Micro Vias Affect the Overall Size of a PCB
As electronic devices continue to shrink and manufacturers strive for increased functionality, the need for higher density has driven a trend toward high-density interconnect (HDI) circuit boards. HDI PCBs allow more components to fit on a single layer, making them essential for devices as diverse as smartphones and medical equipment. Microvias are an important component of this innovation, enabling higher-speed signals to be routed with greater efficiency while also minimizing the overall size of the circuit board.
Microvias are blind structures that feature a 1:1 aspect ratio between hole diameter and depth. They are defined in IPC-T-50M as “a blind structure terminating on a target pad or plane and having a maximum depth of 0.25 mm.”
A micro vias can be either a through-hole or buried via, and it may be surrounded by dielectric material or not. Unlike traditional through-hole vias, which are drilled mechanically and plated with copper, microvias are formed using laser technology. This process produces smaller holes with less variation than through-hole vias and a more uniform plating thickness, minimizing fabrication issues.
How Micro Vias Affect the Overall Size of a PCB
As a result of their small size, microvias have to be carefully designed and manufactured to ensure reliability. For example, the fact that they are more vulnerable to stress fracture — specifically in the barrel region of a buried microvia — has led to concerns about long-term reliability. To address these issues, manufacturers use quality control measures to ensure that microvias meet stringent standards during every step of the manufacturing process.
This begins with inspecting raw materials to identify any impurities that might interfere with proper metallization. Then, the microvias are drilled and subjected to a desmearing process to clean out any residual materials and ensure that they are properly sized for conductive routing paths. Finally, the microvias are plated with pure copper, usually by pulse plating. This process is prone to void formation, so manufacturers often add additives that reduce the risk of voiding.
Another important concern is the quality of copper used in microvias. Poor-quality copper can lead to signal loss, impedance mismatches, and other electrical issues. To minimize the risk of such problems, designers should specify their preferred copper quality to their contract manufacturer.
The bottom line is that while microvias have the potential to greatly improve the performance of a PCB, they are not for everyone. The good news is that a PCB designer can determine if they’re right for their design by leveraging the manufacturability check capabilities of their contract manufacturer. This will allow them to build a proposed stackup in their CAD software and then check with their fabricator to make sure that it can be produced using the desired fabrication process. This way, they can avoid the costly consequences of a bad design decision.