PCB Manufacturing

Key Characteristics of HDI Printed Circuit Boards: A Comprehensive Overview

HDI PCB

Accompanied by the development of electronic industries, especially the requirements for portable devices, electronic products need to be designed and produced more and more compactly, and with more and more functions. As a matter of fact, PCBs need to be produced as small as possible. So a special kind of circuit board with high layer counts, high density wires and holes comes into being, that is HDI printed circuit boards.

What is HDI PCB?

The abbreviation of HDI is “High Density Interconnect, so HDI Printed Circuit Boards means High Density Interconnect Printed Circuit Board. This is considered special technology boards in the PCB manufacturing industry. Compared to traditional or standard PCB boards, HDI Printed Circuit Boards do have higher interconnections “within layer” and “layer to layer” using thin tracks and via’s (Blind, Buried or Microvias) and makes PCB into the smaller form factor. And they also use surface mount components to create ways to miniaturize the size of the PCB.

The Characteristics of HDI PCBs

Let’s get into deeper HDI and how it works. Generally speaking, HDI PCBs have the following characteristics.

HDI technology allows –

  • To place more components in a smaller area
  • Reduce layer count
  • Compact PCB size

In HDI PCB boards below operations can be seen obviously – 

  • Dense track in a smaller area
  • Thin Track width (measuring 75µm minimum)
  • Minimum spacing
  • Impedance traces
  • Blind Vias
  • Buried Vias
  • Laser/Micro Vias

The Different Types of Vias in HDI PCBs

What is VIA in PCB? VIA stands for Vertical Interconnect Access. Via is a mechanical plated drill (Drill size ranging from 0.15mm to 0.6mm). Via drills make the connection between “layer to layer”.

Difference between via and plated through hole: In Through hole drill (Connects from Top to bottom layer), components can be placed (Drill size ranging from 0.6mm to 6mm) and soldering can be done in plated through hole drill. But via’s just an interconnect layer for various operational purposes.  That’s how we differentiate via holes with plated through drills.

The operational purpose of via in PCB:

  1. More number of vias placed inside solid copper feature to make electrical conduction between layer to layer and reduce mechanical stress between vias connected layers
  2. Thermal vias are vias placed under QFN IC for thermal relief (Heat dissipates through thermal via)
  3. In BGA location and dense track location, It is required to get signals and power through lengthy tracks that become design complications. That helps to reduce track length by  carrying signals and power from one layer to other layers
  4. VIP: Via in Pad technique, which is drilled in any smd pad (BGA or any IC package pad). Filled with copper or resin and electroplated. This technique offers an immediate connection from the SMD pad to the next power/ground or signal layer

HDI Printed Circuit Boards

Different Types of Vias in HDI PCBs

Via drills may be classified based on their process, based on mechanical process it may classified as –

  • Through vias (0.15 mm to 0.6 mm)
  • Blind vias (0.15 mm to 0.2 mm)
  • Buried vias (0.15 mm to 0.2mm)

Based on the laser process we have  –

  • Laser or Microvias (0.1 mm)

1. Through Vias:

As said before, Components can’t be placed in via drill. And it connects from the top to the bottom layer. Vias can be filled and plugged. All mechanical drills (including blind and buried if said to be mechanical then filling can be possible) filling can be done with copper filled or conductive epoxy resin filled and non-conductive epoxy resin filled via. Plugging can be done with a solder mask. These are also special operations.

2. Blind Vias:

Blind vias also mechanical drill, can see vias outside of PCB and cannot see through PCB because it is limited to a certain layer (drill connects between the middle layer to any of the outer layers called blind vias).

3. Buried Vias:

Buried vias also mechanical drill, cannot see vias outside of PCB because it connects between any of the inner layers (drill connects between the middle layer to any of the middle layers called buried vias).

4. Micro Vias:

Micro vias are laser drills that can be drilled using a laser and pass through up to the second layer. The laser can go up to 3 mil depth of PCB. Cores or prepreg greater than that thickness are difficult to make laser drill processes.

The Unique Techniques in HDI PCBs

  1. Sequential Build
  2. Stacked vias
  3. Staggered vias
  4. ELIC – Every Layer InterConnection
  • Sequential Build:

The design requires two or more laminations, which is said to be the Sequential build. Even traditional PCBs with multilayers also require more than 2 lamination cycles. But HDI differs from the multilayer lamination technique due to its density of tracks, blind, buried and micro via consideration.

For multilayers starting from 4 layers to a number of layers,  the sequential build technique is used to manufacture PCB. Manufacturing starts from the inner core (where the inner core is processed by imaging, exposing, developing, etching and stripping).

After that with the use of prepreg, it is again added on top and bottom, stacked together and pressed together, and after pressing the result will be 4 layers. A similar operation can be done with 2 laminations (1-press cycle) and it goes on according to layer consideration.

Sequential Build of HDI PCB

The core is considered as 1st lamination and two sets of prepreg  are placed above and below the core,  and copper foil added over above and below like the above image is said to be 1 press cycle and 2 laminations process

  • Stacked vias:

Widely we can see the terms for HDI Printed Circuit Boards as shown below,

  • 1-N-1
  • 2-N-2
  • 3-N-3

Let’s see briefly about 1-N-1 in PCB HDI board

1 x set laser vias (1 set referred to drilled from top side and bottom side) connected to buried drill parallel connection said to be 1-N-1 stacked vias. (Here N refers to a number of layers). This can be achieved by filling buried via with copper or conductive epoxy filling.

Stacked Via of HDI PCB

Laser drills from the top layer & Laser drills from the bottom layer are connected directly to the buried drill like the above snap

  • Staggered vias:

The same stackup build as stacked vias, but laser vias should not directly connect with the buried drill. Then it is said to be staggered vias.

Staggered Via of HDI PCB

  • Every Layer Inter Connection:

Every layer (Layers may be any number) can be drilled through PCB (top layer to bottom layer) using micro via and filled with copper. This technique of every layer interconnection enables more interconnection for less thickness PCB stackup.

ELIC of HDI PCB

The Pros and Cons of HDI PCB

PROS:

  1. HDI Printed Circuit Boards technology offers compact PCB size
  2. Immediate connection from layer to layer through laser or blind or buried interconnected vias helps to reduce track length and improves signal integrity
  3. Lower power consumption (reduces signal losses through interconnect vias)
  4. Usage of material in production becomes less, when boards become tinier or compact

CONS:

  1. Requiring Higher precision than traditional PCB boards
  2. Requiring  Smaller  board, more complex manufacturing and more attention due to involving many technologies

Applications of HDI PCBs

  1. Automotive components: navigation, GPS, consoles
  2. Consumer electronics, smartphones, laptops, computers, digital cameras, wearable electronics
  3. Industrial equipment: control unit, signal module
  4. Telecommunications: 5G/6G network
  5. High frequency and high speed application equipment
  6. Medical devices
  7. Aerospace and avionics
  8. Military applications
Conclusion

HDI Printed Circuit Boards we are widely using now are evolving more with “ULTRA HDI”, which offers reducing traces to work from 75µm to 50µm. The requirement of “ULTRA HDI” technology in 5G Telecommunication will be widely used in the future.

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