Brief Introduction of PCB Microvias

One of the biggest challenges facing PCB designers is not understanding the cost drivers in the PCB manufacturing process. This article is the latest in a series that will discuss these cost drivers (from the PCB manufacturer’s perspective) and the design decisions that will impact product reliability.

8 Types of PCB vias

#1. Through-Hole Vias

The most common and easiest PCB vias are through-hole vias. Though-hole vias are drilled from the PCB's upper layer to the bottom layer. When you take a PCB and look at it facing the light, the holes where light passes through are the through-hole vias.

Though-holes vias are mainly PTH (plating through-hole) vias, and some are the NPTH (non-plating through-hole) vias. PTH vias are used for PTH assembly or electrical connection between different PCB layers, while NPTH is used for mechanical connection with screws or connectors to fix the PCB.

#2. Blind Vias

Blind vias are drilled and electroplated from the PCB's upper or bottom layer to an internal layer. When you look at the blind via on the PCB facing the light, you cannot see the other side through the hole.

Blind vias can be laser-drilled or mechanical drilled. For blind vias, the drilling depth has to be accurate. Blind vias can be directly drilled on the PCB, but this is difficult. And it makes electroplating also difficult. More commonly, advanced PCB manufacturers drill holes on the required circuit board layers and then pile up the layers to create blind vias and electroplate them.

#3. Buried Vias

Buried vias are drilled and electroplated between the PCB's inner layers, and you can't see them from the externality. Buried vias are used to connect the circuit between two or more inner layers.

Unlike the blind via, if a buried via connects 3+ inner layers, it can't be directly drilled on the PCB. The PCB manufacturer can only drill holes on the required PCB layers and then pile them up to electroplate the hole wall.

# 4: Stacked vias

Stacked vias can be blind vias or buried vias to connect circuits between different PCB layers across 3+ circuit layers. For example, via 3-6 in the below image is a stacked via as well as a buried via.

#5. Staggered Vias

When vias of different PCB layers are connected but not overlapping, they form a staggered via. For example, in the below, via 1-2 and via 2-3 form a staggered via.

#6. Skip Vias

A skip via is a PCB via that penetrates multiple circuit layers but makes no electrical connection with a specific layer or layers. It can be an overlapping via, a blind via, or a buried via. For example, the via 3-6 as below is also a skip via that goes across four circuit layers and connects two circuit layers.

#7: Microvias

Microvias are blind/buried via structures with a maximum diameter of 0.15mm, the maximum aspect ratio of 1:1, maximum depth of 0.25mm. It penetrates across only two PCB circuit layers.

#8: Vias in Pad

The least common type of PCB via is the via-in-pad. Only big PCB pads such as the pads of MOSFET and BGA can be drilled with holes, and the hols on big PCB pads are vias-in-pad, which are used for thermal dissipation of the components.

DFM

Design for manufacturing (DFM) is defined as the practice of designing printed circuit boards that meet not only the capabilities of the customer’s assembly manufacturing process, but also the capabilities of the board fabrication process at the lowest possible cost. While not a substitute for early design engagement with the PCB fabricator, these articles will provide guidelines that will help to “design for success.”

Microvias
One of the most important technological advancements that made HDI viable was development of the microvia: a very small hole (typically 0.004~0.006”[0.1~0.15mm] or smaller) that only connects certain layers either as “blind” via holes. This represents a totally new way of making electrical connections between layers on a PCB. Traditional PCB technology has utilized “through-holes,” which by definition, are drilled through the entire PCB connecting the two outside layers with all of the internal layers. The ability to strategically connect only certain pads on certain layers greatly reduces the real estate needed for a PCB design and allows a much greater density in a smaller footprint. Figure 1 shows through-holes and buried and blind vias.

Figure 1: Microvias vs. through-hole vias. (Image credit: Victory PCB)

Microvia Formation

Microvias can be formed through a number of methods, primarily mechanical drilling, laser drilling and sequential lamination.

Mechanical drilling: Uses traditional drilling equipment to mechanically form holes, but typically limited to 0.008[0.2mm]” diameter and dependent on the depth needed

Laser drilling: Special drilling equipment that utilizes a laser to form the hole and can go down to 0.001” in diameter

Sequential lamination: A process where the microvias are drilled all the way through a sub-panel of the layers that need to be connected by the via, which could require multiple lamination, plating, filling and planarization operations (Figure 2).

Figure 2: Sequential lamination. (Source: Victory PCB)

Stacked vs. Staggered Microvias

Stacked: Microvias that are electrically connected and literally stacked vertically on top of each other through various layers of the PCB

Staggered: Microvias that are electrically connected and offset to one another through various layers of the PCB (Figure 3)

Figure 3: Staggered and stacked microvias.

Via-in-Pad Microvias

The via-in-pad production process allows you to place vias in the surface of the flat lands on your PCB by plating the via, filling it with one of the various fill types, capping it and, finally, plating over it. Via-in-pad is typically a 10- to 12-step process that requires specialized equipment and skilled technicians. Via-in-pad is often an optimum choice for HDI PCBs because it can simplify thermal management, reduce space requirements, and provide one of the shortest ways to bypass capacitors for high-frequency designs (Figure 4).

Figure 4: Via-in-pad. 

Understanding the cost drivers in PCB fabrication and early engagement between the designer and the fabricator are crucial elements that lead to cost-effective design success. Following your fabricator’s DFM guidelines is the first place to start.Send your questions or inquiry to us: sales@victorypcb.com