Blind and Buried Vias Are Used in PCBs
As component lead pitches become smaller and small form factor devices dominate a large percentage of the products being developed today, the ability to connect components on both sides of a PCB without using excessive space is becoming more important. With this in mind, it is essential that the PCB designer and manufacturer understands how to use blind and buried vias to reduce component costs while increasing layer density and signal integrity.
blind vias are barrel-shaped vertical conductive holes that make connections between multiple layers on a circuit board. The IPC defines eight different types of vias, but the two most common are blind and buried. These are used to connect outer layers with inner layers on the PCB and don’t penetrate all the way through the entire board.
These are typically used in HDI PCBs, which can offer higher layer densities than traditional PCBs. This allows for the inclusion of more electronics in a smaller, lighter, and more portable device. They also help to improve signal integrity by reducing the distance that signals must travel through the board, making them more reliable.
How Blind and Buried Vias Are Used in PCBs With High-Speed Differential Pairs
Blind and buried vias are complex to design and manufacture, but they offer many benefits. As a result, they are often used in high-speed PCBs that require more rigorous power delivery and signal integrity constraints than standard PCBs. When used correctly, they can allow a much smaller PCB to be made with more advanced features than what would otherwise be possible.
When designing a PCB with blind and buried vias, the first step is to create the desired pad locations on the outer layers of the PCB. Then, the corresponding hole locations must be created on the inner layers. These holes will be filled with copper plating to complete the conductor connections.
To create the vias, the PCB must be sent through a pre-preg and foil fabrication process. This process sandwiches a sheet of the pre-preg and a sheet of copper foil together and then uses heat and pressure to bond them all together. After this, the layers are separated and etched. Then, the copper foil is drilled to make the buried and blind vias.
The drilling can be done mechanically or with a laser. Generally, it is more cost effective to drill the copper and dielectric material with a laser. This also avoids the tool break problems that can occur with mechanical drilling. Depending on the complexity of the board, the number of blind and buried vias can vary. There is also a limit to how deep the vias can be. The maximum depth depends on the thickness of the outer layer.
When you choose to use blind and buried vias, it is important to follow the PCB design rules for high-speed PCBs. These will include specific net classes, differential pairs, trace widths and spacing, impedance controlled routing rules, and other important design requirements. Once these have been checked off, it’s time to start creating your final PCB.