What are Stacked Microvias?

What are Microvias?

Microvias are small holes drilled in a PCB to make electrical connections between layers. They are typically defined as blind vias (connecting an outer layer to an inner layer) or buried vias (connecting inner layers) with a diameter of 150 μm or less.

Some key characteristics of microvias include:

• Small diameter (typically 50-150 μm)
• Laser drilled
• Provide high-density layer-to-layer interconnects
• Enable miniaturization of PCBs

Types of Microvias

There are three main types of microvias:

What are Stacked Microvias?

Stacked microvias take the concept of microvias a step further by aligning and stacking multiple microvias on top of each other to span greater distances through the PCB. This allows interconnections between several or all layers of a multi-layer board.

Some key points about stacked microvias:

• Allow high-density connections between multiple or all layers
• Stacking can be done as a single stack (one via stacked on top of another) or staggered (overlapping vias)
• Require precise alignment and specialized manufacturing processes
• Enable further miniaturization and increased layer count in PCBs

Stacked vs Staggered Microvias

Stacked microvias can be formed as a single stack where each via is directly on top of the previous one, or staggered where the vias are stacked with a slight overlap. Here is a comparison:

Manufacturing Stacked Microvias

The manufacturing process for stacked microvias is complex and requires specialized equipment and precise process control. The basic steps are:

1. Laser drilling blind microvias
2. Depositing copper into the vias to form connections
3. Laminating additional layers and repeating steps 1-2 to form the stack
4. Plating the final through hole to connect the stacked vias

Some key challenges in manufacturing stacked microvias include:

• Precise laser drilling and alignment of vias
• Reliable copper deposition into high aspect ratio vias
• Maintaining stack integrity and reliability through lamination cycles
• Avoiding defects like voids, misalignment or poor copper adhesion

Applications and Benefits of Stacked Microvias

Stacked microvias are used in a variety of applications that require high-density interconnects and miniaturization, such as:

• Smartphones and wearables
• Aerospace and defense electronics
• High-performance computing
• Medical devices
• Automotive electronics

Some of the key benefits of using stacked microvias include:

• Increased interconnect density
• Reduction in board size and weight
• Improved electrical performance from shorter signal paths
• Ability to integrate more functionality in smaller packages
• Cost reduction at the system level from increased integration

Design Considerations for Stacked Microvias

When designing PCBs with stacked microvias, there are several important considerations:

• Stack configuration (stacked vs staggered)
• Layer count and microvia spans
• Pad and antipad design
• Signal integrity and power delivery
• Design for manufacturing (DFM) rules
• Reliability requirements

Working closely with the PCB fabricator is essential to ensure the design is manufacturable and will meet reliability requirements. Following DFM guidelines and simulating the design can help identify and mitigate potential issues.

Frequently Asked Questions

What is the typical size of a microvia?

Microvias typically have diameters ranging from 50-150 μm. The exact size depends on the PCB design requirements and manufacturing capabilities.

How many microvias can be stacked?

The number of microvias that can be stacked depends on several factors, including the PCB thickness, layer count, and manufacturing process. Stacks of 4-8 microvias are common, but some advanced PCB fabricators can achieve even higher stack counts.

Are stacked microvias reliable?

When properly designed and manufactured, stacked microvias can be very reliable. However, the complex manufacturing process does introduce some additional risks compared to regular through-hole vias. Following best design practices, working closely with the fabricator, and thorough testing are important to ensure reliability.

What materials are used for microvia PCBs?

The most common materials for PCBs with microvias are traditional FR-4 laminates. However, some high-performance applications may require specialized materials like polyimide or high-Tg FR-4 to meet reliability and signal integrity requirements.

How much do PCBs with stacked microvias cost?

The cost of PCBs with stacked microvias varies depending on the complexity of the design, layer count, and the specific manufacturer. In general, they are more expensive than traditional PCBs due to the specialized manufacturing processes required. However, the increased cost can often be offset by system-level cost reductions from the miniaturization and integration benefits provided by stacked microvias.

Conclusion

Stacked microvias are a powerful technology for enabling high-density interconnects in multi-layer PCBs. By providing a way to make connections between several or all layers in a small footprint, they are a key enabler for continued miniaturization and integration of electronics.

However, stacked microvias also introduce additional complexity and challenges in manufacturing that require careful design consideration and close collaboration with fabricators. As electronics continue to push the boundaries of performance and integration, stacked microvias will undoubtedly play an increasingly important role in PCB design and manufacturing.