High-Density Interconnect PCB

The amount of technology in a smartphone is truly astounding when you consider all its capabilities. Communication, computing, networking, navigating, photography, and various sensors are just a few examples, and amazingly, they all fit in the palm of your hand. Electronic miniaturization for devices like cell phones isn’t possible without the benefits provided by high-density interconnect (HDI) printed circuit boards. And the need for smaller circuit boards with increased circuitry will only grow as new technologies continue to be introduced.

To successfully build high-density printed circuit boards, you need someone who understands both the manufacturing requirements and the design. At VSE, we regularly partner with electronic innovators who need HDI PCBs to support the amount of circuitry required in their advanced technology. And as our customers have found out, not only does HDI allow for more circuitry to fit into a smaller space, but it offers better signal performance. This page will summarize what you need to know about high-density interconnect PCB design to prepare for HDI design requirements.

Exploring High-Density Interconnect PCBs

A high-density circuit board provides many benefits to product designers, including reducing the size of the circuitry. Smaller circuitry allows for more and, therefore, better product functionality. Here are some other benefits that product designers will realize by using HDI technology:

Key HDI PCB Design Benefits:

  • Lower PCBA costs
  • Increased circuitry density
  • Better PCBA reliability
  • Increased signal integrity and performance
  • Decreased susceptibility to electromagnetic interference (EMI)
  • Better thermal management
  • Overall improved efficiency in the design
One of the chief advantages of HDI technology in printed circuit board design is adding more functionality to the board through compression of the circuitry. This compression is accomplished through several methods:

  • Advanced packaging of newer components allows smaller part sizes than what has been previously available for PCB design
  • Smaller packages have smaller pin pitches which compress the space needed for circuit routing
  • Packages are placed closer together than on a standard PCB design
  • Multiple via types are used in the design including via-in-pad, and microvias
  • Use of smaller trace widths and spaces
  • More layers can be used to build a board for additional routing because of the thinner substrates

Another advantage of designing with HDI technology is the smaller size lends itself to improved signal integrity and reduced EMI. This process involves more care on the part of the designer to lay out the board carefully according to good signal integrity rules, but the benefits can be substantial. For instance, a standard thru-hole via a circuit board will have measurable inductance, which is reduced through microvias in HDI design. Additionally, the trace routing in an HDI circuit board is shorter due to its smaller size and the closer placement of components to each other.

Achieving these improved signal integrity results requires some new PCB layout requirements, which we will look at next.

HDI PCB Layout Requirements

High-density interconnect PCB designs compress their circuitry using smaller trace widths and spacings during layout and many other techniques. Different rules must be followed to design an HDI circuit board successfully. Here is a summary of some of those HDI layout requirements.

Board layer stackups

High-density interconnect circuit boards require a carefully conceived board layer stackup configuration before the layout begins. This process will ensure that the impedance-controlled routing is correctly calculated for the design.

Component placement

With the use of via-in-pad and microvias to conduct the routing into the inner layers, both the outer layers of the board can be tightly populated with components. Additionally, the spacing constraints from one component to another are less on an HDI board than on a standard PCB.

Trace and via routing

Smaller trace widths and spacings are used with HDI designs allowing for greater routing densities on the board. With the use of vias that do not go fully through the board, additional routing channels on inner layers also open up. This configuration allows parts with high pin-counts and low pin-pitches to be routed on an HDI board. The vias used are a combination of regular thru-hole, blind and buried, and microvias instead of only thru-hole vias used in a standard circuit board.

Design rules

Circuit board layout designers must use the full capabilities of their PCB CAD systems to help them design HDI boards. Typically, these CAD systems can be set up to allow or disallow specific via types on certain nets and change spacing rules and trace width values. Design rules are essential in HDI design to avoid EMI and other potential signal integrity problems.

Once the circuit board has been designed, it then gets turned over to the manufacturer for PCB fabrication.

Choosing a Manufacturing Partner that Understands High-Density PCBs

Fabrication of an HDI circuit board can vary from the production methods used for standard PCBs. The fine lines require great care during imaging to prevent errors from being created, and direct laser imaging is the preferred method. Thinner board materials are also used for the substrates, and the microvias require different fabrication processes for laser drilling and sequential build-up.

At VSE, we understand all of the processes involved with high-density interconnect design and manufacturing. We are committed to helping our customers to make the best choices regarding the processes, materials, and design rules necessary for their designs. Our goal is to ensure that you will get the quality you expect in your next HDI circuit board project.

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