3rd-order Blind and Buried Via Technology in flex PCB

As electronic products continue to evolve toward thinner, lighter, and higher-density designs, flexible PCB technology is also undergoing continuous advancement. Among the most critical innovations is the 3rd-order blind and buried via (HDI) technology, which has become a pivotal breakthrough in high-end flex PCB design and manufacturing.

What is 3rd-order Blind and Buried Via Technology?

In PCB or flex PCB manufacturing, blind vias and buried vias are used to create electrical interconnections between different circuit layers:

Blind via: Connects the outer layer to an inner layer without penetrating the entire board.

Buried via: Exists between internal layers only and is invisible from the surface.

The term "3rd-order" refers to a high-density interconnection structure built through three rounds of laser drilling and lamination, forming a more complex multilayer interconnect. For instance, in an 8-layer flex PCB, a 3rd-order HDI process could achieve L1–L4 interconnections, significantly improving routing density.

These via structures can be either staggered or stacked, depending on the product design requirements and manufacturing capabilities.

Why Does Flex PCB Need 3rd-order Blind and Buried Vias?

With the rising demand for flexible, high-density wiring in smartphones, wearables, and medical sensors, flex PCB boards are no longer just simple flexible connectors—they now require integration of complex multilayer structures.

The adoption of 3rd-order blind and buried via technology in flex PCB brings several significant advantages:

Increased Wiring Density

Traditional through-holes occupy space across all layers, while blind and buried vias offer localized interconnection. The 3rd-order structure enables over 40% increase in routing density, helping reduce circuit board area by 30%–50%. For example, the hinge-area flexible PCB in foldable smartphones relies on this technique for efficient signal routing within limited space.

Enhanced Structural Design

Supports sophisticated stack-ups, folding, and hybrid flexible-rigid combinations.

Reduced Signal Interference

Proper layer stacking improves signal integrity. In high-frequency, high-speed applications like 5G mm-Wave and automotive radar, blind and buried vias shorten the signal path, reducing impedance discontinuities and reflection. Bit error rates can drop by 15%–20%.

Space Saving

Ideal for ultra-thin or wearable devices. By reducing through-hole usage, the overall flexible circuit board thickness can be kept under 0.1mm, making it suitable for devices like smartwatches or AR glasses.

Improved Mechanical Reliability

Flex PCBs must endure repeated bending. Blind and buried vias avoid disrupting the structural integrity as through-holes do, allowing the bending life cycle to reach 100,000+ times—a crucial advantage in applications like TWS earbud charging case connectors.

Manufacturing Challenges

Despite its benefits, the 3rd-order blind and buried via process in flex PCBs presents higher manufacturing complexity than traditional rigid PCBs. Key challenges include those frequently encountered by any experienced flex PCB manufacturer, such as:

1. Precision Laser Drilling & Alignment Control

Due to the flexible nature of flexible PCB materials, laser drilling can lead to deformation, hole misalignment, or burn marks—affecting yield.

2. Multiple Lamination Cycles

3rd-order via structures require repeated lamination, drilling, and plating processes, leading to longer production cycles and tighter process control.

3. Reliable Copper Plating

Uniform and firm copper deposition in microvias is essential, especially for stacked structures, to prevent via separation or open circuits.

4. Material Compatibility

Common flex PCB substrates like Polyimide or PET are sensitive to heat and chemicals, requiring strict control of processing environments.

Conclusion

The 3rd-order blind and buried via technology is driving flex PCB manufacturing into a new era of precision and integration. For electronic products that demand ultra-thin profiles and high-density interconnects, this technique is not just an innovation—it is a critical enabler of performance and competitiveness.

Need a custom 3rd-order HDI flex PCB prototype? Contact our technical team today for tailored solutions.