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HDI Routing Challenges and Tips

Small, densely packed components on a circuit board


I grew up in a suburb where the traffic network is relatively simple. There are no flyovers, and vehicles travel at a leisurely pace. When I moved to the city, I was shocked in every aspect, particularly in driving myself to work. In an era where a GPS navigation app was nonexistent, driving in the labyrinth of flyovers where vehicles are traveling at top speed is a nightmare.

Thankfully, I soon adapt to the dense and complicated roads in the city and the blistering speed. Years later, I found myself in a similar situation in PCB design. As speed picks up in semiconductor ICs and the demand for smaller electronics is deafening, the word HDI routing surfaced. 

What is HDI Routing

HDI or High Density Interconnects routing refers to the application of the latest design strategies and manufacturing technologies to achieve denser design without compromising the functionality of the circuit. In other words, HDI involves the use of multiple routing layers, smaller traces, vias, pads, and thinner substrate to fit a complex and often high-speed circuit in a footprint that wasn’t possible before. 

As manufacturing technologies evolved, HDI routing is implemented across designs like motherboards, graphics controllers, smartphones, and other space-constrained devices. When done right, HDI routing not only significantly reduce design space but also reduces EMI issues on the PCB. Cost-reduction, an important motivation for companies, is achievable with HDI routing. 

HDI Routing And Microvias

It’s important to understand that HDI routing goes beyond typical multilayer routing strategies. You may have worked on an 8 or 16 layer PCB, but there are concepts introduced in HDI routing that make these a totally new discipline.

In typical PCB design, the physical printed board is treated as a single entity that is divided into multiple layers. However, HDI routing requires the designer to think from the perspective of bringing multiple ultrathin layers of ultrathin layers of PCB together in a single functional PCB.

It’s fair to say that the key catalyst to the implementation of HDI routing is the evolution of via technology. Vias are no longer a copper-plated hole that is drilled across all layers of a PCB. The conventional vias mechanism reduces routing areas in layers that are not used by the signal trace. 


A set of 5 microvias on a green circuit board

Conventional via has no place in HDI routing.


In HDI routing, microvias take center stage as the enabler of putting multiple layers of dense routing together. It helps to understand that microvias may consist of blind or buried vias but with a different method of construction. Conventional vias are produced with drill bits after the layers are put together. However, microvias are drilled with lasers on the individual layers before they are stacked. Laser-drilled microvias allowed interconnects between layers with the smallest hole and pad size. This favors fan-outs for BGA components where the pins are arranged in a grid format. 

HDI Routing Strategies

With the use of microvias, PCB designers are able to implement complex routing in any layers of the PCB. Such an approach is called any-layer HDI or Every-Layer Interconnect. Thanks to the space-saving microvias, both outer layers can be tightly populated with components as most of the routing is done on the inner layers.


Circuit board ground planes with dense mimcrovias

Low impedance ground planes are vital for HDI routing.


However, denser components and traces in a multilayer design can also increase the risk of EMI emission and susceptibility. When you’re working on an HDI design, it’s important to ensure the PCB stackup are appropriately constructed. You’ll need to have adequate ground planes for a low impedance return path. 

You’ll want to place the inner routing layers between ground or power planes in order to reduce instances of crosscoupling or crosstalk. Keep high-speed signals as short as possible, including the return path. Proper planning and usage of microvias will limit the signal path to a small area and decreases the risk of EMI. 

Of course, it helps to be safe and simulate the HDI PCB with the right software. When working through the layout of an HDI board, you’ll want to trust the tools you’re using. OrCAD PCB Designer is capable of making your layout challenges easy with smart net systems, DRCs, and easy production documentation. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts