High Density Interconnect PCB Fabrication

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HDI (High-Density Interconnector) PCBs are all the rage nowadays because they enable the miniaturization of telecommunication devices and electronic gadgets without affecting the quality and performance. This is enabled using microvias and capture pads, fine pitch, and higher connection pad density.

With HDI PCB, you get higher wiring density with a smaller board than a conventional circuit board. This leads to smaller, lighter, and high-speed electronic gadgets. It’s essential to mention that HDI PCBs offer improved data transmission speed due to a shorter signal path.

Advantages of Using HDI PCBs

Here’s a list of how HDI PCBs are advantageous for multiple industries:

1. Practicality: The higher wiring density of HDI boards allows you to design the board in multiple ways. It also lets you place more components on either side of the PCB.

2. Improved Signal Quality: HDI makes use of via-in-pad and blind via technology. The ability to squeeze in more components in a small space by removing via stubs and decreasing signal reflection enhances signal quality. The resultant shorter path reduces signal loss and drastically improves signal integrity.

3. Reliability: Microvias use a smaller aspect ratio and grant outstanding performance to HDIs due to better materials and parts. The smaller weight and size ensure low chances of harm from mechanical shocks. Laser drilling leaves smaller holes and optimizes the thermal properties of the board, making it a reliable choice for handling extreme environmental conditions.

4. Cost-effective: HDI PCB boards allow designers to pack the functionality of a regular 8-layer through-hole board (standard PCB) into a 6-layer HDI board. You can do this without needing to compromise on quality, which is what happens when you use multiple boards. This reduces overall costs.

5. Easy Installation: In an HDI PCB, the components are easy to solder and can be reworked later. This allows for easy installation and maintenance.

Uses of HDI PCBs

HDI PCBs are ideal for medical devices, including implants and lab and imaging equipment. Not only that, sensors in cars make use of HDI PCBs too. As mentioned in the beginning, all smartphones are created using HDI PCBs. Smartwatches, VR headsets, modern-day warfare, and military communication devices owe their portability, thinness, and accuracy to HDI PCBs.

Features of an HDI PCB

Following aspects should be considered before and during the manufacturing process:

1. Number of Layers

In an HDI PCB, the component count can be increased while keeping the board small in size. In the case of a board with high pin density components, the count can go as high as 20 or even more.

2. Via Type

HDI PCBs use some of these following vias: blind, buried, through-hole, stacked, staggered, microvia, etc. Your choice of vias is one of the determinants of the final cost of your board. The most popularly used via is the microvias. It reduces the number of layers and size of the circuit board. Depending on how they are drilled and electrocuted, they can be used as blind or buried vias.

3. Trace Thinness

HDI PCBs offer a higher trace density. This is made possible by keeping the trace of the board to its thinnest by connecting each layer through the vias.
Steps of Manufacturing

Here’s how the HDI PCB fabrication process goes:

Material Selection
The first consideration is material selection. The actual material depends on the kind of project, process, and manufacturing place. Not just that, the fabricator and designer will have to join heads to answer other questions, too: If the chosen material will be able to withstand the resultant heat? How will it hold through the fabrication steps? Will it be suitable for the lamination process? What's the dielectric constant and loss of the material?

To create a high-speed data transmission device, your selected dielectric material will have to fare against high temperature, thermal expansion, glass transition, etc. Usually, the materials that check all these boxes are expensive.

Your ideal material is from the Fr-4 family. They tend to have average loss and moderate to high speed, which is ideal for analog and digital devices.

HDI PCB Structure

HDI PCBs can be structured in quite a few ways. The basic 3 formulas are described here:
1. 1+n+1
There is one high-density layer of interconnection in this PCB structure type.
2. i+n+i
This type has 2 or more HDI layers on the PCB, with the microvias either stacked or staggered on one or both sides of the circuit board.
3. Any Layer
In Any Layer HDI PCB, all the HDI layers are easily connected. The devices with higher complexity and stronger signal integrity, such as smartphones, digital watches, and military gadgets, require this type of PCB structure.

Type of HDI PCB Stackup

Based on how many layers you have, the order in which you stack them with blind holes is called stackup type. Here are a few of the ways you can stackup the layers:

Sequential Lamination

It is a type of sequential lamination in which the micro vias are placed on one or both sides of the circuit board. The manufacturers first laminate the core and then use a mechanical drilling method. Then micro vias are drilled using laser technology due to higher accuracy and precision. Sequential lamination manufacturing costs are higher than the standard and lamination buildup. Due to its susceptibility to heat damage, it is not recommended for high-density devices with multiple and large BGAs and fine pitch packages.

Standard Lamination

Standard lamination uses through-hole vias and has very limited applications than sequential lamination stackup layers. The manufacturing process is cheap for devices with 20 to 28 layers, but devices with multiple BGAs and fine-pitch components are expensive because the signal routing costs time, precision, and resources. Standard lamination uses limited trace widths, and hence, it is unreliable in its functioning and practicality.

Lamination Buildup

Lamination buildup is the most preferred stackup type for HDI PCBs, and it uses microvias technology. The ability of microvias to have significantly smaller traces and dimensions makes them an ideal choice for optimum signal routing even in fewer layers.

The micro vias can be stacked or staggered n different patterns to increase space for routing. The additional space makes them suitable for large devices with multiple components and fine pitch packages.

Prototype Testing

When designing HDI PCBs for a bulk order, it is customary to create a prototype first. It is a simplified version of your printed circuit for testing design, analyzing the results, and calculating resultant efficiency. The prototype uses multiple visas: stacked, staggered, microvia (blind and buried), so you can create any combination and select the optimal one.

Other Considerations

The materials, vias, stacking type all contribute to the final cost of your project. The project complexity can increase depending on the number of layers and components. The additional layers are helpful and make the HDI PCB more reliable and insusceptible to heat damage, but they also increase the manufacturing cost by up to 50 to 80%.

Final Words

HDI PCBs have proved revolutionary in terms of making our devices smaller yet efficient. With technology such as this, it is best to trust and hire professional HDI PCB manufacturers like Hemeixin.

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