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Blind & Buried PCB

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OVERVIEW

Item Rigid PCB
Max Layer 60L
Inner Layer Min Trace/Space 3/3mil
Out Layer Min Trace/Space 3/3mil
Inner Layer Max Copper 6oz
Out Layer Max Copper 6oz
Min Mechanical Drilling 0.15mm
Min Laser Drilling 0.1mm
Aspect Ratio(Mechanical Drilling) 20:1
Aspect Ratio(Laser Drilling) 1:1
Press Fit Hole Ttolerance ±0.05mm
PTH Tolerance ±0.075mm
NPTH Tolerance ±0.05mm
Countersink Tolerance ±0.15mm
Board Thickness 0.4-8mm
Board Thickness Tolerance(<1.0mm) ±0.1mm
Board Thickness Tolerance(≥1.0mm) ±10%
Impedance Tolerance Single-Ended:±5Ω(≤50Ω),±7%(>50Ω)
Differential:±5Ω(≤50Ω),±7%(>50Ω)
Min Board Size 10*10mm
Max Board Size 22.5*30inch
Contour Tolerance ±0.1mm
Min BGA 7mil
Min SMT 7*10mil
Surface Treatment ENIG,Gold Finger,Immersion Silver,Immersion Tin,HASL(LF),OSP,ENEPIG,Flash Gold;Hard gold plating
Solder Mask Green,Black,Blue,Red,Matt Green
Min Solder Mask Clearance 1.5mil
Min Solder Mask Dam 3mil
Legend White,Black,Red,Yellow
Min Legend Width/Height 4/23mil
Strain Fillet Width /
Bow & Twist 0.3%
Table of Contents
Primary Item (H2)

The need for smaller printed circuit boards (PCBs) to accommodate miniaturization in electronics is increasing. It may not always be possible to add all the PCB components to the PCB. Such a situation can be mitigated using vias in the circuit board. Vias are vertical conductive holes that connect one layer to another in multilayer PCBs. 

These may be present beneath the surface, between two layers, or all the way across the board. There are several types of vias, such as blind, micro, and buried. We will discuss blind and buried vias in PCB in this article. These are the two most often used vias for PCB fabrication. We'll talk about the benefits of vias, how they are fabricated, and their importance. 

What is PCB Via?

Via is the small hole that is drilled through two or more layers in PCBs. Its main role is to allow signals to travel through the layers. The power is also distributed from vias. These interlayer connections, due to vias, impact the functioning of the PCBs in electronic devices. 

Vias in PCB, an abbreviation for Vertical Interconnect Access, play a crucial role in fabricating printed circuit boards. A standard via consists of three key components: the barrel, a conductive tubing linking two layers through a hole in the PCB, and and the pad, connecting to the ends of the barrel and linking it to components, planes, or traces. 

The anti-pad or clearance hole separates the barrel from adjacent copper layers. There are three main types of vias: through-via, blind-via, and buried-via.

The through-via, often the initial thought associated with vias, is the conventional plated-through-hole (PTH) via. This type has drilling holes through the board, connecting both external layers of the PCB.

In High-Density Multi-layered Printed Circuit Boards (PCBs), micro-vias, including blind vias and buried vias, become essential. High-density interconnect (HDI) technology utilizes these micro-vias to create intricate PCBs.

Globalwell Blind & Buried PCB 1

Exploring PCB Via Types

As we know, vias are the conductive pathways connecting PCB layers that serve specific purposes in electronic design. Let's explore common via types:

Blind Via

A blind via goes from one side of the outer layer (top or bottom), connecting at least one inner layer without passing through the entire board.

Valuable for freeing up PCB space, commonly used in Ball Grid Array (BGA) assembly and High-Density Interconnect (HDI) PCBs, enhancing design flexibility.

Buried Via

It connects at least two inner layers, remaining invisible on the outer layer. Primarily designed to connect inner layer signals, reducing the risk of signal interference. Suitable for HDI PCBs, crucial for maintaining signal integrity.

Through-Hole Via

The most common type connects inner and outer layers by passing through the entire board. Standard choice for internal PCB interconnections and used as mounting holes for components, providing stability.

Microvias

Vias, with a diameter under 150 microns, are widely used in High-Density Interconnect (HDI) PCBs. Favored for small hole size, minimizing space usage on the circuit board. Connects layers through copper plating, with a conical shape simplifying copper plating. Stacking multiple microvias is necessary for complex designs.

Micro-vias, tiny holes typically plated to connect adjacent layers, enhance a PCB's circuit density by accommodating more circuit traces. Unlike standard micro-vias linking only two adjacent copper layers, they leave additional space for trace lines.

Via-in-Pad

It involves placing vias directly on Ball Grid Array (BGA) pads on the circuit board. Manufacturers consider this one for space-saving benefits. Incorporating vias into BGA pads minimizes the need for additional space, enabling the design of smaller PCBs without compromising functionality.

Via type, choice depends on specific PCB design requirements. Engineers consider factors such as space constraints, signal integrity, and high-density interconnections when selecting the appropriate via type. The evolving PCB technology landscape pushes design boundaries, emphasizing the importance of understanding via characteristics for efficient and reliable electronic devices.

Need for Via on PCBs

The outer and inner layers of PCBs are connected using electrical components with the help of vias. Via can be used in inner as well as outside layers to make up for the limited space in PCBs for connections. Buried via is present in the inner layers of the PCB. 

They help free up space for other components in the circuit. On the other hand, blind via is present at the surface, ideal for BGA components. HDI PCBs have blind and buried vias for better power and compact surface area. 

The higher connections using vias lead to more density in the circuit boards. They are used in cell phones, smart devices, medical equipment, and laptops. The manufacturing of HDI PCBs is a complex process that can only be done by professionals. 

Benefits of vias

Here are the benefits of vias in printed circuit boards: 

Routing the signal

Vias and micro vias ensure good signal routing with denser PCBs. Both blind and buried vias enhance the signal for further transfer. Vias are also useful for power nets as they carry more current. 

Trace density of multilayer boards

As vias can be added in multiple layers below each other, they increase the density of the PCBs. The overlapping vias provide vertical connections needed in the circuit. Moreover, with several different traces, vias are able to connect with each other. 

Transmitting signal

Vias transfer the signal between several layers of PCBs in a board. Different routes may be required on the board in case you wish to integrate other components. 

Saving space

Vias-in-pads offer an innovative solution for space savings on PCBs. Directly integrating vias into component pads eliminates the need to route signals away, optimizing routing and significantly reducing the overall PCB footprint.

Easier routing

For easier routing, placing vias directly under component pads frees up space and simplifies the process, especially for components with small footprints like those in Ball Grid Array (BGA) packages.

Better heat dissipation

Better heat dissipation is achieved by strategically placing vias in pads near heat sources. This enhances thermal conductivity between components and different PCB layers, facilitating faster and more efficient heat dissipation.

Reduction in inductance

The integration of vias in pads reduces parasitic inductance associated with additional connection segments. This reduction is particularly beneficial for high-speed designs and interfaces, contributing to improved signal integrity.

Cost saving

If the layer count is low with the use of multiple vias, the production volume can be increased at a reduced cost. 

Understanding Blind Via in PCB

Blind vias are visible on one side of the board, connecting an outer layer and passing through two or more layers to connect with inner layers. However, blind vias cannot traverse the entire board to connect directly to the other outer layer. These distinctions among via types provide designers with a versatile array of options for crafting PCBs tailored to specific space, density, and connectivity requirements.

For example, When blind via drilling is done on 2+4+2 layer boards, the outer two layers are drilled first. Another way is to drill 2+4 together while giving special attention to the precise alignment. 

Here are some characteristics of blind vias: 

  • Blind vias only connect the adjacent layers of the board. 
  • It does not pass through the PCB entirely. 
  • Blind via is ideal for high-density interconnections. 
  • It can only connect one layer to another immediate layer internally. 
  • They require skilled fabrications. 
  • Blind vias are not comparable to through holes as they pass the entire board. 
  • It provides connectivity through an internal layer instead of being hidden inside the board. 

Understanding Buried Via in PCB

Buried vias, concealed within the board, remain unseen from the outside as they traverse only between internal layers. These vias can pass through two or more inner layers but cannot extend to any outer layer. Drilling for buried vias occurs during PCB assembly, connecting internal layers without external access.

For example, In buried via, the holes are made before bonding, so only partial bonding is required. Two layers are drilled together before permanently joining into the PCB.

  • Here are some significant characteristics of buried via:
  • Buried vias are enclosed in the PCB. 
  • Plating and holes are not necessary for buried vias. 
  • It only connects the internal layers.
  • Advanced production skillsskills in production are mandatory.
  • It may be used for grounding and power routing. 
  • Buried vias do not show up as openings on the finished boards. 
  • They do not connect external layers as they form an isolation. 
Globalwell Blind & Buried PCB

How are Blind and Buried Vias Manufactured?

Making vias in PCBs involves two main methods: after or before multilayer lamination. For blind and buried vias, cores are drilled, and through holes are plated. The stack is then built and pressed together. For more details on this manufacturing process, refer to the IPC-2221B manual.

When creating blind vias, consider the drill depth carefully. The hole's depth is crucial, impacting the board's performance. Excessive depth may distort signals, while insufficient depth can result in a faulty connection.

For an optimal design and to avoid costly manufacturing issues, consult your PCB manufacturer. Seeking advice on the best approach for your board is essential. Manufacturers can plug the vias with metal or thermal/electrical epoxy and plate copper over them.

It can help prevent internal air bubbles from causing voids or pinholes in the solder joint. Collaborating with the manufacturer ensures well-designed vias, enhancing the PCB's overall reliability and performance.

When and Where to Use Buried Vias?

Buried vias are preferred over blind vias when the PCB interconnects are designed internally. It adds isolations for RF circuits to ensure an interrupted signal. Moreover, buried vias will not show any stubs on the external layers. 

Apart from isolated connections, buried vias are good for connections that may not be applicable from an external circuit board layer. The direct inner connectivity of blind and buried vias will aid in reducing layers, hence making the stack up smaller. 

Techniques Used for Via Manufacturing

Below are some techniques used in manufacturing vias: 

Sequential Lamination

In this method, each layer is laminated along with pre-formed vias that are between them. Here, very thin pieces of laminate are used to create a double-sided PCB. Drilling is done, followed by plating and etching on the laminate. This gives features to the one side of the board that will connect to the second layer.

The top layer, without drilling and etching, works as the top finished layer of PCB. These layers are assembled with others using lamination. If it's a multilayer PCB, other layers are joined with this laminated layer. The internal layer imparts flexibility to the via structure, ideal for flex boards. This method is precise; however, it is expensive to make blind vias. The thin laminates have to be handled with care during the etching and drilling process. 

Laser Ablation

Laser ablation works to create blind and buried vias. Here, the laser will drill holes by removing the material from specific locations. Laser ablation is mostly used for drilling holes in laminated internal layers of the PCB. For instance, if the ablation is done between two internal layers and the outer layer, this is done simultaneously. CO2 laser and excimer laser are two methods used for the ablation of the materials. 

The CO2 laser uses powerful machinery to drill holes in the copper material. Photoimaging may be done to ensure the holes are drilled in the right alignment. One important thing is that laser drilling is done before the etching process. 

The excimer laser is ideal for drilling holes through dielectric materials and copper. So, a blind via can be created in a single-step process. Besides, the copper does not have to be pre-drilled for this method. During the excimer laser process, the copper pads have to be protected so that the laser does not cut through them in buried vias. It is a precise and effective method to create vias. 

Plasma Etching

Thin dielectric layers are plasma etched through dry gas in a vacuum environment. The plasma releases uncharged free radicals to react with the board surface and create blind vias. As the plasma ions release on top of the copper material, the ions are removed, leaving small holes behind. 

Plasma etching is great for precise holes with very small diameters. Moreover, this method is done in an inert atmosphere; hence, no contaminants impact the PCBs. One downside of this method is that it is expensive and may not be good for bulk orders. 

Controlled Depth Drilled Blind Vias

To make controlled depth blind vias, very precise drill penetration is used to create holes on one side of the PCB. The blind vias are made partly through the materials using pads. Copper plating is done in the next step. This method is affordable as it does not need expensive etching or laser equipment. Although, it creates larger holes that may not always fit the demands. Therefore, important circuits have to be kept away from drilled holes for safety.

Photo-Via Tenting

Photo-via tenting is a method that creates both blind and buried vias at precise locations. Selective tenting is done over the annular copper rings with liquid photo-imageable ink. It adds a protective layer over the vias to prevent moisture and dirt from accumulating. 

Photoimageable dielectric layers allow selective tenting over vias, leaving openings only where blind/buried vias are desired. PCBs that are made for harsh conditions and temperature changes require tenting to minimize environmental damage. 

Apart from this, signal interference significantly reduces when the holes are covered. Signal reflections and cross talk also reduce in high-speed PCBs. Improving the performance and reliability.

Globalwell Blind & Buried PCB 2

Differences Between Blind and Buried Vias

Here are some key differences between blind & buried vias: 

  • Blind via fabrication is of moderate complexity. On the other hand, buried vias are highly complex to fabricate. Both of these add to the production cost. 
  • The fabrication process of blind vias involves drilling, tenting, filling, and plating. Buried vias are made using laser ablation and copper plating. 
  • Blind vias have a low inductance, stubs, and resistance. Meanwhile, buried vias have the lowest inductance, stubs, and resistance. 
  • Blind vias are medium to small in diameter. Buried vias are the smallest in diameter and are often confused with microvias.

Design Considerations for Making Vias on PCBs

There are several rules to follow when designing vias on PCBs. The number of vias and their placements can significantly impact the working of the circuit boards. Here are key design considerations for creating vias on PCBs:

Configuration and Clearance Rules

When creating vias on a PCB, use the constraint manager in the physical workspace for precise configuration while following standard clearance rules. This ensures proper spacing to prevent issues related to proximity and interference.

Surface Mount Components (SMD)

Avoid placing vias directly between SMD pads to prevent solder flux entrapment and potential corrosion. Solder flux under SMDs can complicate post-manufacturing inspection, highlighting the need for strategic via placement.

Blind/Buried Via Design

Incorporating blind and buried vias in PCB designs requires attention to guidelines. These vias should span an even number of copper layers and not terminate on the top side of a core or start on the bottom side. Opt for staggered vias over stacked ones to reduce time and cost.

Controlled Depth and Aspect Ratio

Controlled depth for blind and buried vias is crucial. Keep the aspect ratio minimal for high-speed vias to enhance electrical performance and signal integrity and minimize noise, crosstalk, and EMI/RFI.

Size Considerations

Vias' size is critical in PCB design. Use smaller vias, especially in High-Density Interconnect (HDI) boards, to minimize capacitance and inductance. Ensure vias within thermal pads are filled for improved thermal management.

BGA Installations

For Ball Grid Array (BGA) installations, use blind and through vias in thermal pads. Ensure these vias are filled and planarized to maintain solder joint integrity during assembly.

Assembly Considerations

The manufacturing should compensate for the lack of through vias in a thermal pad. They do so by adding a windowpane-like opening around the solder paste stencil above the pad. This prevents solder pooling and outgassing during assembly, enhancing overall solder joint quality.

Clearance and Inspection

Maintain vital clearance, especially for traces and vias near routed/scored edges. Rigorous inspection protocols are crucial, especially for complex components like Ball Grid Arrays (BGAs).

Dog-Bone Design

In dog-bone designs, be mindful of mask clearance for vias under the BGA to avoid interference and maintain connection integrity.

Tolerances

Adhere to critical tolerances, including precise annular rings, drill-to-plane clearances, and preferred diameter ranges. Find the accurate location registration and proper clearance of the solder mask. Considering these factors enhances the reliability, performance, and manufacturability of PCBs.

How is Vias Covered?

Choosing the right via covering method depends on your PCB design's specific requirements. Blind and buried vias, available on boards with a minimum of four layers, contribute to increased multilayer board density, reducing the overall number of layers and board dimensions. Thoughtful consideration of design specifications and intended functionalities guides the choice of via covering processes in PCB designs.

Vias in PCBs can be covered in different ways:

Tenting Vias

Tenting vias means covering the annular ring with a solder mask to insulate it. Ensuring full coverage with a thick layer of solder mask prevents accidental short circuits, enhancing overall PCB reliability.

Vias Not Covered

Choosing "vias not covered" leaves both the via hole and annular ring exposed without a solder mask. Commonly used for debugging measurement signals, this method can also increase the heat dissipation area, supporting better thermal management. However, note the higher risk of short circuits.

Plugged Vias with Solder Mask

Plugging vias with a solder mask prevents solder balls from causing short circuits during wave soldering and avoids flux residue in the via hole. For PCBs with components like a Ball Grid Array (BGA) or Integrated Circuit (IC) assembly, selecting mask-plugged vias ensures a reliable soldering process.

Parting Words

Now you know blind and buried via in PCBs are crucial to incorporate all the components in different layers. Blind and buried vias require professional skills and equipment to ensure quality and precision. Therefore, it's important to choose the right printed circuit board manufacturers with ample experience. Look for someone offering custom prototypes and one-time deliveries for PCBs.

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