How Micro Vias Impact the Design

Technology continues to trend towards miniaturization, and printed circuit boards (PCBs) are no exception. Advanced PCB fabrication techniques have shaved off the space needed to house powerful electronics, allowing them to be packed into a small form factor. One of the key technologies enabling this miniaturization is the use of microvias. Microvias are a type of blind hole or via that is much smaller than a standard through-hole or jumper wire. They are used in place of traditional through-holes for interconnects between layers on a PCB.

As the technology inside electronic devices grows ever more complex, designers must look for ways to increase the number of interconnects on a PCB without increasing the overall size of the board. This has led to the increased adoption of microvias on modern electronic products. However, the use of microvias comes with its own set of challenges and considerations for the designer.

To avoid voiding or compromising the reliability of a PCB, micro vias must be designed with care. This is because the structure of a microvia relies on a combination of geometry, plating method, and stack-up configuration to function correctly. In particular, the shape of a microvia’s copper plating and its volume void fraction can have a direct impact on how the structure performs.

How Micro Vias Impact the Design of RF Antennas on PCBs

Voids in microvias can cause reliability concerns by concentrating stress near the neck of the barrel, and this is particularly true for high aspect ratio microvias. These vias have a steeper slope in the barrel region than do conventional through-hole vias, making them more susceptible to stress concentration at the neck. This can lead to the formation of dimples or voids in the neck, which may result in failure in the via under normal conditions.

Moreover, the diameter of a microvia is critical to its performance. A larger diameter provides lower impedance than a small diameter, thereby enhancing signal transmission. The height or aspect ratio of a microvia is also important to consider. As a general rule, an aspect ratio that exceeds 1:1 should be avoided.

Aspect ratio considerations also play a role in determining whether a stacked or staggered microvia is used on a given PCB. Stacked microvias consist of a stack of buried and blind microvias that are positioned adjacent to each other on the same layer of an HDI PCB. The manufacturer must fill these internally buried microvias with conductive paste and then plate them over to ensure strong contact with the next via in the stack as it is deposited and plated. Staggered microvias, on the other hand, are placed offset from each other on successive layers in an HDI interconnect.

Ultimately, the choice of stacked versus staggered microvias in a design depends on whether the PCB will be subjected to accelerated life testing, and the aspect ratio that needs to be enforced. In addition, the proper materials selection and adherence to IPC-T-50M guidelines are crucial in ensuring microvia reliability. Continuous resistance monitoring during reflow and a careful stack-up configuration can also help to identify any potential problems before they become serious.