Multilayer PCB Design: HDI PCB Stackup Demystified

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If you want to pack more functionality in your printed circuit board, you need to use HDI PCB design considerations. HDI PCBs are miniaturized and advanced PCBs that enable manufacturers to add more components on a smaller board area. This is made possible by using vias, strategic trace routing, and choosing an ideal stackup type for your project.

Stackup types determine how layers are arranged in a multilayer board. It helps improve signal integrity and reduces power consumption. HDI PCBs are used in almost all electronic gadgets and consumer devices. So, let’s find out the correct HDI PCB stackup type for your project.

What is HDI PCB Stackup?

High-Density Interconnect PCB stackup refers to how the multiple layers are arranged using blind, buried, and micro vias. The purpose is to compact the circuit board further and add the same or more functional components. The chosen stackup type provides copper weight and board thickness to fabricate a PCB. Thus, you need to choose a stackup type before designing an HDI PCB.

Primarily, a HDI PCB stackup consists of 3 layers:

• Metal foil is made with copper.
• Prepreg, which is an interwoven glass cloth
• The core is composed of single or multiple layers of prepreg bonded between the upper and bottom metal foil layer to form the core.

There are 2 other layers used in HDI PCB stackup; the ground and power plane layers. The power plane layer is a copper layer connected to the power supply. It provides a steady voltage supply to the PCB. Similarly, the ground plane layer is connected to the ground point of the PCB.

Benefits of Multiple Layers in HDI PCB

There are many benefits of using multilayer HDI PCBs over conventional PCBs.

• High-Density Interconnect PCBs are ideal for complex devices with multiple advanced functionalities.
• The multiple layers in HDI PCBs make the board thicker, thus making them more durable than traditional PCBs.
• The use of vias and strategy trace routing reduces the path the signal has to follow. This improves signal integrity and reduces power consumption.
• Traditional PCBs require multiple connection points. HDI PCBs use a single connection point and reduce the complexity of electronic device design.

There is extreme precision required when designing an HDI PCB stackup. Thus, the manufacturing costs can add up, but they will be higher in quality than conventional PCBs.

Types of HDI PCB Stackups

There are a total of 6 HDI PCB stackup types available. Out of which, only stackup types I, II, and III are used. The other three stackup types are not suitable for HDI PCBs due to costly manufacturing and assembly challenges that haven’t been resolved.

PCB professionals use the following three manufacturing processes to create the stackup types:

• Standard lamination using through-holes and vias.
• Sequential lamination using through-hole and buried vias.
• Lamination buildup using microvias.

Out of these, lamination buildup with micro vias is most commonly used. You can achieve high routing density with fewer layers and improve signal integrity. The fewer layers open up space for routing, which is ideal for devices with multiple large fine-pitch BGAs.

Stackup Type I

The HDI stackup type I consists of a laminated core with one or two micros via layers on one or both sides. It uses through-hole and blind vias. This stackup type is not ideal for fine pitch BGA devices because of fewer layers.

The number of layers is limited due to thin FR-4 dielectrics. They can delaminate under high heat. Similarly, the aspect ratio needs to be less than 10, which is not the case.

Stackup Type II

The HDI stackup type II also has a laminated core and uses blind, buried, and micro vias. It has a single micro via layer on one or both sides. Manufacturers have the advantage of using micro vias as either blind or buried vias and stack or stagger them.

It is slightly better than stackup type I but has more or less the same limitations. Since the micro vias can only be placed on the outer layer, you can’t use ground or power plane layers, which reduces its effectiveness.

Stackup Type III

The HDI stackup type III uses blind, buried, and micro vias around a laminated core. At least two or more micro via layers on one or both sides. It is the ideal choice for electronic devices with multiple fine-pitch BGAs.

It has the same limitations as the above two stackup types. One significant advantage is using the ground and power plane, thus leaving many layers for routing and increasing signal integrity.

How to Design HDI PCB Stackup

Determining the stackup type for your project depends on your requirements and layer count. You need to keep in mind multiple design considerations when designing the HDI PCB stackup type.

1. The first step in designing an HDI PCB stackup is to size traces to the appropriate width and determine layer thickness for controlled impedance. You can do this by using the BGA pitch to set the upper limit on trace width and using that value to determine layer thickness.

2. Once the layer thickness has been determined, you can use that value to determine the space required between each layer. To estimate the board size, multiply BGA components per unit area by the board, and determine the number of nets per area.

3. Use the nets per area value and divide it by your total net count to get the total number of layers. Add the ground and power plane layers, and you have the initial HDI stackup design.

As the number of layers increases, the manufacturing process becomes expensive and complex. Thus, it’s best to consult PCB fabricators when designing an HDI PCB layout. Hemeixin is one of the first companies to provide HDI PCB capabilities to its customers. They offer a wide range of services such as rigid-flex PCB, RF boards, laser-drilled micro vias, and more.


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