DFM 101: PCB Via Structures
One of the biggest challenges PCB designers face is not understanding the cost drivers in the PCB manufacturing process. This article is the last in a series that will discuss these cost drivers (from the PCB manufacturer’s perspective) and design decisions that will impact product reliability.
Design for Manufacture (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 highest cost board manufacturing process. low possible. While not a substitute for an early design engagement with the PCB manufacturer, these articles will provide guidance that will help “design to succeed”.
One of the most important technological advancements that made the HDI viable was the development of microvias: a very small hole (usually 0.006 in. Or less) that only connects certain layers as “blind” or “vias” holes. “Buried”. This represents a whole new way of making electrical connections between the layers of a PCB. Traditional PCB technology has used “through holes”, which by definition are drilled through the entire PCB connecting the two outer layers to all the inner layers. The ability to strategically connect only certain pads on certain layers greatly reduces the space required to design a PCB and allows for much greater density in a smaller footprint. Figure 1 shows through holes and buried and blind vias.
Figure 1: Microvias vs through vias.
Types of microvias
- Blind via: Used to connect a surface layer with at least one inner layer
- Buried Via: Used to create internal layer connections without contact with surface layers
- Via-in-pad: A type of blind via in which the via hole is drilled into a surface mount pad, eliminating the need to run a trace and a via pad from each SMT pad
- Filled Vias: Completely fill the microvias with a non-conductive or conductive paste. Conductive infill is generally used for heat dissipation and non-conductive infill is used to closely match the thermal expansion of the substrate.
Microvias can be formed by a number of methods, primarily mechanical drilling, laser drilling, and sequential layering.
- Mechanical drilling: Uses traditional drilling equipment to mechanically form holes, but generally limited to 0.006 “in diameter and depending on the depth needed
- Laser drilling: Special drilling equipment that uses a laser to form the hole and can go down to 0.001 “in diameter
- Sequential lamination: A process where microvias are drilled throughout a sub-panel of the layers that are to be connected by the via, which may require multiple rolling, plating, filling and planarizing operations (Figure 2).
Figure 2: Sequential lamination. (Source: Siemens EDA)
Stacked or staggered microvias
- Stacked : Microvias which are electrically connected and literally stacked vertically on top of each other through different layers of the PCB
- In a staggered arrangement : Microvias which are electrically connected and offset from each other through different layers of the PCB (Figure 3)
Figure 3: Staggered and stacked microvias.
The via-in-pad production process allows you to place vias into the surface of the flats of your PCB by plating the via, filling it with one of the different types of infill, styling it, and finally covering it. Via-in-pad is typically a 10 to 12 step process that requires specialized equipment and trained technicians. Via-in-pad is often an optimal choice for HDI PCBs because it can simplify thermal management, reduce space requirements, and provide one of the quickest ways to bypass capacitors for high frequency designs (Figure 4 ).
Figure 4: Via-in-pad.
Understanding the cost drivers in PCB manufacturing and early engagement between designer and manufacturer are crucial elements that lead to cost effective design success. Following your manufacturer’s DFM guidelines is the first place to start.
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 is also a co-author of “Fundamentals of Printed Circuit Board Technologies” and is an I-Connect007 columnist. To read previous columns or contact Vardya, click here.