PCB Manufacturing

HDI PCB Technology: From Basics to Advanced Concepts

1+8+1 HDI PCB

In today’s world, the need for compact and powerful performance devices is increasing rapidly. As a result, the need for advanced PCB technologies which can accommodate complex designs in a smaller area is becoming famous. High Density Interconnect (HDI) is the solution for such complex criteria and it offers miniature PCB boards with reliable and highly efficient devices. With the invention of HDI PCB technology, the electronics industry has revolutionized all devices by enabling the usage of smaller areas, lightweight designs and much more sophisticated electronic devices. These technologies are widely used in all fields right from mobile phones, consumer electronics, medical equipment, automobile electronics and even in the aerospace domain. In this article, we will deeply learn about the HDI PCB concepts from basic to advanced including the types, limitations, advantages, disadvantages, the design and manufacturing process requirements.

As the name itself says, HDI PCB boards have the capacity to provide higher wiring density compared to traditional PCB board technology. It mainly focuses on reducing the space and size of the board that is consumed in traditional PCB boards.

Basic Design Considerations for HDI PCB Technology

IPC-2226 provides the standards for designing HDI PCB boards. Designing the HDI PCB boards requires more attention while considering the factors that might affect the performance, durability and reliability of the board during design as well as manufacturing the boards. Let us understand what factors need to be considered during the designing process one by one in detail.

  • Stackup

Based on the functionality and routing complexity, the number of layers required can be determined. Optimizing the layer reduces the signal loss and EMI problems. Also with the use of microvia, routing density can be reduced as these drills are smaller compared to standard via drills.

  • Selecting Microvia Design

Considering microvias in the design helps reduce the clearance between the trace and the adjacent pads to prevent possible short circuits during manufacturing or assembling the components. Care should be taken while calculating the drill size based on the aspect ratio and capability of the fabrication house. It also reduces the routing complexity and the signal skews.

  • Component Placement

While placing the components, group all the related components together to reduce the routing length and signal crossover. Place all the capacitive and resistive components as close to the components to reduce signal path length and to ensure better signal integrity.

  • Thermal Issues

Heat dissipation is one of the reasons for the failure of the boards if not resolved during the design process. In order to overcome this problem thermal vias have to be introduced in the design where there is a component with a high heat dissipation factor. Along with the components getting heated it also transfers the heat to the neighboring traces and components which would result in the malfunction of the board.

  • Manufacturing Capabilities

Communicating with the manufacturer is very important when the circuit is complex and involves the HDI PCB process. If the board is densely populated with components and routed with minimal clearance, it will be challenging for manufacturers to fabricate the board and sometimes it might not be possible to manufacture at all. To avoid such issues and make the board feasible for manufacturing, it is always recommended to interact with the fabricator and know the minimum capabilities that they can achieve.

Different Types of Vias Used in HDI PCB

HDI PCB technology uses several via technologies, a few of which are listed below:

  • Microvias

They are typically very small vias which range from 0.1mm to 0.2mm and are usually laser drilled most of the time. Because of the laser drills, these drills can’t penetrate more than two layers at a time. Due to this, it is further classified into two categories based on the construction.

  1. Stacked Microvias
  2. Staggered Microvias

stacked microvias and staggered microvias

Stacked microvias are vias that fall directly above each other with the same axis. These vias need to be filled with copper and plated over.

Staggered microvias are vias that don’t fall directly on each other through the same axis. These vias needn’t be filled.

  • Blind Via

These are vias that connect the outer layer to one or more inner layers. But they are not drilled through the entire board thickness like thru-holes.

  • Buried Via

These are vias that connect only inner layers and are not to be seen from outer layers.

blind via and buried via

Advanced Design Considerations for HDI PCB Technology

With the development of advanced concepts in HDI PCB, PCB size has been reduced to miniature sizes with an increase in the performance and reliability of the boards.

  • Sequential lamination

Building the boards by laminating multiple cores with alternate copper and prepreg layers is called the sequential lamination process. Doing so eases the manufacturing process when there are micro-vias involved and this enables to achievement of higher layer count manufacturing with tighter routing density.

  • Embedded Components

This technique allows the fabricator to build the components within the PCB substrates to avoid the assembly process in the later stages. This will reduce the component density on the PCB surface, which in turn helps reduce the overall size of the PCB.

  • Flex PCB

PCB that has the ability to bend and twist in any shape is called flex PCB. Considering the flex and rigid-flex technology during designing allows flexibility which is necessary with applications where space is the constraint. It is very thin and light in weight which can be accommodated within a smaller area compared to rigid boards.

  • Material Selection

With the development of HDI PCB technology, the selection of material should also be important as it plays a crucial role in providing thermal stability, better electrical properties and mechanical strength which makes them suitable for high-speed board design.

advantages and disadvantages of HDI PCB

Advantages of HDI PCB Boards

  • Routing density is higher
  • Enhanced signal accuracy
  • Improved electrical efficiency
  • Ability to reduce the overall size of the board
  • Improved reliability

Disadvantages of HDI PCB Boards

  • Manufacturing cost is more
  • Due to complexities, it is quite challenging to design the boards
  • Reworking and repairing the board is complex
  • Problems with manufacturability

Manufacturing Process of HDI PCB Boards

Manufacturing the HDI PCB boards involves several processes which require a high level of technical expertise and advanced equipment to process the boards. Since the trace width and spacing are very tight in HDI PCB technology, special care is taken at each step right from selecting the materials to finally finishing the boards.

  • Selection of Prepreg or Substrate

Selecting the materials is the initial step to start the manufacturing process. The most commonly used materials for HDI PCB boards are FR-4 and polyimide. FR-4 is a fiberglass epoxy material which is used for rigid boards whereas polyimide is used for flexible circuits due to its durability and flexibility properties.

  • Lamination

After selecting the substrate required, the next process is to laminate the substrate and copper foil to form a layered stackup. Lamination provides the bonding between the copper foil and prepreg and ensures the layer is aligned precisely as needed.

PCB lamination

  • Drilling

After the lamination is done, small holes are drilled either mechanically or by laser technology. These small holes act as an interconnection between layers.

  • Copper Plating and Viafill

The microvia drilled are plated with copper to establish the connection between layers and filled with copper or non-conductive material.

  • Plotting the Circuit

In this process, circuitry is imaged on the copper surface and unwanted copper is etched off to get the required circuit pattern printed on the surface. 

  • Multi-Layer Lamination and Final Pressing

For a multi-layer board, the above process steps are repeated and finally, all the inner layer cores are laminated with the outer layers and pressed to form a final stackup. Normally 2 to 4 lamination cycles are done in manufacturing and not more than that.

  • Surface Finish

As the boards are ready with the circuit printed on it, it is now important to provide a better surface finish for the bare copper to avoid oxidation and to provide better solderability between the components and the copper surface during assembly.

  • Testing and AOI

The finished HDI PCB boards undergo several testing and inspection methods to check whether the boards came out well from the production process. It involves an electrical performance check, connectivity check, automated optical inspection etc. to ensure that the quality of the board is not compromised and complies with IPC.

Summary

Overall, HDI PCB technology has transformed the PCB industry by enabling the creation of compact boards, high performance devices without even compromising functionality. With the use of advanced drilling capabilities, manufacturers are able to produce boards with high levels of miniaturization, performance and reliability that are required in today’s electronic world. In order to achieve all these, designer engineers have to communicate with the manufacturer and assembly people to know the capabilities of the manufacturing plant before designing the boards.

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