Blind and Buried Vias PCB - PCB Manufacturing

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Blind & Buried Vias, Stacked & Staggered Vias, Any-Layer HDI
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What is Blind and Buried Vias PCB?

Plated through holes (PTH) are the vias from top to bottom layers. But blind vias are the vias from the outer layers (either the top or bottom layer) to the inner layers. When both sides of the vias are buried in the inner layer of the PCB, they are called buried vias. Thus, buried vias can’t be seen after the PCB is manufactured. We take the following 6-layer HDI PCB as an example: there are blind vias from layer 1 to layer 2, buried vias from layer 3 to layer 4, and plated through holes from layer 1 to layer 6. So blind and buried vias are commonly used in high-density PCBs, also called high-density interconnect PCBs.

So a printed circuit board with blind and buried vias are called Blind and Buried Vias PCB.

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Why Do Blind and Buried Vias Emerge in the PCB Manufacturing Industry?

Why do blind and buried vias emerge in PCB fabrication? This answer should start from a traditional multilayer printed circuit board. The structure of the standard multilayer PCB has outer layers and inner layers, to achieve the inner connection function between every circuit layer, you should use plated through via holes and make the vias metallization. But you know the size of the printed circuit is limited, so if you want to achieve more function and put more high-performance parts in the board in PCB assembly, you have to add more circuit lines to the board.

Except for making thinner circuit line width, the DIP plug-in via size zooms out from 1 mm to SMD 0.6 mm and further zooms out below 0.4 mm. But it still will occupy the surface area of PCB, so blind and buried vias emerges in a few high-tech PCB board manufacturer as the times require.

When Do Blind and Buried Vias Emerge in the PCB Manufacturing Industry?

When do blind and buried vias emerge in PCB fabrication? From the end of 20 century to the beginning of 21 century, the printed circuit board electronics industry has had rapid development in technology. In the meantime, electronic techniques improved rapidly. So to meet the customer’s requirements continuously, the printed circuit board industry must follow the development step of the circuit board electronics industry.

And every person will like more small and exquisite products, so electronic products constantly update. Along with the volume of electronic products becoming smaller, lighter, and thinner, print circuit boards are developed into flex PCB, rigid-flex PCB, blind via PCB and buried Via PCB, etc., to satisfy the requirement of electronic products.

What Makes Blind and Buried Vias Important?

The application of blind and buried vias can reduce the size of the PCB and improve EMC, and making the design work easier and faster. That’s why blind and buried vias become so important.

Introduction to Buried Vias

Buried vias are vias between the inner layers. After the lamination process, you can’t see it. They will not occupy the area of the outer layer. Both sides of the buried vias are in the inner layer of the printed circuit board. Take a 6-layer PCB, for example. Vias in L2-L3, L2-L4, L2-L5, L3-L4, L3-L5, and L4-L5 are buried vias. In other words, it’s buried inside the boards. That’s why you can call them buried via, and the PCB is called buried via PCB or buried vias PCB.
Buried via PCBs are more complicated than traditional multilayer PCBs in the production process so that the cost will be higher from the aspect of the PCB manufacturer. Traditional multilayer PCBs are drilled after lamination, while buried via PCBs are needed to drill the via in the inner layer before lamination. Drilling should be finished in a specific inner layer, first to press the specific inner layer together, then drilling and plating, and lastly make the full lamination for the complete PCB.
Because this operation process needs more time and cost than plate through holes and blind vias, the cost is most expensive for printed circuit board manufacturers. This process can increase the space utilization for other circuit layers, which are typically used in high-precision PCB fabrication.

Introduction to Blind Vias

Blind Vias are located from the outer layer to one or multiple inner layers. One side of the blind vias is in the outer layer, and the other is through to the interior of the board, but it will not extend through the whole board. Take a 6-layer PCB, for example, again. Vias in L1-L2, L1-L3, L1-L4, L1-L5, L2-L6, L3-L6, L4-L6, L5-L6 are blind vias. That’s why they are called blind vias. The PCB with blind vias is called blind via PCB or blind vias PCB.
It’s an excellent way to use buried and blind vias to increase the density of multilayer PCB manufacturing and reduce the electronic circuit board's layer and the board's dimensions. Also, PCB suppliers significantly reduce the quantity of the plated through holes. Almost all the BUM (Build Multilayer PCB) boards use buried via and blind via structures in PCB design.
Please note that most buried and blind vias have small diameters like 0.05-0.15 mm, so we also called this kind of PCBs with micro via in PCB as micro via PCBs. Buried via are in the inner layer. The production of buried via PCB is the same as double-sided PCB. And blind vias are produced by a CNC machine that controls z-axis depth for drilling small holes, but now with the continuous development of technology, universal use of laser drilling, plasma etching holes, and photoinduced hole.
Laser drilling usually uses three types of machine, and one is carbon dioxide laser machine, an Excimer laser machine, and a YAG Ultraviolet laser machine. I want to briefly introduce the carbon dioxide laser machine from Hitachi Limited (a Japanese company). This machine's laser wavelength is 9.4 nm, one blind via drilled in 3 times, and it can drill a large number of blind vias like 30 thousand holes per minute, extraordinarily efficient.
With the development of electronic products to high density and high precision, corresponding print circuit boards also need to be developed to match by custom PCB manufacturers, and the most effective way to increase PCB density is to reduce the quantity of plate through hols. The same time needs to set up the buried and blind via accurately.

What is the Difference Between Blind and Buried Vias?

Blind vias are located on the top and bottom surfaces of the printed circuit board and have a certain depth for the connection between the surface layer of the circuit and the inner layer of the circuit below. The depth of the hole usually does not exceed a certain ratio. Blind vias are the type of vias that connect the routing of the inner layer of the PCB to the routing of the surface layer of the PCB. Blind vias do not penetrate the entire board.
Buried vias are located in the inner layer of the PCB. It does not extend to the surface of the printed circuit board. Buried vias belong to the type of holes that only connect the routing between the inner layers, so they are not visible from the surface of the PCB.

Many Benefits of Blind and Buried Vias

1. The use of blind and buried vias is an effective way to increase the density of multi-layer boards, reduce the number of layers and the size of the boards, and significantly reduce the number of plated through holes.

2. Using blind and buried vias increases reliability, improves the characteristic impedance control, and reduces crosstalk, noise, or distortion.

3. Blind and buried vias provide more space for the PCB. The design of buried vias will help free up space on the board without affecting the surface devices or routing on the top and bottom layers.

How Blind and Buried Vias are Made?

Blind vias in PCB production are designed in the top and bottom layers and have a relative height to connect the outer circuit layer to the inner circuit layer. Normally, the depth of the hole has a specified ratio(the depth can’t be more significant than the diameter of the hole). The drilling depth must be appropriate. If not appropriate, it will be challenging to plate the holes. Thus, very few PCB manufacturers choose this kind of printing method. To achieve this process, you can first drill in some layers of all the layers that should be connected in this way and then press them together, but this method needs a more precise positioning and alignment device for the PCB manufacturer.

The fabrication process of blind via PCB has three different methods, as below:

Mechanical specified the depth drilling:

Produce the PCB production by traditional multilayer PCB process till finished the lamination. Use a drilling machine to set the z-axis depth to drill the desired depth. This method is relatively simple but has a few problems:

Just drill one board every time, which causes very low output, which is not good for batch production for a multilayer PCB manufacturer.
The levelness of the drilling machine table is strictly required. Each spindle drilling depth setting needs to be the same. If not, you want to control the depth of each hole. That’s very difficult and difficult to ensure the same depth of the blind vias.
This approach makes plating in the hole difficult under the condition that the diameter of the hole is smaller than the depth. In this case, the wish to perform good plating inside the hole is hard to realize. According to the limits above, this method is gradually not used anymore.

Sequential lamination:

Take the 8-layer PCB boards, for example:
Step 1: the four inner layers (layers 3-6) are made in the way of a general 4-layer PCB with PTH, and blind layers 1-2 and 7-8 are produced as normal double-sided PCB;
Step 2: Laminate all the layers together: the inner 4-layer PCB and 2 pcs of 2-layer PCB, then we get eight layer PCB;
Step 3: Produce the 8-layer PCB as standard double-sided PCBs, drill the PTH holes, copper plating, solder mask, etc.

But if there are both blind and buried vias in the same layer, sequential lamination is not possible anymore. As the following illustration, there are blind vias from layers 1-3, 6-8, and buried vias from layers 3-6. Then there are blind and buried holes distributed in both layer five and layer 6. In this case, we need to use a laser drill. The steps are：
Step 1: laminate layers 3-6, produce the copper trace and PTH as standard 2-layer PCB;
Step 2: laminate layers 2-7, and laser drill layers 2-3,6-7, produce copper trace, and plate the vias;
Step 3: laminate layers 1-8 and laser drill layers 1-2,7-8, produce copper traces in the outer layers, and plate the vias.

Any layer lamination and drilling

This method is so popular that almost the global blind via and buried via PCB fabrication industry uses it now. A lot of big PCB manufacturers can do that.
This method is an expansion of sequential lamination/drilling mentioned above(the B method), but the difference is to add layer by layer from the inner to the outside of the board. Normally it begins from layer 2 of PCB in the middle and uses the blind holes by laser drilling to connect all the additional layers. These vias are stacked micro vias or staggered micro vias. Stacked via means the via is precisely on the top of the via from the other layer, but staggered via is not. This kind of PCB is called any layer PCB. There are three ways to do any layer drill below:

Photo-Defined type:

Use photosensitive resistance and the permanent media layer simultaneously, then aim at a specific location, make exposure and development with the negative film to expose the bottom copper pad and form a bowl-shaped blind hole, and then thoroughly add chemical copper and copper plating. Then the circuit of the outer layer and the blind via are etched out. Or you don’t need to make copper plating to complete the conduction, fill in copper paste or silver paste also has the same result. Of course, you can add the other layers in the same way in the PCB manufacturing process.

Plasma Etching:

This is the done company’s patent, so they named its business name to the demonstrate method.

Laser Ablation:

In the HDI PCB manufacturing processes, there are three categories: CO2 laser, which is more commonly used, and Excimer laser and Nd: YAG laser.

Laser Drilling Introduction

Laser drilling technology has advantages such as high accuracy, strong versatility, efficiency, low cost, remarkable comprehensive technical and economic benefits, etc. Now, this technology has become one of the key technology in the modern PCB manufacturing field. Before the laser appeared, only can use high hardness material punched holes in lower hardness material. That makes drilling holes in high-hardness diamonds become complicated things. After the laser appeared, this kind of drilling became quick and safe. But, the hole drilled by laser is conical, not cylindrical like mechanical drilling. This is very inconvenient in some places. Programmable control of graphic output through the galvanometer.

Laser drilling is the process that focuses the laser on becoming a high-intensity heat source to heat the material that should be drilled into holes, melting the material in the laser action area or vaporization, then evaporation, then becoming holes. Laser beams are highly concentrated in space and time, and using the lens to focus can reduce the spot diameter to the 5th power of 10 ~ the laser power density of the 15th power of 10 w/cm2. Such a high power density can laser drill holes in almost any material.

Laser drills, as a kind of blind via in HDI PCB, have their characteristic in PCB fabrication:

Blind Vias and Buried Vias Electroplating

A, Under normal circumstances, any layer of copper just can make one-time whole board electroplating + one-time Pattern Plating.

B, Under normal circumstances, after the lamination process, the board thickness greater >= 80MIL, the through hole board will need whole board electroplating + Pattern Plating, so when the blind via plating, the outer layer can’t make whole board electroplating in PCB manufacturing.

C, After meeting the conditions above, the electroplating of blind vias needs to make as below:
1, If the trace width of the outer layer is greater than 6 MIL, at the same time, the thickness of the PTH board is less than 80 MIL, then the outer layer can make whole board electroplating in the electroplating process of blind vias.
2, The trace width of the outer layer is greater than 6 MIL, but the thickness of the PTH board is greater than 80 MIL, the outer layer should film sticking in the surface to protect the outer layer in the electroplating process of blind vias.
3, The trace width of the outer layer is less than 6 MIL. At the same time, the thickness of the PTH board >= 80 MIL, the outer layer should film sticking in the surface to protect the outer layer in the electroplating process of blind vias.

Important Things in Blind and Buried vias Production

Resin plug blind hole: When the size of the blind hole is quite big, and the number of holes is larger, it will need a lot of resin to fill the buried via when pressing the board, so to avoid that, it affects the thickness of pressing board, you can use resin to plug the buried hole in advance before pressing board process.
When there have blind via requirement in the outer layer,
a. Because there will be glue flow out in the pressing process so it will need a glue removal process after pressing.
b. Because the board surface will be cleaned before the outer layer dry film process, there will have a board grinding process. The chemical copper precipitation just has very thin copper, 0.05 MIL to 0.1 MIL, it is very easy to grind off when grinding process, so we will need to add an electroplating process to thickening copper. The relative processes are: pressing board --- removing glue --- drilling --- chemical copper precipitation --- electroplating --- dry film --- Pattern Plating.
In addition, when making the high layers blind via boards, you may need to use PIN-LAM press boards, but we need to note that only when the core thickness is less than 30 MIL the drilling machine can make PIN-LAM holes.
About the side strips of the blind vias board, consider multiple pressing boards, and have many tooling holes, so better keep the side strips size over 0.8”.
When writing the LOT card about the sub-process, you need to write the plate structure of a single sub-process. You also need to point out the plate structure of the main process in the particular requirements for the convenience of the following PCB production process.
When writing the LOT card, you must point out the dry film made in the inner or outer layer if it is a blind hole.
Preparation of engineering documents: when the engineer makes the engineering file, we need to pay attention to the contraposition hole setting between the layers. Otherwise, there will have a mismatch problem when contra position. Even can not distinguish which layer are which layer. Advice uses the way: the second layer has two identification points, the third layer has three identification points, the fourth layer has four identification points, and so on. Also, the identification point in the film needs to be the same as the drilling file.
Cutting: Blind and buried via board need to use better material, 100% 150-degree baking board for 4 hours, to eliminate the material's internal stress and water content. At the same time, the thickness of the laminated plate can’t be over 250 mm.
Drilling: make sure to use the designated drilling file, you can’t use the wrong one, and they must use the brand new drilling bit. The thickness of the laminated plate is reduced by 20% from the standard laminated plate thickness and must ensure the disposal of each drilling, and the correct trace of the relative circuit must be done. In the inner layer drilling process, I don’t think the thickness of the inner layer is thin, so it laminated to many plates. The drilling parameter is slower by 20% than the standard parameter to make sure the quality of the wall of the holes, and can’t have dust and burr. The accuracy of blind via and buried via are very important.
Inner layer: Pay attention to distinguishing the direction hols, differentiate the layer for the inner layer production, can’t mix up and make the wrong layer, and pay special attention to the image of each layer. Otherwise, the association of the circuit will be opposite. Before production, check if all the films are correct, and ensure the inner layer's excellent coincidence and the coincidence deviation is less than 0.05 mm; the film needs to control the expansion coefficient. If the panel is over 12 * 12 inches, the film needs to enlarge appropriately.
Lamination: good Browning, fully consider the relationship between the thickness of the lamination, inner layer copper, and residual copper rate, in case improper configuration PP causes inner layer short. Then rivet positioning over an eight-layer PCB with blind and buried vias better use pin template positioning to produce and avoid deviation and slip; when making the lamination, the glue must be enough to ensure the glue inside buried are buried full and level. When the second press, the key is to fill the glue. The flow-out glue inside buried vias can’t pile around the buried via. Blind vias need to be filled with glue. The accuracy of position holes must require between 0.05 - 0.075 mm, otherwise will cause a short in the inner layer. No matter how to control the glue flow of the outer layer after lamination, the plate will have glue flow out. The process is baking first, removing the glue, grinding right away, and drilling after the oven dries.
PTH & copper precipitation: Backlight over level 8, for holes smaller than 0.3 mm, can choose copper precipitation twice to ensure the holes' copper thickness. All the glue is removed and activated. The copper sink will need to turn on the vibration motor, and thickening the inner layer must strictly control the time and current density. If not have other special requirements, the copper thickness of the inner layer needs to maintain thickness consistency and avoid the inner layer short because of uneven thickness after lamination.
Pattern Plating: Special emphasis on inner layer pattern plating: inner layer better put in one Fly Pakistan, two rectifiers, single side through current, put rack in the same direction, glossy copper side unify through 2.0 ASD dozen current, the other side through 1.2 ASD current and electroplating 60 minutes to make sure the thickness of copper inside hols can reach 12-15 um.
The important point of Control pattern plating

A. electroplating operation parameter: plating liquid: H2SO4: 220 g/L CuSO4.5H2O: 65 - 75 g/L CI-: 50 - 70 PPM The photocatalyst is 200ml/1000A/minute. The temperature of the plating liquid is 26℃.
B. Tin plating on the side of the blind vias must be uniform and thick enough. Otherwise, pinholes will appear easily. Fly Pakistan must be very clean. The groove voltage is around 1.2V.
C, The board surface can’t have a crease. When the bare board is electroplating, the current can’t be massive. Normally it should be controlled in 1.0 A/dm2. The time control should be in 30 minutes. The first copper thickness of the through hole is 7-12 um.

How to Confirm and Recognize Blind and Buried Vias from Gerber Files?

A simple way is that: when you find more than two drilling files in the data, they may have blind and buried vias. For example, Gerber files of 4-layer PCB have multiple drilling files: TXT1, TXT2, TXT3, TXT4, drl1-2, drl3-4, drl1-4, etc. This board has blind and buried vias.
After the traditional multilayer board use the design and fabrication of blind and buried vias, the size of the PCB is significantly reduced. The application of buried / blind holes is bound to become more and more common, but its investment amount is enormous. Particular-scale PCB manufacturers like medium and large-scale factories should aim at large-scale production and high yield rate. The small-scale PCB manufacturer should do what they can to find the corresponding market for sustainable operation.

How Is a Multilayer PCB Built?

Concise process flow of the multi-layer PCB: cutting and grinding → drilling positioning holes →graphics of inner layers → etching in inner layers→ inspection → blackening → lamination → drilling → plated through hole→graphics of outer layers → gold plating, de-filming etching → secondary drilling → inspection → solder mask → silkscreen → punching → testing → inspection

Choosing a Blind and Buried Vias Printed Circuit Board Manufacturer

Are you still struggling with how to choose a suitable blind via and buried via PCB manufacturer? Viasion can be your best choice!

1. Viasion has a strong technical force, a high-quality technical management team, and advanced automatic production and testing equipment. That means we can provide a strong guarantee for our blind and buried vias PCB quality and delivery.

2. Viasion has many years of experience in blind and buried vias PCBs production, our operation and sales service as well as a perfect management system and has won a good reputation among customers.

3. The price of our PCBs with blind and buried vias is advantageous and our raw material supply channel is guaranteed. To pursue the high quality of our products, we have passed the ISO9001 certification and all our products can meet the requirements of RoHS and REACH standards.

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Frequently Asked Questions

What are blind and buried vias?

Blind vias: When only one side of the vias is in the outer layer of the PCB, they are called blind vias.
Buried vias: When both sides of the vias are buried in the inner layer of the PCB, they are called buried vias.

What is an example of when to use blind or buried vias?

Blind and buried via are usually used in high-tech electronic circuit boards that combine very complex functions in a small area, such as cellphone PCBs. We usually call this kind of blind buried via board with small drill holes and trace width as HDI PCBs.

How are blind and buried vias created?

The following three methods can create blind and buried vias:
Mechanical drilling with fixed depth
Sequential lamination and drilling
Any layer lamination and drilling

How are blind and buried vias filled?

Blind and buried vias are filled with non-conductive and conductive material, such as resin, copper or other via filling materials.

What is blind via manufacturing process?

Firstly, we talk about laser drill blind via holes. PCB blind via fabrication process as following:
1) finish all inner layers first;
2) laminate two out layers of prepreg and copper sheet on the ready-finished inner layers of PCB board;
3) drill the controlled depth blind via on the PCB by laser.
Please note accuracy is very important for the blind via in pad.

As for mechanical drill blind via holes, we take 4-layer PCBs with blind vias in layer 1 to layer 2 for example, the process as follows:
1) produce layers 1 and 2 as a standard 2-layer PCB, so there will be drills on layers 1 to 2;
2) laminate two core boards together so we get the 4-layer PCB, and drill the plating through holes;
3) when PCB is finished, you get PTH from layers 1 to 4 and blind vias from layers 1 to 2.

What are the benefits of using blind and buried vias?

The benefits of using blind and buried are stated below:

Reducing the size and weight of the PCB
Reducing the number of layers
Reducing costs of production of several types of PCBs since combining more functions
Improving the electromagnetic compatibility
Increasing the characteristics of electronic products
Making the design work easier and faster

What are the challenges of using blind and buried vias?

The miniaturization of the diameters of blind and buried vias places higher demands on PCB production.
Very highly experienced engineers are needed to design blind and buried vias PCBs.
It is more challenging to assemble blind and buried vias PCBs since they always have tiny pads, such as BGA pads.

What is the normal blind via aspect ratio?

In laser drill blind vias board, the normal blind via aspect ratio is 1:1.

How about blind via cost in a blind buried via board？

It is very difficult to say how much cost will be increased on blind vias, but generally speaking, one round of blind via cost around 10-20% of the total printed circuit price.

What is buried via?

Buried vias in PCB are via holes between inner layers. We take a 6 layer blind/buried via circuit board for example: blind vias can via holes from layer 2-3, 2-4, 2-5, 3-4, 3-5 and 4-5.

Buried via vs blind via

Blind via and buried via are commonly used HDI PCBs, they always exist in high tech high density printed circuit boards the same time. But they are totally different types of vias.

Blind and Buried Vias