With the rapid development of miniaturization and complexity of electronic technology, PCB is also constantly updated and developed. From the original single-sided PCB to double-sided board and multi-layer PCB, the lines are getting denser, and more and more components need to be placed.
High precision, high density, high reliability, and miniaturization have become the major trends in the development of printed circuit boards. Then, while the corresponding circuit board processing apertures are increasing, the apertures are getting smaller and smaller, and the spacing between holes is getting smaller and smaller, micro via will gradually become the new trend of PCB via technology.
This article will explore micro via in PCB and how to design micro via in advanced HDI PCB in detail to meet the needs of the market and customers.
What is micro via in PCB?
Micro vias are very small round holes in PCBs, sometimes simply called through hole, used to connect to various circuits. Micro via can be placed anywhere on the PCB, but they are most commonly found in areas where many signal or power lines cross each other to reduce PCB crosstalk and improve PCB signal integrity.
Micro via can be widely used in high-speed PCB design, power and signal circuit design, and radio frequency(RF) design that needs to pass signals into and out of the structure.
What are the types of micro via?
Stacked micro via (SMV)
Staggered micro via
A staggered micro via PCB is aligned with pads on the same layer but offset from its center by half the pad diameter. The distance from its center to the edge of the board is also half the diameter of the PCB pad. Staggered micro via allow you to place two through-holes within one pad without overlapping them or having to route traces on different layers together at this location. This type of micro via can be used to make electrical connections to surrounding copper pads and traces.
Importance of Micro Via Aspect Ratio in PCB Design
The micro via aspect ratio in PCB design is crucial because it determines the maximum thickness of the PCB layers while ensuring the vias can be effectively drilled and plated, directly impacting the board’s reliability and performance; a higher aspect ratio can lead to challenges in maintaining good plating quality and thermal conductivity, making it essential to balance between layer thickness and via dimensions to achieve optimal signal integrity and mechanical strength.
Are Micro Vias Filled or Unfilled?
Micro vias, which are tiny holes used in PCB (printed circuit board) design to connect different layers, can be either filled or unfilled depending on the design requirements. Filled micro vias have their holes completely or partially filled with a conductive material to enhance electrical performance and mechanical stability, and are often used in high-density or high-performance applications. Unfilled micro vias, on the other hand, typically have a hollow interior that may be coated with a thin layer of conductive material to create electrical connections but do not offer the same level of mechanical strength or thermal performance as filled vias. The choice between filled and unfilled micro vias ultimately depends on the specific needs of the circuit board’s design and its intended application.
Why are Micro Vias Better Than Other Types of Vias?
Micro vias are often preferred over other types of vias because they are much smaller and allow for a higher density of connections in compact spaces, making them ideal for advanced, high-performance circuit boards where space is at a premium. Their small size helps reduce the overall footprint of the PCB, which is crucial in modern electronics where miniaturization is key. Additionally, micro vias can be designed to be blind or buried, which means they can connect internal layers without being visible on the outer layers, further optimizing space and improving reliability by reducing the risk of signal interference and thermal issues. This makes them especially valuable in high-frequency and high-speed applications where efficiency and performance are critical.
What is the function of micro via?
Micro via in a PCB have many functions, but here are some common ones:
1. Heat release from soldered components:
The relatively small surface area of solder balls on standard size through-hole components compared to their volume causes them to take a long time to heat up during the soldering process. This results in cold solder joints and poor electrical performance. Micro vias can be used as heat sinks to allow more heat transfer from the solder balls, reducing the time required for reflow soldering and improving reliability.
2. Connect the power plane:
Power planes help dissipate heat away from high-density PCB(HDI PCB) areas where multiple components draw current. To achieve this, micro vias are usually placed directly above each power plane so that heat can be quickly distributed across the plane, rather than being concentrated in one point on the board like standard vias.
3. Connect the ground plane:
Ground planes are designed to absorb noise and prevent interference from intruding into sensitive parts of the board, such as analog or digital signal paths. The ground planes are usually connected micro vias to share a common connection point on the board. This helps ensure that all ground points are connected and properly grounded.
4. Resonant circuit:
Micro via can be used in resonant circuits to reduce the inductance of the traces and reduce the phase shift between the primary and secondary resonant elements. This helps reduce signal loss in high frequency applications where signal integrity is important.
5. Power integrity testing:
Micro via can be used to test power integrity without removing components from the board or putting them under stress during testing procedures such as burn-in testing or EMI testing procedures such as radiated testing or immunity testing).
Advantages and Functions of Micro Via PCB
Micro vias are often preferred over other types of vias because they are much smaller and allow for a higher density of connections in compact spaces, making them ideal for advanced, high-performance circuit boards where space is at a premium. Their small size helps reduce the overall footprint of the PCB, which is crucial in modern electronics where miniaturization is key. Additionally, micro vias can be designed to be blind or buried, which means they can connect internal layers without being visible on the outer layers, further optimizing space and improving reliability by reducing the risk of signal interference and thermal issues. This makes them especially valuable in high-frequency and high-speed applications where efficiency and performance are critical.
What are types of micro via drilling ways?
With the development of complex electronic products, the lines on the PCB will become more and more dense, and more and more components need to be placed, but the size of the PCB will not become larger, but will become smaller and smaller. Then, from the selection of PCB substrate, PCB layout design to PCB manufacturing and PCB assembly, it is necessary to continuously upgrade and develop.
PCB via is an indispensable part. Facing more and more challenges, PCB manufacturers need to constantly think about how to deal with them. At present, commonly used PCB drilling technologies mainly include laser drilling, mechanical drilling, photosensitive hole forming, laser ablation, plasma etching, chemical etching, etc., of which mechanical drilling and laser drilling are the two most commonly used micro via drilling methods.
1. Laser drilling: Laser drilling refers to the optical structure module composed of lenses and lens groups, which gathers the light emitted by the laser light source into a high-energy-density laser beam, uses the laser beam to heat, dissolve, and ablate local materials, and then processes to form micro via. method. Generally speaking, holes larger than or equal to 1.5mm will be drilled mechanically, while holes smaller than 1.5mm will need laser drilling.
2. Mechanical drilling: Mechanical drilling refers to the method of drilling holes at different positions on the PCB by using numerical control technology to control the high-speed and precise relative movement of the high-speed rotating cutting drill and the PCB board. Mechanical drilling is suitable for a wide range of plate types and drilling diameters, covering almost all PCB drilling fields.
3. Laser ablation: laser ablation in PCB mainly uses carbon dioxide and ultraviolet laser cutting to make PCB micro via. The laser ablation process in PCB is usually used in PCB design with high-density wiring and complex layout to create very small holes. In order to make electrical connections between different layers of the PCB board.
4. Plasma Etching: Plasma etching is mainly used for PCBs with resin copper layers. Oxygen-containing gas is used as plasma. After contacting with copper on PCB, an oxidation reaction occurs, and then the resin material is removed to achieve the purpose of hole making.
5. Chemical Etching: Chemical etching refers to the technology of using chemical reaction to remove excess materials, that is, using chemicals to erode copper foil, resin, etc., and finally form vias on the PCB board.
Similarities and Differences Between Blind Vias, Buried Vias, and Micro Vias
Blind vias, buried vias, and micro vias are all used in PCB design, but they each serve different purposes and have distinct characteristics.
Similarities: All three types of vias are used to connect different layers of a PCB to ensure electrical continuity. They enhance the functionality and density of the circuit board, allowing for more complex designs.
Differences:
Blind Vias: These vias connect an outer layer to one or more inner layers but do not go through the entire board. They are useful for minimizing signal interference and are typically employed in high-density designs where space is limited.
Buried Vias: These are completely internal, connecting only the inner layers of a PCB and not extending to the outer layers. They are used to create complex routing without affecting the external layers, which helps in maintaining a clean surface and reducing signal noise.
Micro Vias: These are much smaller than traditional vias and can be either blind or buried. They are used in high-density interconnect (HDI) boards where space and layer count are tightly constrained. Micro vias allow for more compact designs and improved performance in high-speed applications.
Factors Affecting Reliability of Buried/Blind Vias and Stacked Micro Vias
The reliability of buried or blind vias and stacked micro vias is influenced by several factors, including the quality of the PCB fabrication process, the precision of via placement and drilling, and the materials used. For buried and blind vias, issues such as poor plating or inadequate solder mask coverage can lead to signal integrity problems or mechanical failure. In the case of stacked micro vias, the alignment between layers and the effectiveness of the via fill are crucial; misalignment or incomplete filling can cause weak connections or thermal stress. Additionally, environmental factors like temperature fluctuations and mechanical stress can impact the long-term reliability of these vias, making meticulous design and manufacturing practices essential for ensuring their durability and performance.
What is the minimum size for a micro via?
IPC-T-50M : The micro via is a blind buried hole structure with a diameter <= 0.15mm, the maximum aspect ratio is 1:1, and the maximum depth of the start and stop layers does not exceed 0.25mm.
IPC-6012 definition of micro via structure : Via is a blind hole structure with a maximum aspect ratio of 1:1 and an overall penetration depth of no more than 0.25mm.
IPC-2226 to micro definition of via:
• Pad diameter ≤ 350 um
• Have plated holes ≤ 150 um diameter formed by laser or mechanical drilling
• Holes with a depth-to-aperture ratio of 1:1 (typically 10:1 for through holes (PTH))
How to design micro via on HDI PCB?
The complexity and high-density design of electronic products means that while electronic products achieve more functions, electronic products and circuit boards face the challenge of miniaturization. For PCB design, more advanced solutions such as HDI, flex and rigid-flex PCBs must be used in some cases.
HDI PCB structure
IPC-2226 Design Standard for High Density Interconnect (HDI) Printed Boards
IPC-2226 definition of HDI PCB:
• PCB line width and line spacing ≤ 100 um
• Vias ≤ 150 um and pads < 400 um
• Higher connection pad density than traditional PCB (>20 pads/cm^2)
The definition of HDI structure in IPC-2226 usually has the following three types:
Type 1: Means that one or both sides of the outer layer are single-layer micro via , combined with vias , realize interlayer interconnection . micro The aspect ratio of via is recommended to be kept at 0.8.1, this type does not include buried vias.
Type 2 : Means that there are at least two layers of micro on one or both sides of the outer layer via , using micro via and via in pad realize interlayer interconnection. This type can include buried vias.
Type 3 : There are at least two layers of micro on one or both sides of the outer layer via in pad , using micro via in pad and via in pad, used to realize interlayer interconnection. This type can include buried vias.
Design microvias for an advanced HDI PCB
When designing micro via in advanced HDI PCBS, the following aspects should be considered:
- Micro via: When using HDI technology, copper layers are added on top of multi layer boards or as HDI multi layer. Micro vias are useful in connecting these thin layers. You can drill 5.9 mil vias that are 60 mils in diameter, or you can laser drill holes that are about 2 – 3 mils in diameter.
- Hole size: All elements of a conventionally designed PCB must be adjusted to accommodate the miniature’s smaller size. This is a challenge for PCB layout engineers who are familiar with traditional PCB design. This often happens if the design has holes that are too large, which results in a non-functional or sub-optimal miniature PCB
- Copper thickness: Common plating patterns are useful in fabricating microcircuits. This means the traces don’t need to leave the circuit. Pattern plating helps connect the entire circuit. On the other hand, wirebond plating is either electrical or chemical.
- Electrical test: The lower limit for rigid or flying probes is 2 to 3 mils. Over time, we expect this number to decrease as smaller landing pads are required. If the miniature PCB has smaller points, such as edge strip connectors, then it is desirable for the lines to extend out of the circuit.
- Reliability: Most common laminates used for printed circuits are used for microcircuits or HDI. However, they all have limitations. You can manufacture strong and rigid FR4 PCB laminates with micro double sided or single circuit. However, they must be thin to allow micro vias .
- Safety sign: Can image very small individual barcodes into solder mask for proper identification of printed circuit boards.
Choosing the Best Vias for Your PCB Layout
Choosing the best vias for your PCB layout involves several key considerations tailored to your specific design needs:
1.Board Density and Size: For high-density designs with limited space, micro vias are often the best choice because their small size allows for a higher number of connections without crowding the board. If space is less of a concern, traditional vias might be sufficient.
2.Layer Connections: Blind vias are suitable if you need to connect an outer layer to one or more inner layers without penetrating through the entire board. Buried vias are ideal for connecting internal layers without impacting the outer layers, helping to maintain a clean external surface and reducing potential signal interference.
3.Signal Integrity: For high-speed or high-frequency applications, micro vias can help maintain signal integrity by minimizing trace lengths and reducing parasitic effects. Proper alignment and plating of these vias are crucial to prevent signal degradation.
4.Thermal Management: If your design involves high heat dissipation, you should consider the thermal properties of the vias. Micro vias and stacked vias can help with thermal management by facilitating better heat dissipation through the PCB layers.
5.Manufacturing Costs: Micro vias and blind/buried vias can be more expensive to manufacture compared to standard vias due to their complexity. Balancing performance requirements with budget constraints is essential to choose the most cost-effective solution.
6.Reliability and Durability: The reliability of the vias can be impacted by factors such as via fill quality, alignment, and material properties. Ensuring high-quality fabrication and assembly processes will help enhance the long-term durability of the vias.
By carefully considering these aspects—density requirements, layer connections, signal integrity, thermal management, cost, and reliability—you can select the most suitable vias for your PCB layout to achieve optimal performance and efficiency.
Conclusion
Miniaturization of electronic components, functional components , PCB volume and space reduction, we need to achieve finer line spacing and more interconnection boards on higher density circuits. Therefore, we need more and more pores with smaller and smaller sizes, and these requirements have also promoted the development of micro via technology .
IBE, a professional ODM service provider, provides solutions integrating design, manufacturing and assembly for PCBs in the fields of automotive electronics, industrial control, medical beauty, etc., continues to explore new solutions to meet the product needs of society and customers.
High precision, high density, high reliability, and miniaturization have become the major trends in the development of PCB, smaller and smaller PCB vias are required.
Micro via in a PCB have many functions, and there are some:
- Heat release from soldered components
- Connect the power plane
- Connect the ground plane
- Resonant circuit
- Power integrity testing
A micro via in pad is a tiny hole that connects a pad on the surface of a PCB directly to one or more internal layers, effectively allowing for compact and efficient connections in high-density circuit boards. This type of via is drilled into the pad area itself rather than being placed between pads, which helps save space and improves electrical performance by reducing the length of the trace and minimizing potential signal loss. By integrating the via directly into the pad, designers can achieve more precise and reliable connections in modern electronics where space and signal integrity are critical.
A microvia laser typically creates vias that are very small, usually ranging from about 50 to 200 micrometers in diameter, depending on the specific requirements of the PCB design and the capabilities of the laser equipment used. This small size allows for high-density interconnects, making it possible to fit more connections into a compact space and support advanced, high-performance electronic devices. The precision of the laser ensures that these tiny vias can be accurately drilled and plated, which is crucial for maintaining signal integrity and overall reliability in modern circuit boards.
The primary difference between a via and a micro via lies in their size and application: a traditional via is larger and used for connecting multiple layers in standard PCBs, while a micro via is much smaller, designed for high-density interconnects in compact, high-performance circuit boards. Micro vias, which typically range from 50 to 200 micrometers in diameter, allow for more precise and densely packed connections, which is crucial in modern electronics where space is limited and signal integrity is critical. Traditional vias, on the other hand, are more suited for less dense designs and can handle larger connections but take up more space and may not be suitable for the high-density requirements of advanced applications.
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Last updated on September 2nd, 2024 at 06:39 am