Behind the Scenes with HDI PCB and its Applications

High Density Interconnect (HDI) PCB

The digital word is getting bigger hour by hour whereas the hardware associated with it is getting drastically smaller. Contrary to the popular belief, smaller the better is the way to go!

Your computers are no longer ugly and heavy. They are sleek and sexy. You can work on your laptop and keep your business running while you board a flight. You can also watch your favorite action movie on your smartphone, some of which are way powerful than your laptops. Or you can just plug in your wireless headphones and get into the groove. Mankind is fond of electronics and it has become a part of our lives. In this era, all the electronic devices are designed for portability. Electronic devices are being designed for human convenience. All this was possible because of the miniaturization of the components and PCBs inside the electronic devices.

The PCB industry plays a prominent role in the development of all the electronic gadgets. Let it be a pacemaker in someone’s chest or the motherboard in a programmer’s workstation, PCB made it possible for creating crucial electronic devices.

How did everything get so small? How was this possible?

As the size of the components shrunk smaller, printed circuit board manufacturers accommodated to the changing trend. This led to the birth of HDI design.

What is High Density Interconnect (HDI)?

High-Density Interconnect (HDI) is simply a PCB with more number of interconnections, occupying minimal space. This results in the miniaturization of the circuit board. The components are placed closer and the board space is significantly reduced but the functionality isn’t compromised.

To be more precise, a PCB with an average of 120 to 160 pins per square inch is considered as an HDI PCB. The HDI design incorporates dense component placement and versatile routing. The HDI popularized the microvias technology. A denser circuitry is crafted with the implementation of microvias, buried vias, and blind vias. The drill to copper is reduced in an HDI design.

Sierra Circuits HDI PCB

Why you should opt for HDI

  • Phenomenal versatility- HDI boards are ideal where weight, space, reliability, and performance are primary concerns
  • Compact design- Combination of blind vias, buried vias, and microvias bring down the board space requirement
  • Better Signal transmission – The HDI incorporates via-in-pad and blind via technology. This helps in placing the components closer to each other which cuts down the signal path length
  • High reliability – The implementation of stacked vias make these boards a super shield against extreme environment conditions
  • Cost effective- The functionality of a standard 8 layer through-hole board (standard PCB) can be reduced to a 4 layer HDI board without compromising the quality

HDI PCB Designer Tip

Once you determine that you require HDI, you need to look at the stack-up. Don’t use more than three sequential layers unless there is absolutely no other solution. It is recommended that you use a good fanout strategy for complex ICs, like BGAs and QFNs – this will help you reduce the number of sequential laminations if you plan your fanout strategy properly.

The benefits of HDI PCB

  • Higher circuitry density than conventional PCB
  • Higher connection pad density
  • Finer traces and gaps
  • Smaller vias and capture pads
  • Blind/buried vias and microvias smaller than 4 to 6mil
  • Smaller aspect ratios (1:1)
  • Accurate laser drilling for microvias
  • Remarkably improves signal integrity due to shorter signal paths
  • Lead inductance and trace inductance are minimized
  • Minimized noise and interference leads to optimized EMC

HDI PCB advantages

  • HDI reduces size and weight of the products
  • The microvias connect embedded capacitors and resistors within the layers of an HDI PCB. This reduces the distance and the mass of the conductive material. This results in improved electrical performance in compact and complex devices.
  • Enhances electronic performance of the device – Since the traces are shorter, inductances will be reduced. Hence, electrical performance will be enhanced.
  • Reduces cost by reducing PCB layer count and size

The difference between HDI and standard PCBs

HDI inherently provides a better signal integrity performance than non-HDI because all the stray capacitances and inductances get reduced when using small vias. Since there are no stubs, the impedance of microvias is close to the trace impedance. The stray capacitance of a normal via is much higher, which causes greater discontinuity in impedance than a microvia does. S0me of the significant differences between the HDI and conventional PCBs are listed below:

The benefits of HDI
HDI Vs Standard PCB

 

 HDI PCB

Standard (Through-hole) PCB

 Higher component density per square inch Lesser compared to HDI
 Smaller, lighter boards with more functionality Larger and heavier boards
 Implementation of Blind, buried, and microvias Through-holes with blind and buried
 Laser direct drilling Mechanical drilling
 Lower number of layers Higher number of layers
 Lower aspect ratio Higher aspect ratio
 Compatible with high pin count and low pitch packages

May face compatibility issues with low pitch packages

 

The HDI build-ups (stack-ups)

  • 1+N+1 – PCBs contain 1 build-up of high-density interconnection layers
  • i+N+i (i≥2) – PCBs contain 2 or more build-up of high density interconnection layers. Microvias on different layers can be staggered or stacked
  • i+bN+i (i≥2)- includes buried vias
  • Any Layer HDI – All the layers of a PCB are high density interconnection layers. This allows the conductors on any layer of the PCB to be interconnected freely with copper filled stacked microvia structures

Applications of HDI PCB

Healthcare (Medical):

The HDI PCB is making an impact on the medical industry. The medical devices are usually HDI since they can fit in small devices like implants and lab and imaging equipment. Medical equipment plays a crucial role in diagnosing diseases and also do provide life support. Ex: Pacemaker, Diagnostic and monitoring facilities.

With the miniaturized camera sizes, the inner parts of the patients can be observed and the right diagnosis can be established. The cameras are getting smaller but the picture quality isn’t compromised. The HDI PCB technology made it possible to accommodate these advancements.

The camera’s that need to be inserted from the open ends, which some men find it to be demoralizing, have reduced in the size with greater picture clarity. Getting a colonoscopy won’t hurt much anymore, I guess. Thank God the PCBs are reducing in their size or else colonoscopy would have kept the patients away!

Automotive HDI PCB:

Electric Car
Electric Car. Image credit: evstores

Car manufacturers are extremely attracted to small sized PCBs since they can save more space in the car. With the launch of futuristic cars from brands like Tesla, the integration of the electronic devices to provide a better driving experience has been the prime focus of the carmakers.

Smartphones and Tablets:

All smartphones are HDI PCBs with ELIC (Every Layer Interconnection) construction. The HDI PCBs are responsible for creating thinner are smaller portable electronic devices.

Wearable Technology:

Wearable Technology
Apple Watch. Image Credit: Apple.com

With the launch of Apple watch and other wearable devices like VR headsets, HDI is becoming the major stakeholder in the consumer market. The wearable technology is gaining popularity among the young crowd due to its exceptional functionality.

Military and Aerospace:

HDI is incorporated in military communications devices and other strategic equipment like missile and defence systems. The HDI PCBs are well suited for extreme environment and hazardous conditions making it ideal for aerospace and military applications.

PCBs have revolutionized the electronics industry and are used in every electronic gadget you know. The demand for Printed Circuit Board (PCB) looks promising as it is extensively used in various industries as mentioned earlier. The HDI PCBs have gained momentum in recent years due to their versatility. The 10 layer HDI are major drivers in the HDI PCB market. With HDI design, designers can create smaller products and make electronics fit anywhere they are needed.

Bonus: HDI Design Guide

 

HDI Design Guide

Related Articles