What are Stacked Vias?

A Stacked Via is a type of via structure that consists of two or more vias placed directly on top of each other, traversing multiple layers of the PCB. They are used in multi-layer printed circuit boards (PCBs), and especially in high-density interconnect (HDI) designs. Each via in the stack connects to at least one inner layer of the board. The primary purpose of stacked vias is to provide a continuous electrical connection through several layers of the PCB. This is particularly useful in complex designs where space is limited, and a high number of connections is required. Multiple vias in a stack drilled and plated across two or more adjacent PCB layers create optimized vertical routing solutions that can be achieved within a single compact structure to make connections and electrically link non-adjacent layers of a PCB.

How are Stacked Vias Made?

The stacked configuration can include a combination of different types of vias, such as micro vias, buried vias, and through-hole vias, depending on the design requirements. They can be made primarily by two methods:

• Drilling and Plating: Creating stacked vias is a complex process that typically involves sequential lamination and drilling, where very small holes are first drilled using lasers. These holes are then plated with copper to form the electrical connections. It requires precise alignment of the vias during the PCB fabrication process to ensure a reliable connection through all the layers involved. This technique enables the precise formation of vias or micro vias directly on the PCB substrate, thereby enhancing the ability to implement stacked vias.
• Layer-by-Layer Construction: In this type of construction the PCB is constructed layer by layer, with each layer having its own set of vias. Once a layer is completed, the next layer is added, and vias are drilled and plated, stacking directly on top of the previous ones. The process involves building up the PCB layers by laminating and drilling them in multiple cycles. It allows for complex vias structures but increases manufacturing time and cost.

While creating stacked vias, the quality of copper plating is very important. To maintain reliable electrical connections and to withstand thermal cycling, the plating must be thick and uniform. Materials such as FR-4, polyimide, and other advanced laminates are used as substrate materials. This ensures that the stacked vias are mechanically stable and thermally stable.

Advantages of Stacked Vias

There is a need to make electronic devices smaller without compromising their performance as technology advances. As stacked vias include multiple vias that overlap with one another vertically, they take up less space on a PCB than typical through-hole vias. This helps achieve more compact designs, especially for high-density boards where PCB space is limited. Stacked vias offer a way of increasing density and dealing with size constraints so that signals can be routed effectively within the available space.

Stacked vias also provide exceptional flexibility for layer-to-layer interconnection in PCBs. This allows complex routing configurations to be realized while ensuring efficient use of the board area. It is also possible to have several vias around a single center hence simplifying the path of signal transmission and minimizing any loss along the way. Such structures help lower parasitic capacitance associated with vias thereby improving signal integrity. 

Challenges with Stacked Vias

When it comes to producing PCBs with stacked vias some challenges may be encountered such as aspect ratio control, dealing with voids as well as determining the right stacking arrangement among others. The design of stacked vias requires careful consideration to maintain the integrity and reliability of the PCB. Factors such as thermal management and stress distribution also need to be addressed, as the stacked structure can impact the overall robustness of the board.

Furthermore, the use of stacked vias adds complexity and cost to the PCB manufacturing process. It demands advanced fabrication techniques and precise quality control.

Industry Standards and Compliance Related to Stacked Vias

IPC-6012: This is the qualification and performance specification for rigid printed boards laid down in the IPC standard. Stacked via designs are required to meet stringent reliability requirements based on this standard.

IPC-2221: This is a general standard on PCB design which includes guidance on via structures and their placement.