shape of blind via holes
Printed circuit boards (PCBs) can be comprised of multiple layers, each with different signals and components. Vias, which are barrel-shaped vertical conductive holes that connect the layers, allow for signal transfer between layers. There are several different types of vias available, including blind and buried vias. While these two options may seem similar, there are different rules and limitations that govern their use.
Through hole (PTH) vias have traditionally been the standard for PCBs, and they still serve their purpose well. However, they are limited in their effectiveness due to the amount of space they take up. This is especially true on high-density boards that must be as compact as possible. This is where blind and buried vias come in, and they can help increase a PCB’s breakout channel while keeping it compact.
A blind or buried via can be used to connect the outer layer of a multilayer board to one or more inner layers. It can also be used to reduce the number of signal stubs that would otherwise exist between layers. Whether a blind or buried via is required, it must be drilled at the proper location on the core.
Depending on the fabrication process, a blind or buried via can be made before or after the multilayer lamination. If it is made before, the cores are drilled and then the vias are pasted together. On the other hand, if it is made after, the PCB has already been layered and the vias are plated.
Are there any restrictions on the shape of blind via holes?
A photo defined blind via hole is created by laminating a sheet of photosensitive material to the PCB’s core. This material is then covered with a pattern that covers the area where the holes are to be created. The exposed areas will harden when exposed to the correct wavelength of light. Once the material has been hardened, it will be immersed in an etching solution to create the holes. Once the holes have been etched, copper can be applied to them.
Another way that a controlled-depth blind via can be formed is by drilling the hole to the side of the pad, rather than directly underneath it. This method eliminates the problem of an air bubble forming under the solder ball, which can cause a short between the pad and the component. However, this method requires the product developer to be cautious in ensuring that there are components underneath the percolated hole that will form contact with it.
Creating a blind or buried via can be a challenge for the board designer, but it is possible to achieve a good result by following some simple guidelines. For example, the drill depth should be set at a level that will not interfere with any other layers of the board, and the width of the hole should be limited to the minimum necessary to make a connection between the surface layer and an internal layer. It is also a good idea to use an even ratio between the via height and diameter, because larger diameters can lead to higher signal interference and slower signal transmission.
