HDI (Blind and Buried Vias) PCB Design Rules
In the world of circuit board manufacturing, the use of relevant technology and upgrades to the latest PCB design tools are the order of the day. It is rightly so because the PCB industry is navigating to modernization and the best way to get things working is by using the right tools.
You may have heard about blind vias and buried vias in the circuit board manufacturing process. They are terms that have become prominent as far as PCB design is concerned. That is because they aid in reducing the complex structures of Printed Circuit Boards.
If you are wondering what the two are doing on the circuit board, you couldn’t have come to a better place. This article will show you all you need to know about designing High-Density Interconnector (HDI) circuit boards using blind and buried vias.
What Does Vias Mean?
We want you to understand what those two terms mean. We will start with the meaning of vias, as that will give you background knowledge of what the other two concepts mean.
Vias are the copper-plated holes in Printed Circuit Boards (PCBs). They are there to facilitate the connection of the layers on the circuit board.
Worthy of mention is that the standard via is called Through-Hole. As much as the Through-Hole via is amazing in its way, there are some limitations when it is to be used in Surface Mount Technology (SMT).
It is against the backdrop of that limitation that the duo of the blind and buried vias is used for Surface Mount Technology (SMT) and other methods of placing components on a circuit board.
What Is a Blind Via?
The work of the blind via is to be the interconnector or connection between one layer and another.
What the blind via does is to connect one outer or external layer to one or multiple layers of the circuit board.
That is not all there is to the blind via. The other use case is that the blind via becomes the pathway that enables the interconnection of the top layers and the inner layers.
What Is a Buried Via?
On the other hand, the primary role of the buried via is to connect at least two inner layers of the circuit board. The layers to be connected in this case aren’t to be visible from the outer layers.
Moreover, using a buried via on a Printed Circuit Board (PCB) helps to bolster more functionality on the circuit board without taking up more spaces. That is made possible by the packing density technology that minimizes spaces and improves performance.
Application of Blind and Buried Vias
Indeed, the duo of blind and buried vias helps to improve the performance of circuit boards. Yet, there is a particular use case where the functionalities are optimum. It is on the High-Density Interconnector (HDI) PCB.
The advantages of using blind and buried vias on HDI PCBs are not underestimated. The most obvious advantage is that the vias help to optimize or pack the density of the circuit board without having to increase the number of board layers or board size.
Design Parameters for Blind Via
If you are set to use blind via for your next circuit board design, we would like you to take some time to understand the parameters. These rules are what you need to pull off the job to the desired outcome.
Having said that, below are some of the parameters and rules that must be considered anytime the blind via is being used on a High-Density Interconnector (HDI) circuit board:
Drill File
The first thing to have in mind is that the holes to be used for each connection must be defined on a separate drill file.
Furthermore, the ratio that will be used for adjudging the drill diameter to the depth of the hole is 1:1. Of course, this can be larger depending on the expectations on the circuit board.
Distance
The distance on the circuit board must also be considered. An excellent way of doing that is by using the value of the smallest hole to determine the depth.
When that has been determined, it wouldn’t be an arduous task to calculate and determine the maximum distance between the outer layer and the corresponding inner layers on the HDI circuit board.
Note: It is important to mention here that the de4sign parameters of the blind via varying by the type. For example, the value or the outcome when using laser technology tends to be different from what is obtained when using the mechanically drilled technology.
Design Parameters for Buried Via
On the other hand, there is the buried via, which is important also in the designing of the High-Density Interconnector (HDI) Printed Circuit Board.
Now, there are two angles to determining the design rules and principles of using a buried via for HDI PCB design.
Hole Connection
The first consideration is the size of the hole. Ideally, the holes that are to be used for each connection level on the HDI PCB must be defined as a separate drill file.
Also, the aspect ratio, which is the ratio of drill diameter to the depth of the hole, must be 1:12. It can also be larger depending on the components that will be used on the HDI circuit board.
Depth
The depth of the HDI circuit board can only be determined by the smallest hole. Therefore, the maximum distance that will separate each of the inner layers on the HDI circuit board can only be determined by the size of the smallest hole, which, in turn, influences the depth of the circuit board.
Note: Annular rings have a role to play in the stability of the electrical connection on the HDI circuit board. Therefore, the more annular rings are selected, the more stability of the connection will be enhanced.
Below is a table that details the various types of vias and how they can improve the performance, as well as optimize the use of the HDI PCB.
| Type of Via | Maximum Via Diameter | Minimum Via Diameter | Via Pad | Annular Ring | Aspect Ratio |
| Blind Via (mechanical) | 0.4mm | 150μm | 450μm | 127μm | 1:1 |
| Blind Via (laser) | 0.1mm | 100μm | 254μm | 150μm | 1:1 |
| Buried Via (mechanical) | 0.4mm | 100μm | 300μm | 150μm | 1:10 |
| Buried Via (laser) | 0.4mm | 100μm | 225μm | 150μm | 1:12 |
How to Use Blind and Buried Vias on HDI PCB
Incorporating or adding the blind and buried vias on a High-Density Interconnector (HDI) PCB is worth the effort. You will be amazed at the impressive design of the circuit board, as well as the overall improvement in the space on the board.
If you want to use blind and buried vias on an HDI PCB, some of the steps to doing that are:
Placement beneath the Layers
The hidden or buried layers are the ones that can be placed beneath the layers. When that happens, more spaces will be freed on the circuit board.
That will, in turn, free up space on the surface of the circuit board so that the HDI components that are required for the design can be added.
Note that care must be taken so that the freeing up of the space on the surface doesn’t traces or surface components on the top or bottom layers.
Reducing Signal Stubs
As much as buried via plays an important role in freeing up more spaces, the blind vias can also do more.
What it does here is to free up more spaces that are pivotal to the fine-pitching of the BGA components.
Worthy of mention is that the blind via helps to reduce stubs in the signals, especially during the drilling process. It is because of this that the signal stubs will be terminated at the last connected layer so there would be no alterations.
Compact
The assertion that using blind and buried vias helps to create more space on the HDI PCB is because of the overt role played by the buried vias.
What the buried vias does here is to keep the HDI circuit board compact and light while freeing up more spaces on the surface.
Drawbacks to Using Blind and Buried Vias on HDI PCBs
With the impressive benefits of the blind and buried vias come the downsides. We want you to understand these potential drawbacks so you wouldn’t be taken aback when things don’t work out as planned.
Extra Manufacturing Steps
The first drawback or downside to using blind and buried vias for HDI circuit board design is that more time would be needed to manufacture the circuit board.
Because of the complexities involved, it will take more time to design and manufacture the HDI PCB, as opposed to the shorter timeframe of designing the traditional Printed Circuit Boards (PCBs).
Added Costs
We know you are trying to avoid this, but it seems there is no way to do that. The fact is that designing the HDI PCB using blind and buried vias can trigger more costs because of the lengthy time it will take to get the circuit board ready.
Applications/Use Cases
If there are some use cases or places where the blind and buried vias are used for manufacturing electrical appliances, they wouldn’t surpass the ones we use every day.
The potential applications are:
- Medical devices
- Cell phones/smartphones and
- Laptops
General HDI Design Rules
You have now learned about the design rules that apply to the use of the blind and buried vias on HDI circuit boards.
Now, let us look at some of the design rules that apply to the designing and manufacturing of High-Density Interconnector (HDI) circuit boards.
Signal Integrity
The integrity of the signal strength must be prioritized at all times. The reason is that the transmission on the HDI circuit board needs to be accurate and the best way to determine that is by ensuring that the signal integrity is working as it ought to.
It is pertinent to mention that the same signal integrity rules that apply to traditional Printed Circuit Boards (PCBs) are also usable here. The only difference is that they must be scaled if need be.
Furthermore, the integrity of the signal can be bolstered by ensuring consistency in the impedance as a way of complying with the signaling standard. This is usually the case when the HDI circuit board requires impedance controlled routing as a way of enhancing the signal integrity.
Manufacturer’s Capabilities
Not all HDI PCB manufacturers have the same capabilities. Some may perform more than the others do.
That informs the reason why you need to run a background check on different PCB manufacturers to confirm their capabilities as they relate to HDI PCB design.
Use Fewer Layers
Although the use of blind and buried via can create more surface space on the HDI circuit board, it doesn’t mean that you should pack in all the components.
The rule of thumb is that you should use fewer layers on the board so that the manufacturing time and underlying costs will be reduced.
Don’t Use Staggered Vias
You may be tempted to use ELIC or staggered vias on the HDI circuit board because of the available spaces.
If you don’t need those, you shouldn’t incorporate them into the circuit board. That way, you will save more spaces as provided by HDI routing.
How to Select the Best HDI PCB Designer
Locating and selecting the best HDI PCB manufacturer for the job wouldn’t be difficult if you know what you want.
By defining the specifications for the HDI circuit board, the major factors you should be looking for are the fast turnaround time and the cost of manufacturing the circuit board.
You also want to consider the capability of the manufacturer, as well as the shipping methods so you can decide on whether to design your HDI circuit board with such a manufacturer.
Final Words
Blind and buried vias are important in High-Density Interconnector (HDI) design because they help to provide as much space as possible on the circuit board for routing on the surface and inner layers.
The next time you want to design an HDI circuit board, you already know that the duo of blind and buried vias will be the game-changer, and getting the best PCB designer/manufacturer for the job will make everything easier.