Microvias
A microvia is an extremely small hole in a printed circuit board (PCB). Because most modern PCBs are multi-layer boards, these holes must connect various layers of the board together. Typically, these connections are made through holes that are drilled through the PCB and plated with copper. Depending on the size of these holes, they may be considered through-hole vias or blind/buried vias. As technology continues to shrink while becoming more powerful, it is important to maximize space in the electronics we use. Microvias allow PCB designers to incorporate more components into smaller packages while maintaining the necessary functionality and ensuring quality and reliability.
The role of microvias in enabling higher-density interconnect (HDI) PCBs is crucial in the development of advanced electronic products and devices. Whether used in mobile devices, aerospace electronics, medical equipment or any other application, HDI PCBs rely on microvias to ensure the smooth transmission of electrical signals and power between layers. In addition to facilitating increased component density, these small holes offer several other significant benefits.
How small are microvias typically? As per IPC standards, a microvia is defined as a hole with a diameter of less than 150 um (6 mils). These holes can be mechanically drilled and then plated using standard copper plating processes. However, new advancements in laser drilling are allowing for even more precise holes with diameters of up to 15 micrometers, making them as small as a human hair.
How Small Are Microvias Typically?
When it comes to placing microvias on a PCB, there are two different ways they can be positioned: stacked or staggered. Stacked microvias are built up into a stack to span multiple layers, while staggered microvias are used when a layer transition is too close to the component or trace to permit the formation of a standard through-hole via. Staggered microvias have the advantage of reducing the risk of reliability problems associated with the presence of stubs that resonate and cause radiation.
To be effective, microvias must be properly placed on a PCB in order to perform correctly. As such, it is important to use a modern PCB CAD tool to optimize the placement of these tiny features. In addition to ensuring the best placement of each microvia, these tools can also detect any potential problems that could affect the functionality or reliability of the completed circuit.
After they are drilled, microvias must undergo a desmearing process to remove any debris left behind during the drilling process. This process is vital to ensuring that the underlying surface of the microvia is free of contamination and offers a good conductor for electrical signals. Once this step is complete, the microvias can be metallized with copper to create the electrical connections between layers of the PCB.
While microvias can be fabricated in multiple ways, they typically consist of a target pad and capture pad that are defined by adding the diameters of both annular rings and the required fabrication allowance to the as-formed diameter of the microvia feature. These dimensions are then mapped out on the PCB in a CAD program, allowing the PCB fabricator to place these vias where they are needed.