As electronic products continue to evolve toward higher density and miniaturization, the application of high-density interconnect (HDI) printed circuit boards is becoming increasingly widespread.
HDI technology enables efficient connections between multiple layers through micro-blind vias and buried vias, thereby significantly improving circuit routing density and electrical performance.
In HDI structures, first-order HDI is relatively simple in structure, with mature manufacturing processes that are easy to control.
However, as the number of layers and structural complexity increase, second-order HDI boards face greater technical challenges during design and manufacturing, particularly in critical process steps such as alignment accuracy, microvia processing, and plating quality, which place higher demands on manufacturing capabilities.
First-order HDI boards are relatively simple and easy to control. Second-order processes become more complicated; one reason is alignment issues, and another is drilling and copper plating.
There are many types of second-order designs. First, the positions of each layer are staggered, requiring connection to the next adjacent layer, with conductors connected in the intermediate layer—equivalent to two first-order HDI boards.
Second, two first-stage vias overlap, and the second stage is achieved through this layering. Processing these two first-stage vias involves numerous process points that require special control.
The third method involves drilling directly from the outer layer to the third layer (or the n-2th layer). This process differs significantly from the previous ones, and drilling is more difficult.
Explanation as follows:
The first and second stages of a six-layer board require laser drilling, i.e., HDI boards.
For a six-layer first-order HDI board, the blind vias are: Layer 1 to Layer 2, Layer 2 to Layer 5, and Layer 5 to Layer 6. Among these, the vias between Layer 1 and Layer 2, and Layer 5 and Layer 6 require laser drilling.
For a six-layer second-order HDI board, the blind vias are: Layer 1 to Layer 2, Layer 2 to Layer 3, Layer 3 to Layer 4, Layer 4 to Layer 5, and Layer 5 to Layer 6. This requires two rounds of laser drilling.
First, the buried vias between layers 3 and 4 are drilled, followed by lamination of layers 2 through 5. Next, the laser-drilled holes between layers 2 and 3, and 4 and 5 are created, followed by another lamination of layers 1 through 6. Finally, the laser-drilled holes between layers 1 and 2, and 5 and 6 are created.
The through-holes are drilled in the final step. As can be seen, a second-order HDI board undergoes two lamination processes and two laser drilling operations.
Additionally, two-layer HDI boards can be classified into staggered-hole and stacked-hole types.
A staggered-hole two-layer HDI board refers to blind vias in layers 1–2 and 2–3 arranged in an alternating pattern, while a stacked-hole two-layer HDI board refers to blind vias in layers 1–2 and 2–3 stacked on top of each other, for example: blind vias in layers 1–3, 3–4, and 4–6.
Conclusion
Overall, second-order HDI boards are significantly more complex than first-order HDI boards in terms of structural design and manufacturing processes. The core challenges lie in controlling interlayer alignment accuracy during multiple lamination processes, ensuring the quality of laser microvia processing, and maintaining consistency in multi-layer electroplating.
Different design approaches (such as staggered-hole and stacked-hole structures) each involve trade-offs in terms of reliability, manufacturing difficulty, and cost.
Therefore, in practical applications, a comprehensive evaluation and optimized design must be conducted based on product performance requirements and manufacturing capabilities to achieve the optimal balance between high-density interconnect performance and manufacturability.
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