When it comes to designing and building printed circuit boards (PCBs), there are too many specific design elements to easily list. Each is essential to many functions of a given board, and engineers rely on versatility to provide the wide range of products seen today.
Vias are a particularly important design element for multilayer PCBs because they enable conductivity between the various layers and can reduce the overall PCB size.
A Breakdown of Vias
Vias are integral to many printed circuit board designs. Specifically, “via” is a name given to a hole that is drilled through a material. That hole is then filled with conductive material, and this process creates electrical connections between two or more layers of the material.
In multilayer circuit boards, vias enable the three-dimensional design elements that can shrink the overall PCB footprint.
While the concept is simple on its surface, there are 4 different types of vias used with nearly limitless different applications:
- Blind Vias
- Buried Vias
- Through Holes
- Microvias
Blind Vias
Blind vias refer to vias that connect outer layers of a board to the inner layers. At the same time, the via does not penetrate the entire board.
Therefore, a blind via is formed with careful control over penetration depths, and in many instances, it allows for via drilling after board assembly.
In other cases, blind vias are drilled before the board layers are connected, allowing for a wide range of design options.
Buried Vias
Buried vias are holes that connect inner layers of the board without reaching any of the outer layers. They can connect only two layers or multiple layers. As long as the via does not have external puncture points, it counts as a buried via.
By their nature, buried vias cannot be drilled after the board is fully assembled. Buried vias have to be drilled at prior stages in the PCB’s manufacturing process as a subset of a sub-laminated metal layers. Most often, buried vias are drilled in each layer separately, before any of the board layers are combined.
Through Holes
Through holes are vias that go through every single layer of the board. As such, through holes can be drilled in assembled boards in many instances. Naturally, there are processes that drill the holes in each layer separately as well.
Generally, it is more cost effective to mechanically drill through holes in a multilayer, full thickness PCB. Laser drilling through holes in full thickness boards will have limitations based on the hole diameter and the over PCB thickness.
Laser drilling through holes in thinner sublaminate stackups or single cores laminates are more common for rigid circuits. Laser drilling through holes in flex circuits is a viable method as well because their stackups are thinner with dielectrics like polyimide that are easily laser ablated.
Microvias
Have you ever wondered what are vias vs microvias? Blind, buried, and through-hole vias can exist in many sizes and patterns and for many purposes. When the vias are small enough, they become microvias. Microvias are ideal for many boards because they maximize real estate. They can also help with circuit reliability, impedance control, and RF line termination.
While there is no universal definition for the size of a microvia, the term typically refers to vias between 0.15 mm and 0.025 mm (0.001 inches) in diameter.
Considering the precision necessary to create such holes, microvias are often drilled using laser technologies. Laser drilling is preferable for much of microvia production for a few reasons. The obvious is the aforementioned positional precision that allows for such small hole drilling and landing on small inner layer pads or etch artwork.
Laser microvia drilling is also better at handling diverse materials and depths. Some of this goes back to the precision in laser drills, but variable wavelength drilling options also help laser drills adapt to and overcome any number of material-induced obstacles. Virtually any PCB can be drilled successfully by lasers.
Laser drilling also allows for different treatments that can help prevent or reduce material contamination on the board or within the via itself. Such treatments can minimize resin and carbon contamination that can otherwise complicate mechanical drilling and punching techniques. I
Conclusion
Ultimately, vias and microvias are essential to PCB design and production. Understanding the various via techniques and purposes can help with design and manufacturing decisions, lowering costs, improving efficiency & reliability, and creating additional options along the way.