Stack-Up Strategies for Effective HDI PCB Design

Whether you believe Moore's Law is dead or alive, the strong economic incentive to pack more processing power into ever smaller form factors is unlikely to abate any time soon. With the entry of the HDI stackup—a technology at the cutting edge of multilayer PCB design that promises to help PCB designers make smaller complex boards for years to come. The stack-up in a printed circuit board (PCB) design is a critical element that influences the board's performance, manufacturability, and reliability. In High-Density Interconnect (HDI) PCBs, stack-up strategies become even more essential due to their compact designs and complex layer structures.

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 for ways to pack more components on the board within smaller areas. HDI boards are designed and manufactured as per IPC-2315 and IPC-2226. This blog explores what a PCB stack-up is, why it’s essential, methods to choose a stack-up, commonly used stack-up configurations, and considerations for impedance control in HDI PCBs. To know more about PCB designs, see our recent article on Via's used in PCBs.

What Is a PCB Stack-Up?

A PCB stack-up refers to the arrangement of copper and insulating layers in a PCB. It dictates how signal traces and power planes are distributed across layers, impacting electrical performance and thermal management. For HDI designs, stack-ups often involve multiple layers with precise configurations to achieve high interconnect density.

Why Do We Need a PCB Stack-Up?

HDI PCB design is like a multi-dimensional puzzle. Here are some common design considerations you’ll need to factor into your HDI PCB design:

Impedance Control: You’ll want to maintain tight tolerances (within ±10%) on dielectric layer thicknesses, trace widths, and spacings to ensure impedance does not impact signal integrity.

EMI/EMC: All those radiation considerations such as avoiding accidental antennas and noise apply, especially because HDI is used for high speed signal designs.

Thermal: HDI often leads to improved thermal performance, however you’ll still want to factor in thermal stability of microvias and trace widths in high-speed signal designs.

The need to factor physical, electromagnetic, and thermal considerations into an HDI PCB design adds a lot of complexity to the design process. Fortunately, EDA (electronic design automation) software tools have evolved to make solving multi-dimensional PCB problems easier.

Rules for Proper PCB Stackup Design:

Like any other design or product manufacturing, designers need to follow some rules to produce the highest quality products. As you already know, electronics go through several processes which involve different components before producing the final product. Therefore, designers must ensure they identify and follow proven design PCB stack-up best practices. For PCB stack-up design, some rules should be followed to get the best results.

• The use of ground planes is essential as they allow signals to route in strip lines and significantly reduce ground noise by lowering ground impedance.
• High-speed signals must be routed on an intermediate layer between levels, with the ground plane acting as a shield to suppress radiation at high speeds.
• Signal layers should be placed close to the ground plane for optimal performance.
• Mass planes and power connections must be carefully designed for efficient functionality.
• The PCB configuration must be symmetrical to ensure balanced performance.
• Signal impedance requirements must be met to maintain proper signal integrity.
• The thickness of each signal layer should be considered during the design process.
• The thermal, electrical, chemical, and mechanical properties of materials must be evaluated to ensure they meet the design requirements.

Collaborating with your PCB manufacturer during this process ensures feasibility and adherence to industry standards.

How to Route 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. Some of the most common multilayer PCB design guidelines are given below.

Commonly Used PCB Stack-Ups:

2-Layer PCB Stack-Up: Simplest configuration with one signal layer and one ground/power plane. These can be used in basic applications such as simple IoT devices and consumer electronics.

4-Layer PCB Stack-Up: Features two signal layers and two planes (power and ground). Ideal for low-frequency and moderately complex designs requiring better EMI control.

6-Layer PCB Stack-Up: Adds additional signal and plane layers for enhanced performance. Common in mid-range applications like communication devices and industrial equipment.

8-Layer PCB Stack-Up: Incorporates multiple signal and plane layers for high-density interconnections. Suitable for advanced designs with high-speed signals and tighter space constraints.

10-Layer PCB Stack-Up: Offers extensive signal, ground, and power plane distribution. Often used in HDI PCBs for applications like servers, aerospace systems, and automotive technologies.

Each configuration is tailored to specific design needs, with HDI PCBs favoring higher layer counts to accommodate microvias and other advanced features.

Layer Stack-up:

Like any other advanced PCB, success in HDI design comes from designing the right stackup. While this is certainly true in terms of signal and power integrity, it also matters for manufacturing; the HDI PCB stackup you use must conform to a set of standard processing steps required to build up the board. According to the IPC-2226 HDI PCB standards, there are several types of standardized HDI PCB stackups that help in minimizing the parasitic elements and improves the overall signal integrity.

By choosing the layer stackup according to the requirements of a design, noise can be reduced to minimum. For instance, if a signal path is positioned over a ground plane, the signal propagates more quickly. And by including a power plane in the stack-up, the number of tracks can be reduced, allowing the use of vias to provide VCC or 5V.

The basic PCB stack up for 6 layer PCB with EMI/ EMC considerations:

Signal Layer

Ground plane

Signal Layer

Power Plane

Ground Plane

Signal Layer

The basic PCB stack up for 4 layer PCB with EMI/ EMC consideration for audio applications:

Signal Layer

Ground plane

Ground Plane

Signal Layer

The basic PCB stack up for 4 layer PCB designed for embedded solutions:

Signal Layer

Ground plane

Power Plane

Signal Layer

Component Placement in the HDI PCB Design:

1. Component placement is critical in HDI PCB design as it directly impacts routing density and signal integrity. Proper planning ensures the shortest routing paths.

2. Group components based on schematic workflow into sections such as analog, digital, high-speed signals, mixed signals, high-frequency, and power supply. Separate sensitive and critical components and position them accordingly.

3. Place main components like microprocessors, microcontrollers, Ethernet, and memory devices at the center of the board since they connect to most other components. Position decoupling capacitors, crystal oscillators, and resistors close to these components for efficient signal flow.

4. Ensure all components are oriented in the same direction for effective and error-free routing and assembly.

5. Place all surface-mounted components (SMDs) on the same side of the board (top or bottom) and thru-hole components on the top side to streamline the assembly process.

6. Separate analog and digital circuit sections to minimize noise interference.

7. Position decoupling and bypass capacitors as close as possible to their respective circuit sections to maintain signal integrity.

Impedance Control in Stack-Up Design:

Controlled impedance is vital in HDI PCBs for high-speed and high-frequency applications. The stack-up directly influences impedance by determining:

• Trace Width and Thickness: Adjusted to achieve target impedance values.
• Dielectric Constant (Dk): Ensures consistent signal propagation.
• Spacing Between Layers: Maintains isolation to reduce crosstalk.
• Placement of Power and Ground Planes: Provides stable reference planes for signals.

Accurate impedance control prevents signal degradation, reducing errors and ensuring reliability in advanced circuits.

Conclusion:

PCB stack-up design is essential for both designers and electronic engineers. The ability to come up with high-quality electronics requires several considerations. Without a high-quality PCB design, the product's quality and performance can be significantly affected. Therefore, designers must ensure the right stackup construction and PCB materials are selected to obtain a high-quality product. A high-quality PCB stack-up goes a long way in getting the highest quality PCB yield and productivity. From simple 2-layer configurations to complex 10-layer HDI designs, selecting the right stack-up depends on the application's requirements and the PCB's complexity.

Stackups designed for High-speed designs are costlier than those used for non-high-speed applications. Compensating the stackup quality for cost can result in poor signal integrity which makes the PCB unsuitable for High-speed application.By focusing on proper material selection, layer arrangement, and impedance control, designers can create high-performing and reliable PCBs for modern electronics. Collaborating with a trusted manufacturer further ensures that your design translates seamlessly into production, paving the way for innovative and compact technologies.