What Are Blind and Buried Vias?

Blind and buried vias are types of vias used in PCB design to increase their routing density. A via is a small conductor that connects different layers of a PCB, allowing electrical signals to flow between them.

A blind via connects one outer layer of a PCB to one or more inner layers; however, it does not extend through the board. The via is called "blind" because it does not go through the board. This means it is only visible on one side of the PCB, typically the top or bottom layer. Blind vias are typically used when there is limited space on the outer layers of a PCB and more routing is needed on the inner layers. They also connect high-speed signals that require a shorter and more direct path.

A buried via connects the inner layers of a PCB but does not connect to any outer layers. It is "buried" because it is hidden within the layers of the PCB. Buried vias are typically used in multi-layer PCBs when there is limited space on the outer layers and more routing is needed on the inner layers. They are also used to connect high-speed signals that require a shorter and more direct path.

Blind and buried vias have several advantages over traditional through-hole vias. They increase the routing density of a PCB by allowing for more connections in a smaller area. They also improve the signal integrity of a PCB by reducing the number of through-holes, which can cause signal loss or crosstalk. Additionally, blind and buried vias allow for more flexibility in PCB design, as they can be placed anywhere, whereas through-hole vias must be placed at the edges of the board.

Blind and buried vias are more difficult and expensive to manufacture than through-hole vias. They require special drilling equipment and are typically made using laser drilling or microvia technology. Additionally, they have a higher failure rate than through-hole vias due to the increased complexity of their design and manufacture.

Blind Vias in Your HDI PCB

Blind vias are a crucial component in High-Density Interconnect (HDI) PCBs. HDI PCBs are designed to maximize the number of connections in a small area, which is essential for modern electronic devices requiring many connections in a small form factor. Blind vias are used in HDI PCBs to connect the outer layers of the PCB to the inner layers, allowing for more connections in a smaller area.

In HDI PCBs, a blind via is created by drilling a hole from one surface of the PCB to one or more inner layers. The via is filled with a conductive material, such as copper, and plated over to create a conductive path between the layers. The blind via is then capped with a metal or polymer layer to protect the conductive material from oxidation and other environmental factors.

Blind vias are particularly useful in HDI PCBs because they allow for more flexibility in PCB design. They can be placed anywhere on the PCB, unlike through-hole vias, which must be placed at the edges of the board. This allows for more efficient use of space and a higher routing density. In addition, blind vias improve the signal integrity of a PCB by reducing the number of through-holes.

However, some challenges are associated with using blind vias in HDI PCBs. The drilling process is more complex and requires specialized equipment, which increases the cost of manufacturing. Additionally, blind vias have a higher breakdown rate than through-hole vias due to the increased complexity of their design and manufacture.

What Types of PCB Manufacturing Processes Are Used to Make Blind Vias?

There are several PCB manufacturing processes used to make blind vias, each with its advantages and disadvantages. The most common processes are:

1.      Laser Drilling

This process uses a focused laser beam to drill holes in the PCB. The laser beam is directed onto the PCB surface, vaporizing the material in a small, precise area to create the hole. Laser drilling is a precise method that can create small, high-quality holes with minimal damage to the surrounding material. This process is ideal for creating blind vias in high-density PCBs, where precision and accuracy are critical.

2.      Mechanical Drilling

This process uses a mechanical drill to create holes in the PCB. The drill bit is made of a hard, brittle material, such as diamond or carbide, capable of cutting through the PCB material. Mechanical drilling is a fast and efficient method that is well-suited for creating large numbers of holes in a short period of time. However, the process can be less precise than laser drilling and cause more damage to the surrounding material.

3.      Microvia Technology

This process uses a laser or mechanical drilling process to create small, precise holes in the PCB, similar to laser drilling. However, the holes are typically smaller in diameter, typically less than 100 microns, and are created using specialized equipment. Microvia technology allows the creation of blind vias with the highest density and precision and can be used for creating vias in multi-layer PCBs.

All three processes mentioned above can be used to create blind vias in PCBs, each with its advantages and disadvantages. Laser drilling is the most precise and accurate method but also the most expensive. Mechanical drilling is fast and efficient but less precise. Microvia technology allows for the highest density and precision and the highest cost.

When choosing a manufacturing process for creating blind vias, it is important to consider the specific requirements of the PCB design, such as the number and size of the vias, the density of the PCB, and the desired level of precision and accuracy. Cost and turnaround time also play a significant role in the decision-making process. You must also consider certain factors, such as the number and size of vias, density, precision, and cost, when choosing the best process for creating blind vias.

Blind Vias PCB Fabrication Processes

Blind vias connect one or more inner layers of a printed circuit board (PCB) to an outer layer without extending through to the opposite side of the board. There are several different PCB fabrication processes used to create blind vias, including:

•        Drill and Fill

This process involves drilling a hole through the layers of the PCB where the blind via is to be located. The hole is then filled with a conductive material, such as copper, to create the electrical connection.

•        Laser drilling

 This process uses a laser to drill a hole through the layers of the PCB where the blind via is to be located. The hole is then filled with conductive material.

•        Photo Imaging

 This process involves using a photolithographic process to create the blind via. A photo-sensitive film is applied to the PCB, and the via pattern is exposed to UV light. The film is then developed, and the via is etched into the PCB.

•        Press-Fit

This process involves creating a press-fit connector that is inserted into a drilled hole in the PCB, creating a blind via connection.

Each of these processes has its own advantages and disadvantages. The best process for a particular application will depend on factors such as the size and number of vias required, the type of PCB being used, and the desired performance characteristics.

Let's look at various blind vias PCB fabrication processes in detail:

Method 1- Cut the Copper-Clad Lamination to the Required Size by the Pcb Fabrication

Place a stencil paper on a steel plate and draw the circuit on it using a pen. The stencil is then cut to the appropriate size for the PCB and pasted onto the copper-clad laminate. Create a printing material by mixing a bit of paint with some talcum powder, dipping a brush, and applying it evenly to some stencil paper. Repeat this process multiple times. You can use the printed board to print the circuit. Reusable and well-suited for low-volume production runs, this printed board can be used repeatedly.

For corrosion testing on a printed circuit board, mix 1 gram of potassium chlorate with 40 milliliters of hydrochloric acid to get a solution with a concentration of fifteen percent.

Wash rusted printed boards frequently with water. Wipe off the paint with banana oil, and then wash it several times to clean the printed board without leaving corrosive liquids. Spread some rosin solution on it. Do not drill until it has dried.

Method 2

There are several ways amateurs can make PCB, but they are either time-consuming, not sophisticated in craftsmanship, or of inferior quality. One of the methods with a better overall effect is the following approach to making PCB. Here's how it works:

The electrical schematic of a printed circuit board should be considered. Dots represent pads, and the connection between them need only be a line so long as their relative positions and sizes are correct.

Measure the dimensions of the circuit schematic and cut the copper foil to fit the board.

Copy the diagram onto the printing board with the use of carbon paper. You can skip this step if the circuit is straightforward and the manufacturer has substantial board-making experience.

Paste standard pre-cut symbols (pads) of varying inner and outer diameters on the components as needed. Then, using the direction of the current, adhere to the tape lines of varying widths. Standardized pre-cut symbols and tape are available at electronics retailers. Materials like D373, D266, and D237 are commonly used for pre-cut symbols. Please avoid using things that are black and red plastic. Use a tape with dimensions of 0.3, 0.9, 1.8, 2.3, 3.7, etc. The measurement unit should be millimeters.

It would help if you used a softer hammer, such as plastic, smooth rubber, etc., to ensure the sticker sticks securely to the copper foil. Observe how the line twists and overlaps. In cold weather, heating the surface with a heater improves its adhesive effect.

Just submerge it in ferric chloride to kick off the rusting process. It's important to remember that liquids can't be heated above 40 degrees. To prevent further rusting, remove and rinse at the appropriate time, especially if there are fine lines.

Create openings, smooth the copper foil with fine sandpaper, coat it in rosin alcohol solution, and let it dry. It's safe to say that this printed board is almost as high-quality as the kind used in traditional printing processes. The IC's legs can be connected with 0.3 mm tape, saving time and effort by reducing the number of short jumpers on the front of the PCB.

Method 3# of PCB Fabrication: PCB Fabrication

In a bowl, combine three parts of absolute alcohol with one part lacquer flakes, specifically shellac, obtainable at chemical stores, and stir until completely dissolved. Add a few drops of gentian violet, a medical purple remedy, after the flake mixture has dissolved to achieve the desired color. It can be used as a protective paint during PCB manufacture after thorough mixing.

Before anything else, use fine sandpaper to polish the copper-clad board. Then, draw with the duckbill pen or the ink duckbill pen that can be used to make the compass's visuals. The duckbill pen has nuts that let you change the width of each stroke. An alternative to a regular ruler, a triangular ruler can make extremely fine lines. The drawn line should be uniform and smooth, with no sharp corners, giving the viewer a sense of fluidity and ease. In the blank areas of the PCB, you can also write in pinyin, Chinese, English, and symbols.

If your drawing line can be seen from outside, the concentration is too low, and you should mix in some more paint chips. A few drops of pure alcohol can be used to thin the drawing line back down to a more manageable thickness if it thickens up and stops stretching smoothly. If you make a mistake, it won't matter.

Use a cotton swab dampened in pure alcohol to erase the offending line and start fresh. Corrosion in the ferric chloride solution is a risk after the circuit board has been drawn. Corrosion of the circuit board makes it easy to remove the coating. Wipe the protective paint dry with a cotton ball dipped in 100% alcohol, then apply rosin.

Protective paint, once made, should be stored in a small, tightly sealed bottle, like an ink bottle, due to the evaporative nature of the alcohol. After each usage, replace the bottle's cap. Next time you use it, dilute it with some anhydrous alcohol if the concentration has thickened.

Method 4 of PCB Fabrication - Obtain a Price Estimate for PCB Fabrication

This method involves drawing the circuit on the veneer and then pasting self-adhesive labels onto the copper foil of the copper-clad laminate. First, you'll need to use a cutter to shape the veneer into the necessary circuit prototype. Then, you'll need to remove the non-circuit components. Finally, you'll need to use ferric chloride to corrode the ideal circuit board.

Corrosion can be carried out at temperatures as low as 55 degrees Celsius, and the rate of corrosion is increased. To apply rosin alcohol solution, you must first remove the self-adhesive labels from the corroded circuit board, drill holes in it, wipe it down, and then apply the solution.

Method 5 of PCB Fabrication

Place components on the printed board with a density and placement determined by the schematic circuit diagram's component shapes and the board's total area. The placement of the parts should follow the rule of big first, then little, first globally, then locally. The circuit's components are close together and evenly spaced.

The two traces cannot be twisted at right angles at their corners or crossings, as this would break the connecting trace between the components. They need to make a turn together, but they can't cross over each other. When this is not possible with a given trace, moving the trace to the rear of the printed board may be necessary, and then using stud bolts to link it to the front circuit.

When soldering, you can also use insulated trace. As they are more likely to interact with one another if they are close together, more space should be left between the input and output components.

Method 6 of PCB Fabrication

Use an 80-gram copy of the paper and a printer to create a 1:1 scale version of the circuit board diagram. If you want to use a pen or pencil, you can draw by hand, but make sure the bottom sheet of paper is completely flat.

Locate a fax machine, remove the fax paper tray, and insert a sheet of hot-melt plastic film. To make a copy of the circuit diagram on the hot-melt plastic film, place the original in the fax machine's outgoing fax tray and press the copy button. The printed circuit board's "manuscript" is complete at this point.

Use double-sided adhesive tape to attach the drawn plastic film to the copper-clad board evenly. No creases or dents should be present in the plastic wrap or cardboard. Melted areas cannot be concealed with tape paper. If this isn't taken care of, it'll have a negative impact on the circuit board's final quality.

Apply the paint to the plastic film evenly with a paintbrush. It's important to remember that you should only sweep in one direction. If the plastic sheet wrinkles, the lines on the copper board will also overlap. Remove the plastic covering gently after you've finished brushing the circuit schematics. A printed circuit board has reached its completion stage. It can corrode after it dries out.

Making a wooden frame somewhat bigger than the circuit board is all that's needed to print many copies. Simply set the PCB stencil flat on the frame, and secure it. Then, adhere the permanent plastic film under the screen using double-sided tape. Place the copper-clad board on the table, close the screen frame, make sure the printed picture and the copper-clad board are oriented left and right, paint in one direction, and then open the screen frame. This will help in printing the PCB. Paint and bamboo can hide just about any flaw.

Make sure to follow the process of the letter without making any changes. It's important to alternate between using mild and hard pressure on the brush while painting. The lines will become blurred if there is too much paint or the film is too thick. The lines will snap under insufficient pressure. Furthermore, remember that the glossy side of the plastic wrap needs to be up.

BGA Breakout and HDI PCB Board Thickness

BGA, or ball grid array, is a type of surface mount technology (SMT) used to mount ICs (integrated circuits) onto a PCB (printed circuit board). In a BGA package, the balls, or solder balls, are arranged in a grid pattern on the bottom of the package and are used to make connections to the PCB. One of the main advantages of BGA packages is their higher pin density, which allows for more connections in a smaller area compared to other SMT packages.

A BGA breakout is a method used to connect a BGA package to a PCB. The BGA package is first soldered to the PCB using a reflow soldering process. Then, small vias, or holes, are drilled through the PCB to connect the BGA balls to the underlying PCB traces. This allows for a more compact design and a higher pin density.

HDI, or high-density interconnect, is a type of PCB that uses advanced technology to increase the number of connections in a small area. One of the main features of HDI PCBs is their thinness, which can be as low as 0.1mm. This allows for more layers to be added to the PCB, increasing the number of connections that can be made. HDI PCBs also use smaller vias and traces, which further increases the number of connections that can be made in a small area.

The thickness of a PCB board is an important consideration in the design of a PCB, as it affects the mechanical strength of the board, the thermal performance of the components, and the ability to connect to other boards or devices. PCB board thicknesses range from 0.4mm to 3.2mm, with the most common thickness being 1.6mm. When using a BGA package or an HDI PCB, it is important to choose a PCB board thickness that is compatible with the package and the technology being used, as well as the overall design and requirements of the device.

BGA breakout is a method used to connect a BGA package to a PCB. HDI PCBs are high-density interconnect PCBs that are thin and have more connections in a small area. PCB board thickness is an important consideration in the design of a PCB, as it affects the mechanical strength, thermal performance, and connections of the device.

How Cost and Quality Standards Affect Design With Blind Vias

Blind and buried vias are a type of via that connects one or more layers of a printed circuit board (PCB) without extending through to the opposite side of the board. They are typically used in high-density interconnect (HDI) and multi-layer PCBs to increase the number of connections that can be made in a tiny area. The use of blind vias in PCB design can significantly impact the final product's cost and quality.

One of the main factors that affect the cost of a PCB is the number of layers and the complexity of the design. The use of blind vias can increase the number of layers and the complexity of the design, which in turn can increase the cost of the PCB. This is because creating blind vias is more complex than creating through-hole vias, which require less precision and are, therefore, less expensive to manufacture. Additionally, the cost of the PCB is also affected by the cost of the materials used in the PCB, such as the substrate and the copper, which can be more expensive when using blind vias.

Another important factor that affects the design of a PCB with blind vias is the quality standards that must be met. These standards vary depending on the application of the PCB, but they typically include requirements for the size and placement of the vias, as well as the materials and processes used to create them. For example, in a high-reliability application, such as aerospace or medical equipment, the vias must be very precise to ensure a reliable connection. In contrast, in a consumer electronics application, the vias may be less precise but still must meet certain quality standards.

The design of a PCB with blind vias is affected by the need to ensure that the vias are properly placed and that the connection between the via and the surrounding trace is reliable. This requires high precision in the design process, which can be time-consuming and require specialized software and equipment. The designer must also consider the size and shape of the vias and the materials used to create them to ensure that the final product meets the quality standards for the application.

The fabrication of a PCB with blind vias is a complex process that requires specialized equipment and processes. The vias must be created using a drilling or laser ablation process. They must be plated with a thin layer of metal to ensure a reliable connection. The finished PCB must then be tested to ensure that the vias are properly aligned and that the connections are reliable. This process is more complex and time-consuming than the fabrication of a PCB with through-hole vias, which can increase the cost of the final product.

The use of blind vias in PCB design can significantly impact the final product's cost and quality. The cost of the PCB is affected by the number of layers and the complexity of the design, as well as the cost of the materials used. The quality standards that must be met can vary depending on the application of the PCB, but they typically include requirements for the size and placement of the vias, as well as the materials and processes used to create them. The design and fabrication of a PCB with blind vias is a complex process that requires specialized software, equipment, and processes, which can increase the cost and lead time of the final product.

•        The Use of Via Drilling Equipment

To connect the outer layer to a single inner layer, it is sometimes possible to mechanically drill the vias to a diameter of about.006" once lamination is complete. This approach won't significantly increase costs, but it does have some severe constraints.

These constraints result from the fact that the fabricator will have a hard time properly plating the holes unless the drill diameters are sufficiently large in relation to their depth. If the diameter of the hole is not large enough in relation to its depth, air can become held inside the hole, and the solution will not be able to flow through it as easily as it would if the hole went all the way through. The solution is unable to coat the interior and base of the void due to the presence of air.

Consequently, there is either no link to the internal layer or an unreliable connection. This problem is exacerbated by the hole's depth relative to its diameter. Having a pad big enough to hold a.006" or.008" through can be a problem for smaller items. It's crucial to have thorough planning and process controls in place.

•        Laser-Initiated Microvia Drilling

Laser-drilled microvias can be used in place of conventional microvias in some situations. The primary distinction here is the magnitude. Due to the smaller hole sizes, the dielectric thickness must be decreased to have the same aspect ratio. A 1:1 aspect ratio is typically considered the sweet spot for microvias. When working with dielectrics, this thin, additional design issues arise, such as the need for impedance-controlled signal traces that are wide enough for processing and narrow enough to maintain the correct impedance value.

Unfeasibly narrow traces may be deleted from the external layers and connected via the via to one or more internal layers, where the trace width and dielectric thickness may be more freely adjusted to achieve the desired impedance. Laser microvias need expensive equipment to generate. The holes formed by them must be processed somewhat differently than conventional microvias. Compared to a purely mechanical method of blind drilling, the additional expense is due to the specialized tools and labor involved.

Machine Gun Drilling

When the substance is still in its core form before lamination, blind mechanical drilling can be performed, or specific groups of layers can be laminated together to build substructures before drilling. The next lamination stage can begin after the vias have been drilled completely through the core or substructure and plated.

Because the plating solution may freely flow through the briefly open hole, the quality of the interconnections is quite high when utilizing this procedure. The holes can be plugged before laminating the substructure to the remaining layers.

Because each sub must undergo drilling, filling, plating, and image processing in addition to the several lamination phases, the costs associated with this procedure quickly build up. Processing that returns external layer pads to their undamaged state for surface mount component installation (Via-In-Pad) is more expensive.

•        Microvia Stacking with Sequential High-Density Interconnect Lamination

Stacking microvias and employing successive HDI lamination in many stages are common practices for ultra-dense circuit boards. Vias are produced in this technique by laser drilling through numerous thin layers of dielectric material, then repeatedly treating and laminating the layers until they are all interconnected.

While this approach has clear benefits for sophisticated signal routing in small spaces, the ever-increasing cost of processing is a significant drawback. Before stacking, it is crucial to fill the vias that will be buried once the further lamination stages have been completed to minimize voids. The number of additional lamination stages increases with the number of layers that need to be connected.

Unfortunately, sequential lamination is often the most expensive method since it requires numerous laser cycles in addition to plating and filling before the next layer can be added to build the necessary interconnections.

Creative Design with Blind Vias in Your Next HDI PCB

When designing an HDI PCB (High-Density Interconnect PCB) with blind vias, several creative approaches can be taken to maximize the benefits of this technology.

•        Via-in-Pad Design

Instead of placing the vias on the edge of the pads, the vias are placed directly in the pads. This allows for a more compact design, as the pads can be made smaller. It also improves the thermal performance of the components as the vias are closer to them.

•        Staggered Blind Vias

Instead of placing all of the blind vias in the same location, they can be staggered to increase the number of connections that can be made in a small area. This approach can also improve the reliability of the connections as it reduces the stress on anyone via.

•        Microvias

 Utilizing smaller microvias which are smaller than regular vias, allows for a higher density of connections in a small area and a more compact design.

•        Multi-Depth Vias

Using multiple depths of vias allows for more connections to be made between different layers of the PCB. This approach can also improve the reliability of the connections as it reduces the stress on anyone via.

•        Combining Blind and Through Vias

Combining blind and through vias in the design allows for more flexibility in the placement of the vias. It also allows for a more robust and reliable design.

By using creative approaches, it is possible to achieve a highly dense and reliable HDI PCB design. However, these creative techniques come with additional costs and complexity, so it's important to weigh the benefits against the requirements of the final product.

It is important to note that while these creative approaches can be beneficial in terms of design, they also come with additional costs and complexity in the fabrication process. Therefore, the designer needs to consider the cost and quality standards of the final product before implementing these techniques.

How Blind and Buried Vias Are Made

Blind and buried vias, also known as blind and buried via holes, connect one or more layers of a printed circuit board (PCB) without extending through to the opposite side of the board. These types of vias are typically used in high-density interconnect (HDI) and multi-layer PCBs to increase the number of connections that can be made in a small area.

There are several methods for creating blind and buried vias in a PCB, including drilling, laser ablation, and photoimagable processes.

1.      Drilling

 This is the most common method for creating blind vias. It involves drilling a hole through the PCB layers, typically using a CNC (computer numerical control) drill. The drill typically uses a carbide or diamond-tipped bit to create the hole. The hole is then plated with a thin layer of metal to ensure a reliable connection.

2.      Laser Ablation

This method uses a laser to vaporize the material in the PCB layers, creating a via hole. This method is typically used for creating microvias, which are smaller than regular vias.

3.      Photoimagable Process

This method uses a photolithography process to create the via holes. A photo-sensitive material is applied to the PCB. Then a mask is used to block certain areas from exposure to light. The PCB is then developed, leaving the via holes in the desired location.

All of the above methods have their own advantages and disadvantages, such as cost, precision, and complexity. The choice of the method used to create the blind and buried vias will be based on the PCB manufacturer's design requirements, cost, and fabrication capabilities.

Blind and buried vias are made by drilling, laser ablation, or photo imageable processes. Drilling is the most common method, while laser ablation and photo imageable processes are typically used for creating microvias. The choice of the method depends on the design requirements, cost, and fabrication capabilities of the PCB manufacturer.

Blind Microvia PCB Fabrication

The process of fabricating blind microvias typically starts with the design of the PCB. The designer must consider the size and shape of the microvias and the materials used to create them to ensure that the final product meets the quality standards for the application. The designer must also ensure that the microvias are properly placed, and the connection between the via and the surrounding trace is reliable.

Once the design is complete, the PCB is then fabricated using a specialized process. The most common method for creating blind microvias is laser ablation. This method uses a laser to vaporize the material in the PCB layers, creating a via hole. The laser is typically a UV laser and is precisely focused on the area where the via hole is to be created. The laser ablation process is highly precise, which is critical for creating small microvias.

After the microvias are created, the PCB is then plated with a thin layer of metal, such as copper, to ensure a reliable connection. This is done using a process called electroless copper plating. This process involves immersing the PCB in a copper plating solution, which deposits a thin layer of copper on the PCB. The copper is deposited selectively on the walls of the via holes, ensuring a reliable connection.

Once the plating process is complete, the PCB is then inspected to ensure that the microvias are properly aligned and that the connections are reliable. This inspection is typically done using an automated optical inspection (AOI) system, which uses a camera to inspect the PCB and identify any defects.

Finally, the PCB is tested to ensure it meets the required electrical specifications. This testing is typically done using a test probe placed in contact with the microvias to measure the electrical resistance and continuity of the connections.

It's important to note that the process of creating blind microvias is complex and requires specialized equipment and processes, which can increase the cost and lead time of the final product. However, the use of blind microvias can significantly increase the number of connections that can be made in a small area, which can be beneficial in terms of design and performance.

Conclusion

Blind microvia PCB fabrication is a process used to create small, blind vias in a PCB that connect one or more layers of the PCB. The process starts with the design of the PCB, and then PCB is fabricated using laser ablation, followed by electroless copper plating, inspection, and testing. The process is complex and requires specialized equipment and processes, which can increase the cost and lead time of the final product. However, the use of blind microvias can significantly increase the number of connections that can be made in a small area, which can be beneficial in terms of design and performance.