PCB Design

HDI PCB Design: Comprehensive Guidelines, Process and Considerations

HDI PCB design

In today’s world, electronic products have become more complex than before to achieve better performance to transfer data faster than ever. HDI (High-Density Interconnect) is a type of technology used in PCB to achieve the miniaturisation of boards with higher component density. Therefore, HDI PCB design considerations are particularly important in the manufacturing of HDI PCB. In this article, let us explore some of the considerations and guidelines that need to be followed in the HDI PCB design process.

What is HDI PCB design?

HDI PCB design, or High-Density Interconnect PCB design, is a type of printed circuit board (PCB) design that focuses on maximising the density of electronic components and circuit interconnections in a small area. In order to accommodate more components and increase the wiring density of the board, HDI PCB design uses more complicated features like micro-via, blind vias, and buried vias which allows for designing compact boards.

Fabrication of HDI PCB started in the late 1980’s. The first HDI production began in 1984 with the sequential buildup of PCBs. Since then, designers and manufacturers always look ways to pack more components on the board within smaller areas. HDI boards are designed and manufactured as per IPC-2315 and IPC-2226.

The Considerations and Guidelines that need to be followed in the HDI PCB design process

Stackup in the HDI PCB design

Designing Layer Stackup in the HDI PCB design

HDI PCB design (High Density PCB design) has tighter signal integrity requirements and designing the board with proper stackup is important to ensure signal integrity in HDI boards. Choosing the right material is not only important for signal integrity issues, but also for impedance and heat dissipation characteristics too. Also, the number of layers and the order of the layers built in sequential plays a vital role in the HDI PCB design.

Usually, HDI stackups are made in sequential order with symmetrical construction. It consists of prepreg, core, and copper foils. Designing the stack-up depends on the component used, the impedance requirement, and the signal integrity requirement in the HDI PCB design. For HDI boards, micro vias, blind vias, and buried vias are used to reduce the thickness of the board and to accommodate more wiring density on the layers.

Improper stackup build can result in poor performance causing EMI issues, crosstalk, and more vulnerability to surrounding noise. To overcome such issues, stack-up has to be built in such a way that all high-speed signals have to be provided with ground planes above or below to facilitate signal return path.

A few considerations while designing the HDI board stack up are as follows:

  • Aspect ratio in HDI PCB design: Maintain the aspect ratio of a thru-hole drill (1:10) and HDI drills (1: 0.75)
  • Lamination cycles in HDI PCB design : Limit the lamination cycles to less than 3 cycles.
  • board thickness in HDI PCB design: Use microvia, blind & buried via to reduce board thickness.
  • The return path for signal: Place ground or power plane layer to provide a return path for signals.
  • The least inductance path: The return path of signals should be determined as it takes the least inductance path to return.

By considering all the above factors, a balanced stackup built with proper signal and power ground layers will minimise the crosstalk and EMI effects in the boards.

Implementing different types of VIA technology in the HDI PCB design

Any conductor that is part of the HDI board is considered a part of that circuit. The size and depth of the vias and the length of the traces, all these factors are considered in HDI PCB design. Via size is the main factor that needs to be considered prior to the high density PCB layout process. The smaller the vias, the better the performance of high-speed signals.

Trace length should be shorter for high-speed design & to achieve this vias are placed close to the pads, or partially falling on pads or even falling inside the SMD pads.

Now, let us explore the different types of via technologies used in HDI PCB design based on the functionality

  • Blind via

Vias that connect external layers to one or more internal layers are called blind vias. This kind of connection may be from the top side or the bottom side in the HDI PCB design, but in either case, these drills cannot be seen from the other end of the board after lamination is done. The IPC standard for blind via is 6 mils & less.

  • Buried via

Vias that connect only internal layers of the board are called buried vias. They cannot be seen from either side of the board as it is completely covered after the lamination is done. The IPC standard for buried via is 6 mils and less.

  • Microvia

These are very small vias and are often called laser vias or laser ablated vias or micro vias. The aspect ratio for the ideal microvia is 0.75:1 with a maximum depth of 0.25mm between the surface and target pads. The standard drill size as per IPC is 75-150 microns, which can be drilled mechanically or by using laser technology.

Usually, the depth of these drills is not more than two layers since plating the copper inside these via hole walls are tedious process.

Furthermore, these microvias are classified into two types as follows:

  1. Staggered via: Vias that are offset to each other and have a separate axis.
  2. Stacked via: Vias that are falling on top of one another on the same axis.
  • Via-in-pad technology

Via-in-pad is used when the density of the component is high and the PCB board size is limited. Unlike traditional vias, these via are placed on the SMD component and then it is filled and plated over by copper to avoid solder flow inside vias during the assembly process. This method is called VIPPO (Via-in-pad plated over). This kind of method is suitable for fine-pitch BGA components.

Routing guidelines in the HDI PCB design

Routing guidelines in the HDI PCB design

  • The common routing guidelines for routing the HDI boards

As the component density increases, routing the HDI boards will also become very complex. Trace width, via size, and spacing have to be minimized to accommodate high-density routing in the HDI PCB design. All critical signal components, decoupling capacitors, and ICs are routed first and then the remaining components have to be routed completely in the HDI PCB design.

The best practice is to create multilayer boards and provide ground and power layers as inner layers to minimize the noise and crosstalk that are caused by high-speed signals. These ground and power layers are placed in such a way that the ground layer is placed right below the signal layer which acts as a return path and reference layer for the differential signals on the top layer. The power layer is placed after the ground layer to reduce the impedance.

Copper has to be poured in a split plane concept in order to provide each signal with a separate ground plane. This will avoid the noise generated by different signals & components, to perform better without any interference from neighbouring signals in the HDI PCB design.

  • The other  routing guidelines for routing the HDI boards

Also, when routing the differential pair signals, they have to be routed symmetrically with a specified distance between them to achieve the specified impedance. Improper routing of these differential pairs will result in a delayed signal at the receiver end. There should not be any components or vias in between these signals. Placing components or via between these signals would cause EMI/EMC problems and impedance mismatches in the HDI PCB design.

In addition to this, differential pair requires very tight delays between positive and negative signals. In order to achieve these tight delays, traces have to be length-matched by using serpentine geometry in the HDI PCB design.

Component placement in the HDI PCB design

The placement of components plays an important role in the HDI PCB design (High Density PCB design) as it directly affects the routing density and signal integrity of the board. Planning the placement of components is necessary to maintain the shortest path. Always group the components based on the schematic workflow and separate the components that are sensitive, and critical and place them accordingly.

As an initial step, read the schematic and separate the circuit into different sections based on the following factors: Analog, Digital, High-speed signal, Mixed signals and high-frequency, power supply etc.…

After grouping the components, place the main components like the microprocessor or microcontroller, ethernet, and memory devices at the centre of the board as they connect to most of the components in the circuit. Next place all the decoupling capacitors, crystal oscillators and resistors as close as possible to these components for a smooth signal flow between them.

Some major considerations that need to be followed while placing the components are listed below:

  • Direction of components in the HDI PCB design: Components have to be placed in the same direction to facilitate effective and error-free routing during the HDI PCB design process and also to provide an error-free soldering process in assembly.
  • The layout surface of the components: Place all SMD components on the same side, either on top or bottom. Place all thru-hole components on the top side to ease the process during assembly.
  • Separate circuit analog and digital circuit sections: Separate analog and digital sections to avoid interference of noise created by them.
  • Placement of decoupling and bypass capacitors: Place all the decoupling and bypass capacitors as close as possible to the respective circuit section.
  • Placement of sensitive components: Place all the sensitive components away from the board edge to reduce the influence of EMI radiated from the peripheral devices.

Now, place all the peripherals at the edge of the board which are used for communication purposes from external devices.

Signal integrity

Signal integrity is the measure of the quality of the signal passing through the transmission line without distortion.

Boards which operate at low and medium frequencies are very rarely affected by signal integrity issues, whereas in high-speed design which usually operates at a high-frequency range with a short signal rise time, signal integrity issues are relatively prominent. Signal integrity issue causes PCB to malfunction and degrade the performance of the boards.

Below are a few design considerations that have to be implemented during HDI board design:

  • SI effect: HDI signals require controlled impedance routing to minimize the SI effect. To achieve this, trace width, spacing and proper dielectric have to be maintained to get the desired impedance value in the HDI board design.
  • Signal reflection and impedance mismatches: Reduce via stub length to avoid signal reflection and impedance mismatches in the HDI PCB design. Route signals using blind & buried vias to connect inner layers directly.
  • Signal crosstalk: Use ground shielding and proper spacing between traces to avoid signal crosstalk which occurs due to coupling between adjacent signal traces.
  • Power noise: Place the decoupling capacitor as close to HDI components to reduce power noise. Separate ground and power planes effectively to distribute power smoothly across the board.
  • Signal loss at the receiver end: Maintain proper spacing between differential pair signals and match the length of the trace to avoid signal loss at the receiver end.
  • EMI and ground bouncing effect: Maintain proper return path for high-speed signals to reduce EMI and ground bouncing effect. Use solid ground below high-speed signals and maintain low impedance to prevent signal distortion in the HDI board design.
  • Potential issues: In the HDI board design, use simulation to identify signal integrity issues at the design level to avoid potential issues and fix them accordingly.

Thermal management

Thermal management in the HDI PCB design

Dissipating the heat produced by components is very challenging in HDI boards where the components are placed densely during the HDI PCB design. In order to produce a reliable and durable board, it is essential to manage thermal issues in the  HDI PCB design phase.

Let us explore some of the considerations to be done in HDI boards for thermal management:

  • Thermally conductive materials: In the HDI board design, use thermally conductive materials like metalcore or thermally conductive dielectric materials to dissipate heat in critical areas of the PCB.
  • Thermal vias: Implement thermal vias to dissipate heat from high-power components to inner layers or outer layers. Place these vias close to the heat source to transfer the heat to the other side of the board.
  • Placement of heat-generating components: In the HDI board design, place all the heat-generating components close to the edges of the board to dissipate the heat and optimize the airflow.
  • Suitable enclosure: Design the enclosure to facilitate proper airflow to the PCB, especially around the heat-generating components.
  • Optimizing placement and design: In the HDI board design, use thermal analysis simulation tools to identify temperature distribution across the board which will help in optimizing component placement, via placement and stackup design to achieve maximum thermal performance.

Design for Manufacturing (DFM)

Design for Manufacturing (DFM) in the HDI PCB design

In order to ensure the error-free production of HDI boards, collaborating with the PCB manufacturer prior to releasing the Gerbers is necessary to avoid any issues on the production floor.  Some of the key considerations to implement while designing HDI boards are as follows:

  • Communication before designing the board: Communicate with PCB manufacturers and get familiar with manufacturing guidelines prior to designing the board.
  • Design-related issues: Get to know the minimum trace width, space, annular ring and via sizes to avoid causing manufacturing issues.
  • Aspect ratio: Ensure the aspect ratio is met, as they are the key factors which can affect manufacturing. Improper aspect ratio may cause plating issues and drilling problems during fabrication.
  • Selection of materials: Choose materials that are compatible with high-speed signals.
  • Selection of vias: Based on the  HDI PCB design complexity, choose the proper type of visa (blind, buried or microvias) and avoid placing more vias at fine-pitch components to avoid plating issues.

 

Summary

Finally, incorporating all of the abovementioned strategies into the design of the HDI board results in reliable and high-performance PCB production. With the use of modern  HDI PCB design tools and manufacturing compatibility, PCBs with high component density and complicated designs may be produced without issue. Collaboration with cross-functional teams and manufacturers is critical for a successful HDI PCB design process. Not to mention that all industry standards and IPC specifications must be followed throughout the HDI PCB  design process, from the  HDI PCB design phase until the time the board is manufactured.

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