Blind and Buried Vias

Before you start any PCB fabrication process, you need to know the difference between blind and buries vias.

And, this guide will answer all questions you have been asking about blind and buries vias.

Keep reading to be an expert in PCB vias.

What is the Difference Between a Blind Via and a Buried Via?

Blind vs buried vias

Vias are interlayer connections with conductive capabilities.

They are employed to provide a conductive path between multiple layers in a multilayer PCB configuration.

The use of vias has allowed continued miniaturization of electrical and electronic products by allowing increased circuitry.

The blind vias offer connections between inner layers and any of the two outer layers.

A blind via does not cut across the board’s thickness but can traverse multiple layers.

Visibility of blind vias is possible only from one side: the top or bottom surface.

The buried vias offer interconnections between the inner layers only.

Consequently, you cannot identify buried vias from a circuit board’s surface.

These vias also only traverse an even layer count.

What are the Guidelines for making the Blind and Buried Vias?

When making blind and buried vias, it is important to note that you might be limited in the combinations possible.

Furthermore, the process of making these vias will significantly add to your overall circuit board cost.

Nonetheless, when making the blind and buried vias, there a few aspects worth noting.

• The blind via must begin from the outermost surface of a PCB.
• The length of a blind via cannot be across the board’s entire thickness.
• The length of a blind or buried via is made to cover an even count of conductive layers.
• The termination of a blind or buried via cannot be at a core’s top.
• The start of a blind or buried via cannot begin at a core’s base.
• A blind or buried via cannot start or terminate at the bottom of another blind or buried via. For this to happen, you have to furnish the blind or buried via within a blind or buried via.

How can Blind and Buried Vias be Furnished in a 4-layer PCB?

A 4-layer PCB is a board with four conductive layers.

The design of a four-layer PCB can be made in various ways.

A four-layer PCB can be made with a single core or two cores.

For a single-core 4-layer PCB, the core is provided between the inner layers.

The outer layers are furnished over prepreg material.

4 Layer PCB

The two-core design has a core between the outermost layers and the inner layers.

Therefore, you will find a core between the top layer and the first inner layer.

Alternately, another core will be between the second inner layer and the bottom layer.

For a single-core 4-layer PCB, you can only make buried vias between the inner layers.

Fabrication of blind vias will be impossible due to the guidelines for making vias.

For instance, vias have to cut through an even layer count and cannot terminate at the top or bottom of a core.

However, you can fabricate blind vias in a dual-core 4-layer PCB configuration.

The blind vias can be between the top layer and the first inner layer.

Additionally, you can make vias between the second inner layer and the bottom layer.

Nonetheless, in such a scenario, you will have to do without the buried vias.

Why is having Buried Vias in a Two-core 4-layer PCB not possible?

The 4-layer PCB can be furnished with either a single core or two cores.

With a single-core four-layer P, buried vias can be provided between the inner layers which sandwich the core.

However, in a two-core scenario, the inner layers are separated by prepreg.

Buried vias are typically made by separately drilling the conductive layers and the core.

Drilling of prepreg material separately is impossible, making it impractical to provide an interlayer connection.

Can you use Buried and Blind Vias in a Single PCB Construction?

Yes, you can.

The use of both blind and buried vias is possible in multilayer PCB configurations with a higher layer count than four.

In such a construction, achieving the via design guidelines is possible.

These include the even layer count length and non-core termination and start.

How do you make the Blind and Buried Vias?

Making of the blind and buried vias heavily depends on the drilling process.

Drilling is a fabrication process that involves making holes through the conductive layers and cores of a PCB.

The drilled holes can be plated or metalized to make them conductive or left bare.

For bling and buried vias, the holes are metalized.

In making blind and buried vias, laser drilling, guided by depth, is not employed.

Rather, the conductive layers and cores are separately drilled.

Where there are multiple plates and cores, they can be piled and drilled.

Upon completion of the separate drilling process, the layers are stacked together and pressed, a process that can require multiple procedures.

What are the Advantages of Blind and Buried Vias?

Blind and buried vias have mainly allowed the possibility of achieving higher densities on circuit boards.

The achievement has been accompanied without having to increase the board size.

Subsequently, the performance of applications employing circuit boards with buried and blind vias has improved.

Blind vias also reduce parasitic capacitance in circuit boards that can greatly hamper the signal quality.

By making blind vias with reduced depth and width, you offer a clear path for signal transference.

What are the Disadvantages of Blind and Buried Vias?

The main disadvantage of blind and buried vials is not related to their design or functionality but their associated costs.

Compared to a similar traditional board, the cost of fabricating a PCB with blind and buried vias is significantly higher.

Blind vias that are fabricated after the lamination process can also cause a large depth to diameter ratio.

Concerns associated with this problem include difficulty in executing the plating process.

Furthermore, controlling the depth of the blind via is difficult when the drilling process is performed after the lamination process.

Precise calibration is thus required for the success of the procedure, which is significantly costly.

Moreover, the use of blind and buried vias are limited to boards where only three lamination cycles can be employed.

Board configurations requiring more than three lamination results in functionally unreliable blind and buried vias.

What are the Types of Blind Vias?

There are four common blind vias used in printed circuit boards.

Types of vias

· Photo Defined Blind Vias

A photo defined via is made through a series of processes.

These processes are preceded by the binding of a photosensitive resin film to a core.

The film is then overlain with a hole pattern exposed to radiation that hardens the parts outside the pattern.

An etching process follows by which the holes are made before plating and fabrication of the conductive layer.

The process is simultaneously carried out for the surface layers, while other layers are added after.

The main benefit derived from using these vias is their fixed cost for a single or multiple vias.

Thus, it is a disadvantage when making a few vias.

A common application of photo-defined blind vias is in the construction of cellular PCBs and ball grid packages.

· Sequential Lamination Blind Vias

Creation of a sequential laminate blind via the same procedure for a double-sided board with a thin laminate.

The procedure involves a series of lamination processes hence the name.

The drilling process is carried out on the laminate before it is plated and then etched.

Etching assigns the piece the topography of the second board layer.

The alternate side is provided as a copper film to serve as the outer layer.

A lamination process combining with other similarly furnished layers follows.

The sequential lamination vias are costly to procure due to the various procedures involved.

· Controlled Depth Blind Vias

These blind vias are furnished in a way similar to through-hole vias.

The difference in case being the drill is fashioned to achieve a defined depth.

The path of the drill is precisely selected to avoid board features.

A plating process subsequently follows.

Making controlled depth blind vias has the least attached costs since no extra materials and processes are required.

However, since mechanical drills are employed, the hole sizes have to be large to allow their use.

There is also the danger of interfering with other underlying board features in the process.

· Laser Drilled Blind Vias

Making laser drilled blind vias is carried out on completion of the lamination process of the entire board.

However, the outermost layers are usually unplated and unetched.

You can employ a carbon (IV) oxide laser or eximer laser to remove the external copper features and substrate.

While using a CO₂ laser is fast, you have to etch the copper plate beforehand.

Thus, an extra process is created.

An eximer laser requires no pre-etching with the capability to cut through the copper and substrate.

Is it Possible to Overlap a Blind and Buried Vias?

You can overlap a blind via and buried via in a high layer count PCB.

You achieve more interlayer connections with reduced via count.

However, to achieve this, you need to entirely furnish the buried via the blind via.

You find this measure to require an additional press cycle, which adjusts PCB construction’s overall cost upwards.

What is the Function of Blind and Buried Vias?

Blind and buried vias provide an increased circuitry for printed circuit boards.

By interconnecting the conductive layers in PCBs, vias allow for the attachment of more components.

Furthermore, internal board traces can be furnished across the circuit board through the blind and buried vias.

What are the Parts of Blind and Buried Vias?

The structure of the blind and buried vias is composed of three parts that are:

• The barrel is used to refer to the metalized conduit that is fitted in the drilled hole.
• The pad provides a connection of the via with the conductive surface, track, or component. The pad is located at the ends of the conduit.
• The anti-pad is a gap you find between the conduit of the blind or buried vias. It is typically unattached to the metal plate.

Is the Size of a Blind or Buried Via important?

Structure of vias

You find the size of a blind or buried via to be significant in PCB board design.

When determining the size of the via, the length and diameter of the via are considered.

An established approach ensures the ratio of the height of the via and its diameter should not exceed one.

Consequently, you find when a blind or buried via is made to a larger depth, it requires a larger hole diameter.

When you furnish a PCB with a large blind or buried via, then the resulting dielectric cavity is enlarged.

Can the Blind Vias affect a PCB’s aspect Ratio?

You can use blind vias to reduce the circuit board’s aspect ratio.

The PCB aspect ratio is an important feature when employing surface mounted components such as ball grid arrays.

You find these components with diverse pitch sizes.

A circuit board’s aspect ratio is determined by the ratio of the board thickness and the diameter of the blind via.

Using blind vias rather than through-hole vias and decreasing the layer count will lower your board’s aspect ratio.

What are the Features Observed in the use of the Blind and Buried Vias?

When you use blind and buried vias in your circuit board construction, you identify the following features:

• The hole count on your PCB construction will be significantly more than conventional boards.
• You will have an increased pad count and circuitry than conventional board designs.
• The trace parameters of space and width will be decreased.
• The fabrication approach of drilling will be comparatively different than normal board designs.

Why do Blind and Buried Vias require at most Three Laminations Steps?

The blind and buried vias are vital elements in providing interlayer connections in a printed circuit board.

Therefore, ensuring their functionality is not handicapped is vital.

One way of guaranteeing the reliability of the vias’ performance and quality is by regulating the lamination process.

Blind and buried vias are typically made by the separate drilling of layers.

These layers are then stacked together before being pressed.

While undertaking fewer than two lamination processes is possible for low layer counts, the difficulty increases with layers.

Are Blind and Buried Vias used in Flex-rigid PCBs?

Flex-rigid boards are made such that they encompass elements of rigid board designs and flexibility.

These boards possess attributes such as being lightweight, slim, and small, allowing for their use on miniaturized gadgets and devices.

Since buried and blind vias allow for increased circuitry without increasing the board size, they find enormous use in Flexi-rigid PCBs.

Their use in Flexi-rigid boards allows medical applications like biomedical gear where high performance and reliability are desired.

What Lamination Methods are used for Blind and Buried Vias in Flex-rigid Boards?

You find two lamination types employed when working with blind and buried vias for flex-rigid circuit boards.

One-step Lamination

In this lamination process, the layers internal to the PCB are all laminated or bound in one press.

The process has a little cost involved and is carried out in a relatively short period.

Nonetheless, positioning the overlay during the process is difficult.

Furthermore, you cannot immediately identify flaws in the lamination process.

Such flaws can only be observed in the subsequent process of etching.

These flaws include cases of delamination and occurrence of deformation to the layers.

Step-by-step Lamination

In step-by-step lamination, the layers exhibiting flexibility, and those that are rigid are separately laminated.

Consequently, you have no overlay problems and the likelihood of deformation in the internal layers.

Furthermore, with this lamination type, there is timely identification of lamination-related flaws, ensuring functional reliability.

However, carrying out a step-by-step lamination takes considerably longer than the one-step approach, requiring more procedures to achieve lamination.

Additionally, the costs involved are significantly higher, requiring more material usage.

What Drilling Techniques can be used for Blind and Buried Vias?

Blind and buried vias are fabricated through a series of drilling procedures and lamination.

There are several approaches to drilling that can be employed, including:

• Numerical Machine Controlled (NMC) drilling.
• Ultra Violet (UV) drilling.
• Laser drilling

Among the drilling techniques, UV drilling is the most complex requiring advanced technology that comes at a cost.

How are the Blind and Buried Vias Cleaned?

Cleaning of the blind and buried vias is undertaken by a plasma cleaning method.

The plasma process is an intricate combination of physical and chemical elements.

Executing a plasma cleaning process requires a highly active reaction generated by a gas and solid combination.

In this instance, the gas and solid combination are initiated by the employment of acrylic acid, fiberglass, epoxy, and polyimide.

The gas released will be removed by pneumatic pumps cleaning the vias in the process.

Why are Blind and Buried Vias Preferred over Through-hole Vias in HDI PCBs?

The use of vias in circuit boards has been propelled by the increased performance demands of circuit boards.

As a result, PCBs have been furnished in multiple layers to increase their performance without increasing their size significantly.

Vias offers a connection between the multiple layers that allow the signal transfer.

Through-holes offer connection across the thickness of the PCB.

However, you find this via unsuitable for high density interconnect due to the via stub’s superfluous nature.

Subsequently, the signal quality is negatively affected.

But in high density interconnect PCB application, signal quality and high-performance levels are highly desired.

To counter the effect of the superfluous stub, a combination of blind and buried vias is employed.

These via types still offer interlayer connection from top to bottom and disseminate better signal quality.

Types of vias

What Causes an Empty Cave in Blind Vias?

An empty cave in blind vias occurs in when non-filled vias collapse due to several reasons such as:

• Bubble formation in the vias due to oxidation derived from the solution used in the plating process.
• Improper combination of elements added to the plating solution.
• Presence of foreign matter in the via.
• The dielectric constant of the materials used in the via.
• The thickness of the blind via wall plating.
• The type of blind via and its parameters related to the electric properties of the plating.

What Additives are used in Plating Blind and Buried Vias?

When plating blind vias, different components are employed in the plating solution such as sulfate of copper, chloride, and sulphuric acid.

Additives such as a brightening agent, delivery agent, and leveling agent are also employed.

The brightening agent influences the plating’s interface’s characteristics and provides the desired finish to the plating surface.

The delivery agent works in conjunction with the chloride ions to buffer the brightening agent.

It has smoothening qualities that allow it to give the plating a uniform surface.

You find the leveling agent is an electropositive element absorbed by electropositive positions in the acid concentration.

It also controls the deposition of copper ions due to its shared requirement for electron charges.

How are Additives Controlled to Prevent the Failure of a Blind Via?

Additives influence several features of the plating used in blind and buried vias.

When using the additives in wrong combinations is employed, you find the chance of failure increases.

Consequently, the use of additives needs to be closely monitored.

• When using additives, the dispensing utensil should be calibrated to provide a precise amount of the additive used.
• The plating solution should be consistently checked for carbon presence due to the adverse effects of pollution.
• The Hull cell experiment should be employed on the agents from time to time to determine their optimum contents and plating influences.

How are the Sources of Foreign Matter in Blind and Buried Vias Determined?

Foreign particles will find their way into the blind and buried via cavities in different ways.

Their presence is unavoidable due to the conduction of the plating process in an open environment.

Large particles are easy to deal with due to their visibility; micro particles, on the other hand, are challenging to handle.

Controlling the sources of foreign particles includes taking steps such as:

• The plating solution is enclosed to prevent the entry of foreign particles from external sources.
• The materials used in the plating process should be checked for their purity levels, which should be within the accepted limit.
• The agents used as additives in the plating process should be filtered periodically to check their purities.

Can Bubble Formation be Prevented in Blind and Buried Vias?

Vias in multilayer PCB

Bubbles originate from external sources in the surrounding environment.

A major instigator of bubble formation is oxidation, which occurs due to prolonged exposure to air.

The manifestation of bubbles is typically at the bottom of the cavities in blind and buried vias.

The following strategies can guide the prevention of bubbles:

• The PCB should be stored in a controlled environment before the via plating process. Exposure to elements such as temperature, oxygen, and humidity is thus reduced.

It is preferable also to avoid an environment with acid presence.

• Preprocessing of the copper filling and the employment of devices for bubble removal is essential. You can employ an acid-based degreasing agent with a water flow of at least fifteen degrees centigrade.
• You should select material with high anode performance for the copper container with a reduced additive inflow. Furthermore, you can add a protective layer to the anode to restrict bubbles’ direct formation by the anode.

Can Blind and Buried Vias be Intersected?

When blind and buried vias are intersected, they have to adhere to meet the following descriptions.

• The vias have to be multiple with passage through at least two layers.
• There should be at least a single layer that is common to the vias.

In case you have any question about PCB vias, feel free to contact Venture Electronics team.