Microvias in HDI PCB: Smaller Vias, Better Connections

Introduction

Microvias help define the performance of High-Density Interconnect (HDI) PCBs. They make it possible to build both fast and tiny circuit layouts, which are well-suited for small devices. With microvias, engineers can build PCBs that are more compact and have extra features. In the following section, we’ll guide you to the types of microvias, processes, and challenges to help you learn more about microvias in HDI PCBs.

What is PCB Microvias?

The term microvia refers to tiny holes in a PCB that help connect layers, usually having sizes between 0.1mm and 0.15mm. Microvias help establish efficient signal and power pathways in high-density PCBs. Microvias make it possible to increase the number of connections on a PCB without making it larger. That is why they are well-suited for the needs of small, high-performance devices.

Types of Microvias

Microvias are key features in HDI PCBs, which support high-density connections. Various microvias are employed depending on the PCB’s design needs. The correct type to select depends on considerations such as signal integrity, routing space, and manufacturing complexity. It is essential to know these types to maximize your PCB design.

Buried Microvias

Buried microvias are employed to interconnect the internal layers of an HDI PCB. Since their names describe them as they are wrapped up within inner layers, they suit high-density routes. By buried microvias, you’re able to attain a minimized board with greater room for routes at the surface level. They can save signal loss and EMI while resulting in overall high performance within your PCB. They’re most suited when you’d like to uphold signal integrity and enhance your PCB’s space utilization.

Blind Microvias

Blind microvias interconnect the outer layer of a PCB to inner layers but aren’t through-plated. These vias provide perfect connectivity to attach the outer layer to an individual or set of internal layers. Blind microvias provide space savings and a similar set of benefits to buried microvias, i.e., enhancing signal integrity. Nonetheless, they call for careful examination of stress levels during assembly due to the exposure to external forces. Their thermal management and aspect ratio are also important factors to be considered at the design phase.

Stacked Microvias

Stacked microvias are placed one upon the other to join various layers in the PCB. It is used to facilitate intricate and space-saving multilayer designs that form a key element of miniaturization. Copper fills each microvia, enhancing conductivity and providing solid connections. Stacked microvias are ideal for joining non-adjacent layers, and they are therefore well-suited for sophisticated HDI PCB designs. Stacked microvias enable greater design flexibility with dense PCBs.

Staggered Microvias

Staggered Microvias are microvias in a non-linear array, offset from one another. This alignment minimizes the possibility of delamination, enhancing the mechanical stability of the PCB. The staggered pattern aids in distributing stress evenly during assembly, avoiding areas of weakness in the PCB. This via is also helpful in enhancing the overall durability of the PCB. By designing staggered microvias in a careful arrangement, you can make your PCB reliable even under stress and mechanical pressure.

Microvia Manufacturing Processes and Hole Shapes

Microvias are small holes applied in HDI PCBs to interconnect various layers. The vias are usually smaller than 0.15mm in diameter, and the sizes of pads are less than 0.35mm. Microvias can be of different types, such as blind, buried, or stacked, depending on the design specification. Since lasers are often involved in producing microvias, circuit boards that are both space-efficient and reliable can be used. Understanding the different types of lasers and techniques used to produce microvias allows you to create more effective and accurate PCBs suitable for high-density applications.

Advantages of Microvias

The application of microvias in HDI PCBs helps make the boards smaller and lighter, which is very useful for things like smartphones and tablets, since their size and weight are important. Microvias let you fit more things on your PCB in less space, so they are a good choice if you want to make something small like a smartphone. Microvias help keep the signals strong, reduce resistance, and keep the PCB sturdy. They also help lower costs by using fewer materials and keeping things smooth during production.

Smaller Size and Weight

Microvias let you make your PCB smaller and lighter than big boards, so it’s easier to fit smaller devices. This is important, especially for smartphones and tablets, since they must be light and small. The compact nature of microvias lets you fit many parts onto the PCB in the same size area, making smaller electronics possible. This compactness helps us build smaller, lighter electronics while improving performance. Microvias help make electronics more compact, so you can keep making devices smaller while still keeping them working well.

Improves Performance

By using microvias, you can increase the efficiency of your PCB. Microvias are smaller than standard vias, which helps to make the electrical paths on your PCB shorter. This makes the whole circuit more efficient due to the reduced inductance and resistance. Microvias help you send faster and more accurate data on your PCB. This is rather important for applications that work with fast signals and devices with a high frequency, such as 5G.

Enhanced Signal Integrity

Microvias make it possible to shorten signal paths, improving overall signal communication and stability, particularly in 5G or high-speed applications. The use of microvias allows faster communication in high-frequency devices. They also decrease unwanted induction and capacitance in the board’s structure, which could alter the quality of the signal. By reducing signal paths, electromagnetic interferences are minimized, maintaining a quality factor in the electronics. Microvias in the PCB allow for improved communication speed and increased accuracy.

Mechanical Stability

Having microvias on your PCB strengthens the layers and makes the PCB more resistant to stress. Microvias hold the layers together more firmly through lamination so that the PCB can resist more stress. By having microvias evenly distributed, less damage occurs due to even stress. For this reason, your PCB will not fail, even in difficult environments. With more durable layers, the PCB is able to withstand more stress.

Optimized Cost

Microvia technology may be complex, but it is more cost-effective for production. Switching to microvias can reduce the cost of a PCB by wasting less material. Because microvias help cut the number of layers, the design is simpler. The process becomes efficient and less expensive because assembly is less complex and involves fewer parts. It reduces the total cost, especially for large-scale production runs.

Challenges in Microvia Design

When designing HDI PCBs with microvias, some challenges affect both performance and reliability. Matters like aspect ratio control, thermal management, and material selection must be treated carefully. Selecting the appropriate via type and designing the HDI stack-up correctly are also essential to a successful PCB. Here’s how to handle these challenges successfully.

Aspect Ratio Control

Aspect ratio in microvia design is the depth-to-diameter ratio of the via. A high aspect ratio can cause a number of issues, such as cracks, poor copper plating, or mechanical stress. You must restrain the aspect ratio to avoid these. The ideal ratio is 1:1, according to IPC standards. Having the correct aspect ratio guarantees improved reliability and a long-lasting PCB.

Thermal Management

Thermal management is critical in HDI PCBs, particularly with microvias. The tightly packed component layout produces heat, and microvias can be hot spots. Heat generation can cause stress and damage to the PCB. To control this, use the appropriate material for the PCB and implement thermal vias. Installing heat sinks also enables heat dissipation and optimal performance.

Material Selection

Materials must be carefully selected in HDI PCBs that make use of microvias. Since microvias are fabricated through lasers, the selected materials should be good in both thermal and electrical aspects. Materials like dielectrics are best because they work well in terms of heat and electricity.

Here are some common and recommended materials we often use that you can refer to when making a choice:

• Ceramic-filled PTFE (e.g. Rogers RO3003/RO3006): Very low dielectric loss (Df ≈ 0.0013-0.004) and good thermal conductivity, ideal for RF cases such as 5G antennas and microwave filters;
• Low thermal expansion ceramic composites (e.g. Panasonic Megtron 6): With extremely low CTE (coefficient of thermal expansion) and Dk ≈ 2.6-2.7, ideal for high-reliability multi-layer HDI structures.
• Polyimide Film: combining excellent flexibility and thermal stability, it can be used to make double-sided or multi-layer flexible HDI boards. It is suitable for wearable devices and bending applications.

Via Type Selection

Selecting the right type of via for your PCB depends on the specific application. Stacked microvias save space but are more complex and expensive to fabricate. On the other hand, staggered microvias offer better mechanical stability. The choice of via type will affect your PCB’s performance, cost, and complexity.

HDI Stack-up Design

How the HDI stack-up is designed is very important for the good functioning of microvias. Microvias of different layers must be selected in a stack-up that fits the requirements. Layer thickness, the qualities of dielectric, and the number of microvias should be the main things you prioritize. It is necessary to remember that the type of microvias affects performance.

When (and When Not) to Include Microvias

Microvias help save space and enhance PCB design, yet at greater costs and manufacturing complexity. You must weigh when it is absolutely necessary to use them within your design. Look at items such as BGAs, routing channels, and enhancements to board performance. However, it’s also important to ensure that the stack-up design can handle the constraints of microvia integration in HDI PCB manufacturing and application.

• BGAs: The dense nature of BGAs requires that standard vias can’t fit in the close spaces. Microvias enable you to route connections where standard vias can’t proceed, especially when you’re working with small pitch and restricted real estate. The via structure in this case usually follows the layout of the BGA, which usually contains the closest pitch. This renders microvias the ideal solution to route components in such close designs without sacrificing performance.
• Routing Channels: By reducing the size of the via, you also reduce the size of the pad, which makes the clearance between features narrower. This frees up space for routing channels between arrays of vias that otherwise couldn’t exist. Microvias enable you to add routing density, making creating more efficient and smaller designs possible.
• Improve Board Performance: Microvias eliminate choke points within power and ground planes, eliminating electrical and thermal hot spots. Hot spots can lead to problems such as EMI and signal interference through poor return paths. Staggered microvias, for instance, eliminate congestion by advancing drill positions away from crowded spots. This results in an efficient and stable design, enhancing board performance overall.
• Risks of Microvia Adoption: Even though microvias have a lot of advantages, there are also some risks involved. Having microvias can cause failure issues that should be planned for ahead of time. It has been noted that microvias can fail more quickly in high temperatures. It is important to evaluate the possible risks before deciding to use microvias in HDI, since the IPC task force will still refine reliability. You should always do a full risk analysis before choosing microvias, so your HDI design justifies the extra effort required in production.

FAQs

What is the Difference Between Vias and Microvias?

Microvias are small, dense vias drilled centrally through the board, whereas regular vias are drilled at the top or bottom. Microvias are appropriate for high-density designs and connect adjacent layers, whereas blind and buried vias are not.

How Large Should You Make Your Microvias?

Microvia size relies on the aspect ratio, thickness of the dielectric layer, and pad size. To calculate the size:

1. Select the needed HDI stack-up.
2. Determine the largest drill size for the tightest routed area.
3. Compute the required layer thickness.
4. Choose a laser-drillable prepreg that satisfies the thickness.

Verify microvias satisfy the aspect ratio limit and materials requirements.

Conclusion

For HDI PCBs, microvias help save space, improve the design’s performance, and make the PCB more reliable. But, there are challenges related to the size of microvias, the material used, and warm-up. Being aware of your design needs and planning for those needs is essential. Designing a proper via structure and stack-up can help you enjoy the advantages of microvias without running into any issues.

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