In the world of PCB design, we don't have many items to keep track of. However, there are a lot of design objects (such as vias) that do need to be managed, especially in high-density designs. While earlier designs may have used only a few different vias, today's high-density interconnect (HDI) designs require many different types and sizes. Each of these vias needs to be managed [link to management constraints] in order to use it properly to ensure optimal board performance and error-free manufacturability. Let's take a closer look at the need to manage high-density vias in PCB design and how to do it.
Considerations for Driving High Density Printed Circuit Board Designs
As the demand for small electronic devices increases, the printed circuit boards that drive them must shrink along with them to fit into boxes. At the same time, electronics must respond to requests for enhanced functionality by adding more components and circuits to the board. Further complicating the issue is the fact that the size of PCB assemblies continues to decrease while the pin count continues to increase, forcing the use of smaller pins with tighter pitches. For PCB designers, bags are getting smaller and everything that goes into them is getting bigger, and soon the traditional approach to circuit board design is reaching its limits.
To accommodate the need to increase circuits on a smaller board size, a new approach to PCB design, known as high-density interconnect or HDI, has been introduced. These designs utilize newer manufacturing techniques to build circuit boards with smaller line widths and thinner materials, as well as blind and buried vias or laser-drilled microvias. Together, these high-density features enable more circuits to be fabricated on a smaller area of the board and provide a viable connectivity solution for high pin-count ICs.
The use of these high-density vias brings several other benefits:
Routing channels: Since blind and buried vias and microvias do not go all the way through the board stack, additional routing channels are opened up in the design. By strategically placing these various vias, designers can now route parts with hundreds of pins on them. A component with this many pins would typically block all inner layer routing channels if only standard vias were used.
Signal Integrity: Many signals on these devices also have specific signal integrity requirements that can be affected by the overall length of the through-hole through-hole barrel. These vias can act as antennas for radiated EMI, or affect the signal return path for critical nets. However, the use of blind and buried or microvias can eliminate signal integrity issues caused by vias that are too long.
To better understand the vias in question, we will next examine the different types and applications of vias that can be used for high-density designs.
The via list in the PCB design tool shows the different via types and configurations
High-density interconnect by type and structure
Vias are holes on a circuit board that are used to connect two or more layers in a stack. Typically, vias conduct a signal that travels on the trace from one layer of the board to the corresponding trace on another layer. To conduct signals between trace layers, the vias are metallized during the manufacturing process. Vias have different sized holes and metal pads depending on the application. Smaller vias are used for signal routing, while larger vias are used for power and ground routing, or components that help mitigate heat.
Here are the different types of vias you will use on your board:
Through Holes: Through holes are the standard through holes that have been used since the first introduction of double-sided printed circuit boards. Mechanically drilled and plated throughout the board. However, the drilling capacity of mechanical drills is indeed limited, depending on the aspect ratio of the diameter of the drill compared to the thickness of the plate. In general, any vias smaller than 0.15 mm or 0.006 inches cannot be drilled or plated reliably.
Blind Hole: Also like a through hole, the hole is also mechanically drilled, but only part of the board layer is drilled from the surface using an additional manufacturing step. These vias also have the same drill size limitations as vias, but they do allow additional routing channels above or below them, depending on the side of the board they're on.
Buried: Like a blind via, a buried via is drilled mechanically, but it starts and stops on the inner layer of the board, rather than coming out of the surface layer. Since the via is buried in the layer stack, additional manufacturing steps are required.
Microvia: Laser drill this via to create a hole smaller than the 0.15mm limit of a mechanical drill. Since the microvias will only span two adjacent layers of the board, their aspect ratio allows the use of smaller plated vias. It can also be placed on or inside the board. Microvias are typically filled and plated to essentially hide them so they can be placed in surface mount component pads for parts such as ball grid arrays (BGAs). Due to their smaller via size, microvias also require much smaller pads than regular vias, measuring about 0.300 mm or 0.012 inches.
These different via types can also be configured to work together in different patterns, depending on the needs of the design. For example, microvias can be stacked with other microvias, or they can be stacked with buried vias. The vias can also be staggered with each other. As mentioned earlier, microvias can be used within the pads of the leads of surface mount components. This further relieves trace routing congestion by eliminating the traditional traces that extend out from surface mount technology (SMT) pads to escape vias.
These are the different types of vias that can be used in HDI designs. Next, we'll examine how PCB designers can best manage their use.
Although only a few types of vias can be used in PCB design, there is no end to the ways in which different via sizes and shapes can be created. Power and ground connections typically use larger vias than regular routing, but large BGA parts have hundreds of pins on the underside. For those, microvias in surface mount pads may need to be used with other BGA pads. While some of these larger parts will benefit from the use of micro vias, regular surface mount parts with fewer pins will not, and standard vias can be used for their routing. The vias for these will be smaller than the power and ground vias, and the vias for heat dissipation will be even larger. Then, blind and buried vias of all different sizes can be used.
Needless to say, on an HDI design, dozens of different vias may be required to meet all design requirements, which can cause some confusion. Although a designer may keep track of several of them, vias become increasingly difficult to manage as the number of different sizes increases. Not only do designers have to keep track of all these vias, but depending on the area of the board used, different vias can be used on the same net. For example, a clock signal might be routed out of a BGA pin using a microvia in an SMT pad, but then returned to the burial through the lower end of the line. But you don't want to use a thru-hole via for this net, as the extra length of the thru-hole barrel may create an extra antenna on that line.
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