PCB Blog - How to Manage High Density HDI PCB Vias

High-Density Interconnect PCB Board Design

Vias for HDI PCB design, use design rules for effective via management, just as hardware stores need to manage and display various types, metrics, materials, length, width, and pitch, etc. Nails, screw mountings, PCB board design Design objects such as vias also need to be managed in the field, especially in high-density designs. Traditional PCB board designs may use only a few different vias, but today's high-density interconnect (HDI) designs require many different types and sizes of vias. And each via needs to be managed so that it is used correctly, ensuring improved board performance and error-free manufacturability. This article will detail the need to manage high-density vias in PCB design and how to achieve it.

Factors Driving High-Density Interconnect  PCB Design

As the demand for small electronic devices continues to grow, the printed circuit boards that drive these devices also have to shrink so that they can fit into the device. At the same time, electronic equipment has to add more components and circuits to the circuit board in order to meet the performance improvement requirements. The ever-decreasing size of PCB components and the increasing pin count complicate the problem by having to design with smaller pins and tighter pitches. For PCB board designers, this is equivalent to a smaller and smaller bag with more and more stuff inside. Traditional circuit board design methods quickly reach their limits.

PCB vias under the microscope

In order to meet the demand for adding more circuits on smaller board sizes, a new PCB board design method has emerged - high-density interconnection, or HDI for short. The HDI design uses more advanced circuit board manufacturing technology, with smaller line widths and thinner materials, with blind and buried vias or microvias drilled with a laser. Thanks to these high-density features, more circuits can be placed on smaller boards and provide a viable connectivity solution for multi-pin integrated circuits.

Using these high-density vias also brings several other benefits:

Routing Channels: Since blind and buried vias and microvias do not penetrate the layer stack, this creates additional routing channels in the design. By strategically placing these various vias, designers can route devices with hundreds of pins. Devices with so many pins would often block all inner-layer routing channels if only standard vias were used.

Signal Integrity: Many signals on small electronic devices also have specific signal integrity requirements, and through-holes cannot meet such design requirements. These vias can form antennas, introduce EMI problems, or affect the signal return path for critical nets. Using blind and buried vias or microvias eliminates potential signal integrity issues caused by the use of vias.

To better understand these vias mentioned above, let's take a look at the different types of vias and their applications that can be used in high-density designs.

The via a list in the PCB board design tool shows the different via types and configurations

Types and structures of high-density interconnect vias

Vias are holes on a circuit board that connect two or more stacked layers. Typically, vias transfer the signals carried by the traces from one layer of the board to the corresponding traces on another layer. To conduct signals between trace layers, vias are metalized during the manufacturing process. Via sizes and pads vary depending on the specific application. Smaller vias are used for signal routing, while larger vias are used for power and ground routing, or to help dissipate heat from overheated devices.

Different types of vias on circuit boards

1) Through Holes: Through holes are the standard vias that have been used in double-sided printed circuit boards since they were first introduced. Holes are drilled mechanically through the entire board and electroplated. However, there is a limit to the hole diameter that a mechanical drill bit can drill, depending on the aspect ratio of the drill diameter to the plate thickness. Generally speaking, the diameter of the through hole is not less than 0.15 mm.

2) Blind vias: Like through-holes, these vias are also drilled mechanically, but with more manufacturing steps, only part of the board is drilled from the surface. Blind vias also suffer from drill size limitations; however, depending on which side of the board it is on, we can route above or below the blind via.

3) Buried vias: Like blind vias, buried vias are also drilled mechanically, but start and end at the inner layer of the circuit board rather than the surface. Such vias also require additional fabrication steps due to the need to be buried in the board layer stack.

4) Microvia: This via is ablated by laser and the diameter is smaller than the 0.15mm limit of a mechanical drill. Since the vias only span two adjacent layers of the board, their aspect ratio makes the holes available for plating much smaller. Microvias can also be placed on the surface or inside the board. Microvias are usually filled and plated and are essentially hidden so they can be placed in the surface mount component solder balls of components such as ball grid arrays (BGAs). Due to the small pore size, the pads required for microvias are also much smaller than normal vias, about 0.300 mm.

Typical Microvias for High-Density Designs

According to design requirements, the above different types of vias can be configured to work together. For example, microvias can be stacked with other microvias or with buried vias. These vias can also be staggered. As mentioned earlier, microvias can be placed within the pads of the leads of surface mount components. The problem of routing congestion is further alleviated by eliminating traditional traces from surface mount pads to fan-out vias. The different types of vias above can be used in HDI designs. Next, let's look at how PCB board designers can effectively manage the use of vias.

High-Density Via Management in PCB Design CAD Tools

While there are only a few types of vias available for PCB board design, there are many ways to create different sizes and shapes. Vias used for power and ground connections are usually larger than those used for conventional routing, with the exception of vias placed on the bottom of large BGA components with several hundred pins. For these, microvias in the surface mount pads may be required in addition to the BGA pads. While larger components will benefit from the use of microvias, microvias are not suitable for conventional surface mount components with fewer pins; standard through holes are recommended for this routing. These vias are smaller than power and ground vias, and larger for heat dissipation. In addition, blind and buried vias of various sizes can be used. Obviously, in an HDI PCB design, it's easy to get overwhelmed with many different vias needed to meet all the design needs. While designers can keep track of a few of these vias, vias are becoming more and more difficult to manage as the size of the vias increases. Not only do designers have to manage all these vias, but depending on the area of the board, different vias can be used for the same net. For example, a clock signal can be routed out of a BGA pin through a microvia in the SMT pad, but then goes back to the buried via on the next segment of that trace. But for this net, don't use traditional vias, as the extra barrel walls may create unnecessary antennas on the HDI PCB.