Different Types of Microvias
Vias are essential to ensuring that signals and power flow seamlessly across the layers of a printed circuit board (PCB). They also allow for a denser arrangement of components within a limited space, enabling smaller and more powerful electronic devices. As the world moves towards miniaturization, PCB designers are continually seeking to optimize design space and improve signal integrity, efficiency & reliability. To do this, they must be knowledgeable about the various via types and their implications.
Microvias are a type of blind or buried via that carries electrical connections between the layers of a multi-layer PCB. They are most commonly used in high-density interconnection (HDI) technologies. Unlike traditional through-hole vias, which are typically drilled entirely through the board, microvias use small-diameter holes that connect the different layers of the PCB without passing through the copper surface.
They are smaller than through-hole vias and provide the flexibility needed for modern high-density PCB designs. They are also less expensive to produce and more reliable than through-hole vias, as they do not expose the conductive copper. However, like through-hole vias, microvias are susceptible to failure at the plated interfaces where they connect to the pads on the bottom layer of the PCB. This is particularly true when the PCB is subjected to thermal cycling, as the copper of the microvia will expand and exert high stress on the plated via walls.
Are There Different Types of Microvias?
To avoid this potential failure, PCB designers must ensure that the aspect ratio of a microvia is not too high. Aspect ratios of 1 or higher are more likely to fail due to the stresses placed on the plated interfaces. However, this can be difficult to determine during the early stages of PCB design, as the exact aspect ratio is dependent on several factors, such as the thickness of copper foil and the drilling and plating processes.
Another factor that impacts the reliability of a microvia is its metallization process. In order to minimize voiding, manufacturers need to use a precise drilling process and a carefully chosen copper foil that balances conductivity with heat dissipation. The drill process should also be controlled to minimize vibration and oxidation, which can lead to poor hole quality. Finally, the copper plating process should use additives to prevent the formation of voids in the body of the microvia.
The shape of a microvia can also impact its reliability. While a frustum-shaped microvia is more resilient than a straight barrel, it is still susceptible to failure at the neck of the frustum. This can occur as a result of thermal cycles, strong vibration, or mechanical shock. A frustum-shaped microvia has a more gradual curve in its walls, allowing the copper to absorb and distribute stress more evenly.
In addition, the size of a microvia and its frustum-shaped wall can impact its ability to transfer heat. A large frustum-shaped microvia can cause localized hot spots and circuitous return paths that can negatively affect EMI performance. To avoid these issues, PCB designers use staggered microvias to spread the load across multiple layers.