What are Staggered Vias?

A Staggered Via is a type of via structure that consists of multiple (two or more) vias in an arrangement such that they are never in direct contact with each other as their drill axes are separate which offsets their position on adjacent layers. As staggered vias have a different position in each layer they form a zigzag pattern when the PCB is viewed from the top or bottom of it. They are most often used in multi-layer printed circuit boards (PCBs), and high-density interconnect (HDI) designs.

How are Staggered Vias Made?

The staggered 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: This process 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. Staggering the vias involves fewer design steps. Because the drilled hole does not need to be directly above the one below it, laser-drilled staggered vias do not need copper filling. This makes the design less complicated.
• 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. The process involves building up the PCB layers by laminating and drilling them in multiple cycles. This method uses staggered vias to connect different layers of the PCB i.e., the vias on the first layer might be connected to those on the third, but they aren't aligned vertically and are positioned at different horizontal coordinates instead.

Advantages of Staggered Vias

There are several reasons why designers choose to use staggered vias in PCB designs. Here are some of the key advantages: 

1. Increased routing density: Routing density is the number of vias that can be routed on a PCB without them causing interference to each other. Higher routing density means more vias can be packed into a given area, which is crucial for complex circuits with many connections. Staggering enables the placement of vias with an offset alignment allowing more flexibility and also prevents them from occupying the same vertical space on all layers. Spreading out the vias while placing them also allows the creation of additional routing channels in the spaces between them, further increasing the overall routing density.
2. Reduced layer count: There is a possibility that the use of staggered vias will significantly lower the need for layers in some PCB designs. By capitalizing on the space between vias more efficiently, designers may do the trace routing using fewer layers of the board, which can save cost and make it easier to manufacture. 
3. Signal integrity: If the length of a via is too long (or when multiple stacked vias act as a long via), a signal entering it can bounce back, causing reflections that can distort the original signal. Staggering the vias helps keep their length minimum, thus reducing reflections and maintaining better signal quality, especially at high frequencies. Staggering also helps keep the signals of different vias separate, reducing the chance of crosstalk which is when signals in nearby vias interfere with each other.
4. Thermal management: In high-power dissipation designs, staggered vias are utilized for thermal management as they are efficient and provide better heat dissipation paths. The position of the offset vias is decided carefully to help evenly distribute the thermal energy to make the process more efficient and at the same time redirect heat away from critical components.
5. High-density interconnect (HDI) designs: Staggered vias are used to increase routing density and provide wiring clearance for HDI PCB designs. Though designers lose accessibility through staggered vias, HDI designs enable designers to perform required minimal clearances.

Challenges of Staggered Vias

Staggered vias in PCB designs are tougher to design and manufacture. While designing a staggered via structure, the spacing between the laser-drilled holes is the primary concern. The possibility of the staggered via design depends on the vertical distance between the centers of two adjacent vias. A viable staggered design requires the vertical separation to be more than the diameter of the via. This needs to be done precisely to avoid any issues, making the process more challenging.

Although less complex, staggering makes them use up more space on the board. Hence, if the PCB design has space constraints, staggered vias may not be the right choice. Staggering also introduces more discontinuity in the signal path. This makes it difficult to maintain a uniform via impedance requirement in a high-speed design.

HDI designs already have a high aspect ratio of vias. A staggered via structure may cause additional complexity in these designs resulting in manufacturing errors. There can potentially be more drilling and plating errors also due to the offset vial scoring which can result in higher production costs.

In some instances, staggered vias may even boost the complexity of a heat control system by introducing more thermal interfaces and potential hot point locations mainly because of the positions of staggered vias.