How Micro Vias Are Used
A microvia is a small hole that is drilled with a laser through a thin dielectric layer on a PCB, followed by metallization to create a short electrical connection between two adjacent copper layers. This technology is an essential part of high density interconnect (HDI) PCBs, and it has a significant impact on the quality of a board’s electrical performance and thermal management capabilities. The design of a microvia requires careful attention to a number of specific technical specifications, as the structure’s dimensions are critical to its reliability.
During the PCB manufacturing process, the microvia’s body is filled with pure copper or an epoxy+copper resin to eliminate voids. Plating is typically done using conformal and pulsed processes that fill the entire body of the via to avoid stress concentration at the bottom and top edges. The via’s shape and volume void fraction also contribute to the reliability of the structure. Voids that are too large can concentrate stress near the copper edge, causing the via to crack in the neck region. Smaller voids and lower aspect ratios tend to be more reliable, but these numbers vary widely by manufacturing process and materials.
To ensure the microvia will be able to conduct current without stressing the neck, the micro vias manufacturer needs to make sure the copper wrap plating is thick enough. The copper must also be deposited and plated without gaps or other defects. This can be difficult to accomplish if the manufacturer uses a plating method that does not provide adequate conformal coverage.
How Micro Vias Are Used in Multi-Layer PCBs
Another factor in the reliability of a microvia is how it is positioned on the circuit board. Stacked vias tend to have a higher failure rate than staggered ones, so it’s usually best to use them in staggered arrangements. In addition, the placement of a microvia within a pad can influence its ability to accommodate solder balls. In-pad microvias require a special type of solder mask to prevent “dog-bone” fanout, which can lead to poor connections between the pad and the ball.
As with blind microvias, the PCB designer must limit the aspect ratio of buried and in-pad microvias to improve the structure’s reliability. The easiest way to do this is by building a proposed stackup and sending it to the fabricator for review, where they can tell you the maximum via size that will work with their fabrication process.
To determine the best aspect ratio for a particular microvia, designers can use a tool like Altium Designer’s PCB stackup editor to determine the desired layer pairs and materials. Then, by calculating the target pad and capture pad diameters, along with the required via fabrication allowance, they can calculate the microvia’s land size. This will help to determine an appropriate size for the feature and avoid any issues with manufacturability. To learn more about how to get the most out of your HDI PCBs, download the free trial version of Altium Designer. This advanced PCB design software includes all the tools you need to maximize the performance of your next project.
