High Density Interconnect

The electronics industry today is characterized by a strong miniaturization trend. This trend also places new demands on the PCBs that connect the components. To meet these requirements, engineers require PCBs with higher wiring density and thinner materials. High density interconnect (HDI) PCBs are able to provide these capabilities. They are a significant improvement over conventional PCBs and are found in most modern electronic devices including mobile /cellular phones, tablets, laptops, digital cameras and 4/5G network communication devices as well as military equipment such as drones and avionics.

The key to high density interconnect is a specialized multi-layer structure called microvias, or blind vias. These are tiny plated-through holes that are used to connect layers of the PCB. The holes can be drilled mechanically or with lasers. They can be located in any layer of the stackup and are commonly stacked to form staggered pairs that allow for more efficient routing of fine-grade components.

To make the best use of these tiny holes, designers must carefully plan how to couple them together and route traces to them. This requires careful consideration of design rules, hole wall tolerances and electrical constraints. Early assessment of these factors through modeling helps avoid costly gotchas during manufacturing and assembly.

How Thin Can High Density Interconnect Be Made?

Using HDI technology in your PCB designs will yield significant advantages in terms of manufacturability and cost. But you must work with a PCB manufacturer that is experienced in delivering successful HDI solutions. Identifying the right materials to strike the right balance between price and manufacturability for your specific construction is crucial. NCAB Group PCB experts will help you select the best material and process for your project.

The core of a HDI board has a layer count of 1+N+2 or 2+N+3. The outer layers have thin dielectrics and are typically good choices for ground planes. The inner layers are then fabricated with either a 2+N-2 or a 3+N-3 stack-up. The 2+N-2 option allows for the use of a single outer layer for the microvias while the 3+N-3 version requires one additional trip to the press and increases the overall stack-up thickness.

Regardless of the stack-up chosen, a good strategy is to place the shortest possible vias on the core. This will minimize the number of layers required to make the connection and reduce cost. To achieve this, a common technique is to fabricate the core with a 2+N-2 plus configuration and then add the microvias in the second lamination cycle. This is referred to as the 2-N-2 plus approach and offers the advantage of requiring one fewer trips to the press.

In addition, there are several other ways to optimize your designs for HDI fabrication. These include grouping devices minimizing overall area fitting tightly spaced parts and planning connectivity layer usage decoupling critical traces. It is also important to consider heat dissipation densities and management and to assess thermal, mechanical and assembly constraints through simulation. This ensures a reliable and successful product for your end-customer while maximizing your ROI.