Blind and Buried Via in PCB: The Complete Guide
Blind and buried vias are a sort of salvation when plating through hole (PTH) is not an option. For example, when you fabricate high-dense printed circuit boards (PCBs).
As a matter of fact, most high-performance boards cannot go without blind and buried vias. So let’s learn more about them, compare them, and overview fabrication costs.
What is Blind and Buried via in PCB?
These are connective, plated-through holes between PCBs’ layers. They are used to route a trace between outer and internal layers. It is especially vital for multi-layered boards with complex designs where the provision of sufficient flow of both electrical and thermal current is an issue.
Vias are drilled mechanically and then plated to create an electrical connection. You can see them as simply channels that may vary in diameters/sizes and types. For example, in the image below, there is a conventional, non-optimal PTH via, a buried via, and a blind via.
It is apparent that the first type goes through the entire PCB connecting all the layers at once. But let’s focus more on the other two types as they are way more functional and complicated to use.
- Blind via. Ensures a connection of the surface layer with several next ones. It is called blind because it makes it impossible to see through the PCB.
- Buried via. Ensures a connection between several inner layers. It is called buried because it is not visible on a PCB’s surface.
Understanding what blind and buried via is actually is a no-brainer. But, the ways they are constructed and fabricated and the rules for their fabrication are more complex matters.
Here is an example of universal components of a via detailed in the infographics below.
- Barrel. It is the conductive pole tube that fills the pre-drilled/percolated hole.
- Pad. These are small, metallic structures that link the barrel’s ends to traces.
- Antipad. It is a round, clearance hole separating the barrel from the non-connective layer.
Regardless of the method used to fabricate a via, these elements have to be present for a connective hole to function correctly.
Difference Between Blind and Buried Vias
Let’s have a look at concrete factors that differentiate these types of vias:
| Factor | Blind via | Buried via |
|---|---|---|
| Form of connection | Outer layer to one or more inner layers. Does not go through the entire stuck-up | Inner layer to one or more inner layers. Does not go to an outer layer |
| Fabrication costs | Lower cost because no extra press cycle is required | Higher cost because of the extra press cycle. But it is a case only when a buried via was not an optimal design decision. Otherwise – the same cost. |
| Fabrication complexity and versatility | There are 4 fabrication methods, 2 of which are rather simple | There are two fabrication methods that are slightly more complex |
| Purpose | To free up a PCB’s surface and to eliminate signal degradation and signal discontinuity. | To free up a PCB’s surface without deteriorating traces or connections between the mounted components. |
| Compatible PCB | Any multilayer PCB, but especially prevalent for high-frequency applications because it can connect high-speed signal lines. | Any multilayer PCB |
| Design constraints | There are some limitations | There are major limitations |
| Technical specifications | Mechanical blind via: max diameter – 0.4 mm. min diameter – 150 μm. via pad diameter – 450 μm. Annual ring size – 127 μm. Aspect ratio 1:1 Laser blind via: max diameter – 0.1 mm. min diameter – 100 μm. via pad diameter – 254 μm. Annual ring size – 150 μm. Aspect ratio 1:1 | Mechanical buried via: max diameter – 0.4 mm. min diameter – 100 μm. via pad diameter – 300 μm. Annual ring size – 150 μm. Aspect ratio 1:10 Laser buried via: max diameter – 0.4 mm. min diameter – 100 μm. via pad diameter – 225 μm. Annual ring size – 150 μm. Aspect ratio 1:12 |
Here is the infographic showing the essential measures:
Pros and Cons of Blind Via
Let’s start with reviewing the strengths of blind vias:
- It saves space in other PCB layers, leaving more opportunities for routing.
- Make PCBs more functional.
- Improve the functioning of slow PCBs.
- They are the only way to optimize the use of layers in multilayer PCBs.
- It is a widespread technique, and most manufacturers offer such services.
- Minimal interference between signals.
- They ensure better pad density.
And here are three considerable limitations of blind vias:
- It adds more manufacturing steps and makes PCB production time-consuming.
- It is a bit more expensive (compared to conventional PTH ones).
- There is a possibility of impedance discontinuities and substantial energy losses for large currents.
Blind and Buried Vias Rules
It may seem like blind and buried vias are a universal solution to all the design constraints. However, it is not the case. Actually, there are some rational and technical limitations that you should consider:
- Not all combinations are possible.
- A via always has to connect an even number of copper-plated layers.
- A via cannot end at a core’s top side.
- A via cannot start at a core’s bottom side.
- Both types of vias cannot start and end inside each other unless completely enclosed.
- Clearance design rules are applicable here as well.
- Use blind and buried vias only when necessary as they add to cost significantly.
These are some of the must-to-follow rules. Several of them are even integrated in software for creating digital PCB designs.
PCB Fabrication Process for Blind Via
Let’s start by reviewing a conventional process for making a via, as it is applicable to several more advanced methods (like sequential lamination). It is also commonly used in the DIY fabrication of PCBs:
- Drill holes through copper pads on a PCB.
- Apply a chemical into the drilled holes to liquefy epoxy and uncover the inner pad layers.
- Apply copper into the hole using the electro-coating method.
A small tip is to place a sacrificial via on a corner of a board. Then, slice it and determine if electro-coating was executed successfully. This method is used even by large manufacturers for testing their PCBs and for rapid manufacturing of PCB prototypes.
There are in total 4 methods to fabricate a blind via. Let’s take a look at them.
Sequential Lamination
A thin piece of laminate, such as FR-4, Telfon, or Rogers, undergoes the conventional PCB production stages. The purpose here is to manufacture a two sided PCB first. Then, a detailed-above process is applied, i.ePCB is drilled, etched, and the holes are platted.
Eventually, this layer is laminated with all other layers to comprise a uniform multilayer board. The process is expensive because it basically implies the need to machine a PCB twice.
Photo Defined Fabrication
It is simply laminating a thin layer of photosensitive resin to a core. Holes shall already have been drilled in the PCBs. After that, the photosensitive side is exposed to UV light so the material can fully harden. Next, the material in the holes is removed with etching solution/acid.
When these steps are finished, it is possible to perform copper plating so the thin layer of this material covers both the hole’s internal surfaces and a PCB’s outer surface. This production method is slightly less expensive compared to sequential lamination. But, it suits better designs when a PCB accommodates many holes at once, as the cost per via would be much lower.
Controlled Depth Fabrication
It is the same drilling approach as detailed previously. A hole is drilled to a certain depth into a PCB and then plated. It is a cost-effective and simple method, but it has a severe limitation. The smallest width depends on a drill size which rarely can be smaller than 0.15 mm.
Laser Drilled Fabrication
This procedure can be implemented after the lamination of a PCB but before it has undergone etching and outer layer lamination stages. In this case, a via is created by one-stage laser drilling of both the copper and the dielectric material. This method requires a high initial investment in equipment units but offers a better cost per via than drilling-based approaches.
How Much do Blind Vias Cost?
You have already had an initial insight into how much one or another method of fabrication may cost. Here are all the cost factors for your consideration.
- Fabrication method. As detailed, sequential lamination is the most expensive technique overall. At the same time, photo defined fabrication offers a superior price per via. Controlled depth fabrication is simply the most affordable method.
- Aspect ratio. Typically, manufacturers charge more for aspect ratios that are rather than 10. It is so because such design decisions may require non-conventional approaches to fabrication. But what is even more important is the pad clearance which is extremely hard to keep at higher aspect ratios.
For example, an aspect ratio of 12 on a 4 mm board means the needed via size is 0.3 mm, pad size of 0.56 mm, and a drill hole size of 0.36 mm. In this case, via pad-tp-BGA pad clearance is only 0.09 mm., which is difficult to ensure.
- A number of vias. It is a very basic consideration that impacts some methods and the amount of time needed to fabricate all of them.
- Via size. A machine with higher revolutions per minute is needed to get a high-quality hole. Not necessarily, but it may affect the cost per via. And the differences in via sizes will absolutely result in higher cost. It is so because either several machine units will be required, or the same equipment unit will be subject to changing its tooling multiple times.
Typically, you will not be charged for standard designs in board shops, i.e., for designs that can be performed with standard drill set-ups. But it is because the average via cost is already in the final price. But large suppliers offering custom PCBs at affordable costs do charge depending on the factors detailed above.
Blind Vias vs Micro Vias
Micro vias in PCBs are similar to conventional vias but have different sizes and forms. Micro vias have a conical frustum shape, so it makes a smooth layer transition and terminates at a pad in the next layer. You can see an example in the image below.
Ideally, micro vias span a single layer only. But designers also use stacked micro vias to form connections through several layers. However, it is rather an exception as connecting multiple layers and other types of vias suit better.
Micro vias are small in size (typically about 150 μm in diameter). And they are simply drilled mechanically and plated. Then, layers are stacked, pressed, and laminated. Alternatively, these vias can be formed with the help of a laser.
Indeed, micro vias are an entire type of vias, and they have moth buried, blind, but also stacked and staggered types. So, from the perspective of the difference between a blind conventional via and micro via, the latter serves the purpose of optimizing a single layer. At the same time, conventional vias are needed for making multi-layer stuck-ups more functional.
Summary
Blind vias are a single type of such connection. Others are through-hole vias, buried vias, and micro vias and their subtypes.
You typically need vias to save space for extra routing and, as a result, for extra components mounted. There are four different fabrication methods to fabricate a blind via. You should stick to the one that will be more cost-effective based on your PCB design.
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