High Density Interconnect Boards and How They Can Benefit Your Design

Altium Designer
|  Created: March 23, 2018  |  Updated: September 25, 2020

HDI board

In the world of PCB design, high-density interconnect boards are among the fastest growing designs that are surfacing. Offering lighter weight, faster transmission times, more real estate to place components, and a reduction in signal loss there are few downsides to implementing these designs into your device.

In any industry, the push for better, faster, stronger is usually a consumer-driven force. It is no different here. The push for devices to become smaller, more efficient, faster, lighter (iPhones), stems from this demand. The response to this is HDI designs—put as many components into as little space as possible (high-density) and connect them all (interconnect) in a fast and efficient way. Let us talk about this high density interconnect!

What is a High Density Interconnect Board?

I often think of PCBs as a cake of sorts. For our purposes, let’s pretend it is a delicious multilayer mint chocolate-chip ice cream cake. The fluffy cake layers act as our copper surfaces, and the ice cream layers as the non-conductive structural material (usually a fiberglass of sorts).

These cakes can be made in all sorts of combinations whether it be straightforward ice cream adjacent cake, or a ten-layered combo of sugary goodness, the world is our oyster. A similar approach is employed in the production of high density interconnect or a HDI PCB. Utilizing multiple layers of copper substrates and fiberglass routing of the otherwise component-littered single-layered boards becomes internal and clears up space on the surface to place more components.

Vias and Their Use in a HDI PCB

With the sheer number of inputs and outputs that goes with each component, you could soon end up with a flurry of routing requirements that will soon fill the surface level of any circuit board out there.

In the early days of HDI PCB, we came up with an ingenious way to ‘hide’ the routing between each component freeing up space on the surface level. This method was to place the routing portion of our circuit on the underside of the board. The pins of each component would then shoot through the fiberglass and be usable on the rear of the circuit board. Genius! But what about an even denser packed circuit board?

We soon realized that we could take this school of thought and essentially sandwich a board together to then utilize the same method. This, in effect, gave us a multilayered PCB that had routing schemes both on the surface level, the backside, and also the middle of the board.

Ice cream cake
Thinking of HDIs as ice cream cakes, the layers are then connected by vias.

This was the birth of extremely densely packed HDI boards. The presiding thought was if we can make a three-layer board, why not four? Why not six? Is twelve unrealistic? Vias are simply the means in which to connect internal-layered routing schemes to one another. This allows you to bury layers and layers of routing within a PCB without anybody even knowing.

Vias in Pad Placement Techniques

As HDI boards became more and more utilized with via routing schemes they had to begin adopting parallel technology, namely, pad processes. Pad placement technology offered an additional benefit not only to the production of these ICs as they made it more seamless to route the pins, but the lack of surrounding pins allowed for more placements directly around the IC making it even more of a densely packed circuit. Holy smokes.

Marrying vias to pad ICs allowed for the internal routing of these circuits to take place, again, without anyone ever suspecting they were there. These HDI boards sure can keep a secret, I bet I could even trust them with my social security number.

The Production Side of Things

Obviously, with all this technology floating around, and within, our board questions will be raised of the process behind the actual production of these boards. Tolerances of these boards will have to be spot on the higher the density of your components. Annular rings, lamination cycles, drilling, routing, and soldering will all have to be able to be manufactured to perfection to achieve desired results.

Going back to our mint chocolate ice cream cake (which is about half gone now… sorry about that), the manufacturing is somewhat similar in theory. The more layers you add and the more internal routing schemes you introduce, the more sophisticated the equipment has to be. I’m imagining a baker’s clean room that requires the use of vacuum sealed lab coats, laser imagining machines, and high tech lamination stations now that’s a cake I’d like to see.

The cost and efforts associated with creating these boards can easily outweigh the benefits, but also, can easily be justified. This simply depends on your application. Creating a radio kit for kids will probably not justify these expenses. Creating a military grade RF signal jammer to be placed inside a fighter jet? Perhaps you should take a look.

Whatever the case may be, keep in mind that the added equipment and processes needed to produce these HDI boards will be a large factor at play.

HDI Manufacturing
The more technology you introduce, the greater the manufacturing challenges, the greater the cost.

HDI boards are just one solution being utilized to meet the rising demands of consumer expectations. Utilizing vias to solve clever routing schemes and pad processes, we’re able to achieve unheard of densities in today’s devices.

With the rise in tech, of course, comes the rise in cost, but with the amount of time and effort already established in the arena, HDI PCB manufacturing costs can be expected to begin to level out and even dip into the accessibility of the hobbyist designs.

Using software features such as those found in Altium Designer® can greatly ease your HDI design issues as they are essentially born for today’s printed circuit board design demands. If you would like to explore how Altium Designer can assist in your HDI design, talk to an Altium Designer expert today.

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PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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