DFM 101: PCB Via Structures

Estimated reading time: 3 minutes

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.

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 0.006” or smaller) that only connects certain layers either as “blind” or “buried” 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: NCAB Group)

Types of Microvias

• Blind via: Used to connect one surface layer with at least one internal layer
• Buried via: Used to create connections of internal layers with no contact to the surface layers
• Via-in-pad: A type of blind via in which the via hole is drilled in a surface mount pad, eliminating the need to run a trace and via pad from each SMT pad
• Filled vias: Completely filling the microvia with either a non-conductive or conductive paste. Conductive fill is typically used for heat dissipation and non-conductive fill is used to closely match the thermal expansion of the substrate.

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 0.006” 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: Siemens EDA)

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.

This article originally appeared in the October 2021 issue of Design007 Magazine.

Anaya Vardya is president and CEO of American Standard Circuits; co-author of The Printed Circuit Designer’s Guide to…Fundamentals of RF/Microwave PCBs and Flex and Rigid-Flex Fundamentals; and author of Thermal Management: A Fabricator's Perspective. Visit I-007eBooks.com to download these and other?free, educational titles. He also co-authored “Fundamentals of Printed Circuit Board Technologies,” and is an I-Connect007 columnist. To read past columns, or contact Vardya, click here.