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The abbreviation HDI PCB might not mean a lot to you. However, you will understand better if we tell you that we are talking about high-density interconnect printed circuit boards.

HDI technology is becoming more popular due to its particular specifics. The main idea of using an HDI board is to optimize the size of the PCB while ensuring maximum performance and reliability.

What Are the Benefits of Using HDI Technology?

Here is a brief overview of why you should consider using this approach:

• You can keep the board small and light while adding more components to it. The main reason for this is that high-density boards allow you to place pieces on both sides.

• It has a relatively low power consumption, making it suitable for applications that use batteries, such as smartphones and tablets.

• It has a durable and robust design, and it slows down thermal degradation, which also contributes to durability.

An HDI Stackup type can significantly affect the specifics, performance, and reliability of your printed circuit board. That is why we will discuss different types and how to choose the right one for your PCB.

HDI PCB Stackup Types

If we discuss PCB Stackups, we should mention that the manufacturers have three approaches that they use during the assembly process.

These include:

• Regular lamination, which involves using plated through-holes or vias

• Using Micro vias for lamination buildup

• Using bling plated through and buried vias for sequential lamination

Institute of Printed Circuits Standards

IPC (Institute of Printed Circuits) has adopted a standard that recommends manufacturers to use one of six different types of Stackups for high-density interconnect boards.

These types have categories from I to VI (marked with Roman numbers). However, the classes marked with IV or higher might not be an excellent choice for HDI PCB. One of the reasons is the production cost, which makes it unsuitable for mass production. On top of that, there are specific design and assembly challenges for which the manufacturers still haven’t found the right solution.

Which Type Is the Right Choice for Your PCB?

Due to the price and design challenges, we will immediately eliminate an HDI PCB Stackup types IV, V, and VI. That leaves us with three choices for your application, and here is what you should know about each of the types.

Should I Choose Type I?

If you take a look at the structure of the board, you will notice a laminated core with one or more Micro vias layers. These layers can be on both sides, but they can also be kept on a single side.

The Type I standard doesn’t allow you to use buried vias, but you are welcome to apply bling and PTH vias.

As for the total layer number, you should consider that going with FR-4 dielectrics that are thin may lead to delamination under extreme temperatures, which may be needed to apply the lead-free method of soldering.

Additionally, the ratio between length and hole diameter may be crucial for reliability, and you should always aim to keep that ration under 10 for the plated through-hole.

HDI PCB Type II Details

In this type, you can use buried and blind vias and Micro vias on a laminated core. You also need to place one or more Micro vias layers on the sides. You do not have to put them on both sides, but at least one is mandatory. Manufacturers sometimes go with stacking or staggering them depending on the buried vias, or they can sway Micro vias from others.

The experts consider that this method is far more suitable for high-density boards, but you should keep in mind the same limitations as with the type I approach.

You can only place Micro vias on outside layers, which implies certain limitations for some applications. It would help if you also kept in mind that the experts recommend placing multiple trace routing buildup layers to improve effectiveness.

Why Type III Can Be a Good Choice?

The crucial difference between type II and III is that this approach requires you to place two or more Micro vias layers on the sides (you do not have to put them on both sides). Apart from that, you also have buried and blind vias and Micro vias on a laminated core.

The configuration of this Stackup can be an excellent choice for high-density PCBs that have multiple layers and use several big BGAs that have fine-pitch features. As for FR-4 thin dielectrics and PTH holes, the same limitations apply.

The significant advantage of type III is that you can use the outside layers for power and ground plane. You can achieve that by placing Micro vias in the inner layers, ensuring that there are sufficient layers the manufacturers can use for signal routing.

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