What is a blind via?
It is a connection from an outer layer of a board to one or more inner layers without exiting out the opposite side of the board. When a via starts on one side of a PCB but does not pass all the way through, it is called a blind via.
Why are Blind Vias used?
Blind vias reduce parasitic capacitance by decreasing the overall via stub length and width. As a signal travels through a via, signal degradation and reflections occur. The signal discontinuity (capacitive and/or an inductive discontinuity) created by a via can affect signal and power integrity in high-speed designs.
Blind vias are a good way to connect high speed (above 5 Gb/s) signal lines.
What types of PCB manufacturing processes are used to make blind vias?
- Sequential lamination
- Photo defined
- Controlled depth
- Laser drilled
1. Sequential lamination blind vias
In this process, an extremely thin piece of laminate undergoes all the production steps that would be required to manufacture a 2 sided PCB. It is drilled, etched & plated. Subsequently, this layer is laminated with all of the other layers of the PCB.
The number of manufacturing steps involved in this method of manufacturing blind vias makes it very expensive.
2. Photo defined blind vias
This manufacturing process Involves laminating a sheet of photosensitive resin to a core. The pattern covered photosensitive sheet is exposed to light causing the residual material to harden. The material in the holes created is removed using an etching solution. Copper is plated in the hole and on the outer surface to create the outer layer of the PCB.
This blind via production method is cost-effective when there are a large number of blind vias on a PCB.
3. Controlled depth blind vias
This approach uses the same drilling process as used for through-hole vias. Except a hole is drilled to a certain depth into the PCB, which is then plated.
Generally, drilling is a cost driver but this method is the least expensive way to create blind vias, but the smallest width that can be drilled is dependent on the minimum available drill size, which is usually 0.15 mm.
4. Laser drilled blind vias
This process is done after all layers of the PCB have been laminated but before the etching & lamination of the outer layer. A via can be created by laser drilling the copper and the dielectric material in a single stage.
This is a cost-effective method. To reduce cost and production time, laser-drilled vias could be a better choice than through-hole vias.
What are the potential problems with blind vias?
- Not observing typical clearance rules from other pads or traces within the inner layers
- Drill depth
- Unsuccessful plating – not getting the copper deposit into the bottom and sides of the hole (to avoid this issue, the diameter of the hole must be as large as its depth
- Drilling the correct depth
- Occurance of bubbling when soldering a component with a blind via in pad (to avoid this problems, via can be drilled to side of the pad, or completely filling the hole with copper.