Designing with Microvias: Stack-Ups, Reliability, and Via Filling

Have you ever wondered how designers pack so much functionality into such a small space? You can thank HDI design techniques and microvias in printed circuit design. High-Density Interconnect (HDI) PCB technology is at the forefront of modern electronics, enabling compact and powerful devices. These structures have been around for years, but they are becoming more common in a variety of systems that require multiple functions on a single circuit board.

If you've done a size study and you've determined you'll need 6 mil or smaller traces to fit all your components in your printed circuit board. This article explores what microvias are, their importance in HDI design, and the advantages they bring to advanced PCB manufacturing.

What Are Microvias?

A microvia is a blind hole drilled in conductor-insulator-conductor multilayer to provide electrical connection through insulators in electronic circuits. The maximum aspect ratio of 1:1 between hole diameter and depth, with a total depth of no more than 0.25 mm, when measured from the surface to the target pad or plane. Typically NCAB considers the dielectric thickness between surface and reference pad to be 60 – 80um. The diameter dimensions of the microvia have a  range of 80-100 microns. The typical RATIO is between 0.6: 1 to 1: 1.

Unlike traditional vias, which pass through multiple layers of a PCB, microvias are laser-drilled and are usually limited to interconnect only two adjacent layers. Laser drilling of microvias of diameter less than 150 μm is displacing mechanical drilling and now occupies around 94% of the market. They can be classified into three types:

• Blind Vias
• Buried Vias
• Stacked or Staggered Microvias

Let's know more about them in a separate section below.

Comparison with Traditional Vias:

Traditional vias lack precision and space-saving features. They are less suitable for HDI applications due to their larger size. Which limits fine pitch component placement and increases signal loss. In contrast, microvias are tailored for the miniaturized and high-performance demands of HDI PCBs.

Role of Microvias in HDI PCB Design

Microvias are an integral part of HDI PCBs, they increase interconnect density without occupying extra space. Here are some main features:

1. Space Optimization: Microvias allow components to be placed closer together, reducing the overall PCB size.

2. Improved Signal Integrity: Their shorter length reduces signal loss and unwanted parasitic components, making microvias ideal for high-frequency applications.

3. Thermal Management: Microvias can be integrated into designs for efficient heat dissipation, ensuring the reliability of high-power devices.

How are Microvia's Fabricated?

Exactly how small are microvias? If you ask most designers, they might tell you that they would consider a microvia to be a via with a diameter less than about 150 µm (6 mils). Depending on the size of the vias, they can be mechanically drilled and plated (followed by stacking and pressing each layer), or they can be formed with a high-power laser. The latter process is constantly being improved and is preferred in high volume PCB manufacturing thanks to its high throughput. New advances in laser drilling techniques are pushing microvia sizes down to as small as 15 µm. The two main processes in fabrication are:

1- Drilling and Cleaning of Via

2- Plating of Via

Microvia fabrication involves precise drilling, cleaning, and plating processes. Some of the common plating methods are sputtering, electrolytic deposition, or electroless copper plating. Which is  aimed to eliminate defects like voids and bumps that can compromise via durability under stress. Laser-drilled microvias exhibit fewer defects compared to mechanically drilled ones, which are limited by drill wear and size constraints.

Key Steps in the Manufacturing Process of Microvias:

1. Substrate Preparation: PCB substrate areas are cleaned and treated to ensure optimal adhesion.

2. Drilling: Laser drilling is employed for precise microvia creation, supporting high-density interconnects with diameters as small as a few micrometers.

3. De-smearing: Residual material from drilling is removed to ensure a clean conductive path.

4. Metallization: A thin layer of copper is deposited, usually via electroless plating, to form electrical connections between layers.

5. Quality Control: Advanced materials and optimized drilling parameters are used to prevent defects like incomplete drilling or poor adhesion.

Different Type of Microvia's in the PCB Design:

Advantages of Microvias

Microvias offer significant benefits for PCB design, including enhanced thermo-mechanical reliability and space efficiency. Their smaller size allows miniaturization, ideal for compact devices, while saving board space by reducing the need for larger through-holes. Microvias improve signal integrity in HDI boards by shortening trace lengths and minimizing radiation in high-speed circuits. Laser drilling reduces fabrication defects, enhancing overall reliability. Additionally, microvias boost RF and EMC performance, providing designers with a robust solution for advanced, high-density PCB applications. Some main advantages in summary are:

Miniaturization: The small size of microvias allows for compact designs with more functionality.

Enhanced Electrical Performance: Microvias minimize inductance and capacitance issues, making them ideal for high-speed circuits.

Flexibility in Layer Stacking: Microvias enable advanced stackups, such as multi-lamination processes, essential for complex designs.

Improved Reliability: Laser-drilled microvias offer higher precision, reducing the risk of defects.

Design Considerations for Microvias:

There are several aspects about drilling microvias for PCBs. When designing a circuit board, keep in mind that when you are looking for a hole that can be reliably plated, the microvia should not go above the 1:1 aspect ratio. While it is true that it can be drilled at a larger ratio, it simply will not be reliable as it will likely result in plating defects. Some other design considerations include:

• Drilling Technology: Laser drilling is the preferred method for microvia creation due to its precision and repeatability.
• Aspect Ratio: The aspect ratio (via depth to diameter) should ideally be 1:1 to ensure structural stability and manufacturability.
• Copper Plating: Ensuring proper plating in microvias is essential to maintain electrical connectivity and reliability.
• Layer Stackup: Proper planning of layer arrangements, especially for stacked or staggered microvias, is vital to avoid mechanical stress.

When placing in multiple layers, each layer that receives a via will need to have an additional lamination step. Holes that will not be used can be non-conductively filled or plated shut if desired. Since microvias are so small, they may not need to be filled. Copper is used for the plating, as a specialized bath and chemistry created for the process is produced. Using this process, the fabricator gains more flexibility in the design process.

What is Copper Filled or Unfilled Via?

Microvias can be either filled or unfilled with copper. For buried microvias, it's important to fill the via hole with copper, especially if there will be any stacking. If any voids are left over in the interior of the via plating, then the via will experience high stress concentration along the via wall, which could lead to premature fracture during reflow or during operation. Blind microvias can be left unfilled, and this was typical in the early days of microvia fabrication and implementation. If blind microvias will be used in-pad, then they should be filled with a standard process.

The plating process for microvias typically involves the use of pure copper or a copper-epoxy resin. It begins with conformal plating, followed by pulsed plating to ensure the microvia is filled with solid copper, reducing the risk of voids. Plating without additives often leads to void formation within the microvia. Even when fully filled with copper, the distribution may be uneven, particularly along the walls and surface, if additives are not used. Conformal plating can further exacerbate uneven copper deposition, contributing to potential voids.

Conclusion:

Microvias offer PCB designers flexibility with the fabrication process as customers obtain reliability when connecting multiple layers. When selecting materials for the printed circuit boards, there are some materials that are more suitable for the microvia drilling process. Microvias are a cornerstone of HDI PCB design, enabling the miniaturization and performance required in cutting-edge electronic devices.

Microvias are an advanced method of PCB fabrication, and if your board doesn’t have the need for them you will obviously want to stay with standard vias to reduce cost. But if your design is dense and you need the extra room, take a look whether or not using microvias could help. As always, it is a good idea to check with your contract manufacturer first on their capabilities before you design a PCB with microvias in it.