃/s). The remaining molten substances (such as glass fiber debris and resin carbonized layer) will form a “carbonized layer” (usually 0.5-2μm thick) on the hole wall, which needs to be removed in the subsequent “desmear” process to ensure the reliability of hole wall metallization.
Application Scenarios: FR-4 substrates (ordinary HDI boards), PI flexible substrates (flexible HDI boards), ceramic-filled resin substrates (high-power HDI boards), accounting for more than 90% of laser drilling applications in HDI; the commonly used laser types are CO₂ lasers (wavelength 10.6μm, suitable for resin removal) and ultraviolet lasers (wavelength 355nm/266nm, suitable for high-precision, low-heat-impact processing).
(2) Photochemical Ablation Mechanism: Suitable for High-Frequency/Special Substrates
For PTFE (polytetrafluoroethylene) substrates used in high-frequency scenarios in HDI boards or ultra-thin substrates sensitive to heat (such as PI films with a thickness of ≤25μm), the thermal ablation mechanism is likely to cause substrate deformation and hole wall carbonization (affecting high-frequency signal transmission). Therefore, the photochemical ablation mechanism is required, whose core is “laser energy directly breaking the chemical bonds of substrate molecules without high-temperature heating, achieving heat-free damage drilling”:
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Photon Absorption and Chemical Bond Breaking:
A deep ultraviolet (DUV) laser with an extremely short wavelength (wavelength 193nm) or an excimer laser is used, whose photon energy (about 6.4eV) is higher than the bond energy of C-C bonds (3.6eV) and C-F bonds (4.8eV) in PTFE molecules. When the laser irradiates the substrate, photons are directly absorbed by the substrate molecules, and the molecular chemical bonds can be broken without converting into thermal energy, decomposing PTFE into small-molecule fluorides (such as CF₄) and gaseous carbon.
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Heat-Free Damage Material Removal:
There is no obvious temperature rise during the entire process (the local temperature of the substrate is usually below 100℃), so there is no melting, carbonization, or thermal deformation. The hole wall has extremely high smoothness (roughness Ra ≤0.1μm) and no heat-affected zone (HAZ), which perfectly meets the requirements of high-frequency signals for “low loss and low interference”.
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Precise Depth Control and Forming:
By controlling the number of laser pulses (the thickness of material removed by a single pulse is only a few nanometers to tens of nanometers), “atomic-level” precise depth control of the PTFE substrate can be achieved, which is especially suitable for the processing requirement of “blind vias penetrating the insulating layer without damaging the underlying ground copper foil” in high-frequency HDI boards.
Application Scenarios: PTFE substrates for high-frequency HDI boards (such as 5G millimeter-wave radar HDI, satellite communication HDI) and PI substrates for ultra-thin flexible HDI boards; due to the high cost of equipment (the price of DUV laser equipment is about 5-10 times that of CO₂ lasers), it is currently mainly used in high-end HDI scenarios.
2. Key Technical Parameters of Laser Drilling for HDI: Determining Drilling Quality and Reliability
HDI boards have strict requirements for “aperture accuracy, hole wall quality, and depth control accuracy” of laser drilling. The following core parameters need to be accurately controlled to ensure that the drilling meets the requirements of subsequent hole metallization and signal transmission:
| Technical Parameter | Definition and Function | Typical Requirements for HDI Boards | Impact on HDI Performance |
|---|---|---|---|
| Aperture Accuracy | Deviation between the actual aperture and the designed aperture, including diameter deviation and roundness | Diameter deviation ≤±5μm, roundness ≥0.9 (i.e., the ratio of major axis to minor axis of the hole ≤1.1) | Too small an aperture will lead to poor conduction after hole metallization; too large an aperture is likely to cause short circuits between circuits, affecting high-density wiring |
| Hole Position Accuracy | Position deviation between the actual hole center and the designed hole center | Position deviation ≤±3μm (2nd-order HDI), ≤±1.5μm (3rd-order HDI) | Hole position deviation will cause “misalignment” between blind vias and inner circuits, resulting in “broken holes” or “edge contact”, reducing interconnection reliability |
| Depth Control Accuracy | Deviation between the actual depth and the designed depth of blind vias | Depth deviation ≤±5% (e.g., designed depth 50μm, actual depth 47.5-52.5μm) | Too shallow a depth will prevent the hole from penetrating the insulating layer after metallization, failing to achieve inter-layer interconnection; too deep a depth will penetrate the underlying copper foil, causing short circuits |
| Hole Wall Roughness | Degree of unevenness on the hole wall surface (expressed by Ra) | Ra ≤1.5μm for ordinary HDI, Ra ≤0.5μm for high-frequency HDI | Excessively high hole wall roughness will lead to uneven copper layer coverage during chemical copper plating, increasing the transmission loss of high-frequency signals; it may also leave drill debris, causing poor insulation |
| Heat-Affected Zone (HAZ) | Width of the area where the substrate’s performance changes due to thermal effects after drilling | HAZ ≤10μm for ordinary HDI, HAZ ≤5μm for high-frequency/flexible HDI | Excessively large HAZ will cause brittleness of the substrate (such as PI), reducing the bending life of flexible HDI; HAZ in high-frequency substrates will cause fluctuations in Dk/Df, affecting signal integrity |
3. Typical Application Scenarios of Laser Drilling in HDI: From Ordinary HDI to High-End Special HDI
The application of laser drilling in HDI boards mainly solves the pain point that “traditional mechanical drilling cannot process micro-apertures and is incompatible with special substrates”. According to the type and application field of HDI boards, it is mainly divided into the following three scenarios:
(1) Ordinary HDI Boards: Mainstream Application in Consumer Electronics
In HDI boards for consumer electronics such as mobile phone motherboards, tablets, and smart watches, laser drilling is mainly used to process 1st-order/2nd-order blind vias to achieve “high-density interconnection between surface circuits and inner circuits”. The typical application characteristics are:
- Substrate Type: Mainly FR-4 substrates (PI substrates are used in some flexible areas);
- Aperture Specification: Blind via diameter 50-100μm, depth 20-50μm (corresponding to the thickness of the HDI board insulating layer);
- Laser Type: Mainly CO₂ lasers (moderate cost, high processing efficiency), and ultraviolet lasers are used in some high-precision areas;
- Core Requirement: Meeting the high-density layout of “hundreds of holes per square centimeter” to adapt to the “miniaturized chips and dense components” in mobile phone motherboards (for example, the laser blind via density in iPhone motherboards can reach more than 500 holes/cm²).
Case: In the “BGA (Ball Grid Array) area” of a mobile phone motherboard, the chip pin pitch is only 0.3mm. Laser drilling is required to process blind vias with a diameter of 60μm to connect the surface BGA pads to the inner power/signal layers, realizing reliable interconnection of “narrow-pitch pins”. Traditional mechanical drilling (minimum aperture about 150μm) cannot meet this requirement.
(2) Flexible HDI Boards: Adaptation to Dynamic Bending Scenarios
In flexible HDI boards such as folding screen mobile phone hinge circuits and smart watch strap circuits, laser drilling needs to balance “micro-aperture” and “low damage” to avoid substrate brittleness caused by drilling. The typical application characteristics are:
- Substrate Type: PI flexible substrates (thickness 25-50μm), some matched with ultra-thin copper foil (12-18μm);
- Aperture Specification: Blind via diameter 30-60μm, depth 15-30μm, with strict control of the heat-affected zone (HAZ ≤5μm);
- Laser Type: Ultraviolet lasers (wavelength 355nm) are preferred, which have lower thermal effects than CO₂ lasers and can reduce thermal damage to PI substrates;
- Core Requirement: The bending life of the flexible HDI board after drilling is ≥100,000 times (180° bending), with no cracks or carbonization on the hole wall, ensuring that the hole metallized copper layer does not fall off during dynamic bending.
Case: For the “hinge flexible circuit” of a folding screen mobile phone, it is necessary to process blind vias with a diameter of 40μm on a PI substrate with a width of only 2mm to connect the surface and inner circuits. The low thermal damage characteristic of laser drilling can prevent the PI substrate from cracking from the hole wall during bending, ensuring the folding life.
(3) High-Frequency HDI Boards: Low-Loss Requirement for High-End Communication
In 5G millimeter-wave radar HDI and satellite communication HDI boards, laser drilling needs to meet “low loss and high signal integrity” to avoid hole wall defects affecting high-frequency signal transmission. The typical application characteristics are:
- Substrate Type: PTFE high-frequency substrates (Df ≤0.001) or hydrocarbon composite substrates (such as Rogers RO4003C);
- Aperture Specification: Blind via diameter 25-50μm, depth 10-30μm, hole wall roughness Ra ≤0.5μm, no carbonized layer;
- Laser Type: Deep ultraviolet (DUV) lasers (193nm) are used in high-end scenarios to achieve heat-free damage drilling through photochemical ablation; ultraviolet lasers (355nm) are used in mid-to-high-end scenarios;
- Core Requirement: No impurities or heat-affected zones on the hole wall, ensuring that the insertion loss of high-frequency signals (24GHz/77GHz) during transmission in the hole is ≤0.1dB (10GHz) and the return loss is ≥-25dB, avoiding signal reflection or attenuation.
Case: For a 77GHz millimeter-wave radar HDI board of an automobile, it is necessary to process blind vias with a diameter of 30μm through laser to connect the RF chip and the antenna layer. The high smoothness of the hole wall can reduce the “scattering loss” of high-frequency signals, ensuring that the radar detection distance is ≥200m and the accuracy is ±0.1m. The carbonized layer formed by traditional thermal ablation drilling will increase the signal loss by more than 30%, which cannot meet the requirement.
4. Comparison Between Laser Drilling and Traditional Mechanical Drilling: Advantages Highlighted in HDI Scenarios
In the manufacturing of HDI boards, there are significant differences between laser drilling and traditional mechanical drilling (using cemented carbide drills). The advantages of laser drilling are particularly prominent in “micro-aperture, special substrate, and high-density” scenarios:
| Comparison Dimension | Laser Drilling | Traditional Mechanical Drilling | Conclusion (HDI Scenario) |
|---|---|---|---|
| Minimum Aperture | Down to 10μm (DUV laser), conventional 25-100μm | Minimum about 150μm, hard to break through 100μm | Laser drilling fully meets the micro-aperture requirement of HDI |
| Hole Position Accuracy | ±1.5-3μm (optical positioning) | ±10-15μm (mechanical positioning, easily affected by tool wear) | Laser drilling meets the high-precision requirement of multi-layer alignment in HDI |
| Substrate Compatibility | Compatible with FR-4, PI, PTFE, etc., no tool wear | Only compatible with rigid substrates such as FR-4, tools are easily worn (PTFE wears drills) | Laser drilling covers all substrate types of HDI |
| Processing Efficiency | Single-hole processing time ≤1μs (pulsed laser), suitable for mass production | Single-hole processing time ≥10μs, efficiency drops sharply for small apertures | Laser drilling is more efficient in high-frequency HDI and high-density HDI scenarios |
| Thermal Damage/Tool Wear | Slight HAZ in thermal ablation (controllable), no tool wear | No thermal damage, but tools are easily worn (tools need to be replaced after processing 10,000 holes) | Laser drilling reduces consumable costs and is suitable for long-term mass production |
5. Conclusion
Through precise control of the “energy focusing and substrate interaction mechanism”, laser drilling solves the core problems of “micro-aperture, high-density interconnection, and special substrate adaptation” in HDI boards, and is a key process for the upgrading of HDI boards from “1st-order to 3rd-order and above” and from “ordinary consumer electronics to high-end high-frequency communication”. The evolution of its principle from “thermal ablation” to “photochemical ablation” adapts to the characteristics of different HDI substrates; and the strict control of “aperture accuracy, depth control accuracy, and hole wall quality” directly determines the interconnection reliability and signal performance of HDI boards.
随着HDI板向“更高密度（孔径≤20μm）、更高频率（≥100GHz）、更薄基板（≤25μm）”方向发展，激光钻孔技术也将迭代向“更短波长的激光（如极紫外EUV）、更高的定位精度（≤1μm）、更低的热损伤”发展，进一步推动HDI板在消费电子、汽车电子、航空航天等领域的应用突破。
