How to Optimize Layer Stack-Up in HDI PCB Boards
6 min
With the introduction of the HDI stackup, a cutting-edge technique in multilayer PCB design, PCB designers should be able to create increasingly complex, smaller boards in the years to come. Establishing your project's needs precisely is the first step in designing a PCB stackup. Determining the number of layers required is the first stage; this depends on the circuit's complexity, signal density, power distribution requirements, and if the design calls for RF or high-speed signals.
In the late 1980s, HDI PCB fabrication got underway. The successive building of PCBs in 1984 marked the start of the first HDI manufacture. Since then, producers and designers have been constantly searching for methods to fit more components into fewer spaces. In accordance with IPC-2315 and IPC-2226, HDI boards are designed and produced. This article will cover the following topics: what a PCB stack-up is, why it's important, how to choose one, typical stack-up configurations, and impedance control issues for HDI PCBs.
1. What is a PCB Stack-Up?
The arrangement of copper and insulating layers on a PCB is known as a stack-up. It determines the distribution of power planes and signal traces among layers. This has a direct impact on temperature control and electrical performance. Multiple layers with exact configurations are frequently used in stack-ups for HDI designs in order to obtain high interconnect density. However, some questions still remain as they are. For example, how do you pick the layers? Is there a formula? Should I use four or six layers? By the end of this piece, all of these questions will have been answered. Although there is no set formula, there are basic design principles that must be adhered to; otherwise, problems could arise like:
⦁ Resistance Mismatch
⦁ Issues with thermal lagging EMI/EMC
2. Design Guidelines for Proper PCB Stackup:
To create the best-quality products, designers must adhere to certain guidelines, just like in any other design or product manufacturing process. As you are already aware, before the final product is produced, electronics must pass through a number of processes involving various components.
⦁ Because they enable signal routing in strip lines and greatly minimize ground noise by lowering ground impedance, ground planes are crucial.
⦁ The ground plane serves as a shield to reduce radiation at high speeds, and high-speed signals must be routed on an intermediary layer between layers.
⦁ For best results, signal layers should be positioned near the ground plane.
⦁ For effective operation, mass planes and power connections need to be carefully planned.
⦁ To guarantee balanced performance, the PCB configuration needs to be symmetrical.
⦁ Maintaining appropriate signal integrity requires meeting signal impedance criteria.
⦁ When designing, each signal layer's thickness should be taken into account.
⦁ To make sure the materials fulfill the design specifications, their mechanical, chemical, electrical, and thermal qualities must be assessed.
3. Design Guidelines for Effective Signal Routing:
HDI board routing will get increasingly complicated as component density rises. For the HDI PCB design to support high-density routing, trace width, via size, and spacing must be kept to a minimum. The HDI PCB design requires that all remaining components be routed after all key signal components, decoupling capacitors, and ICs have been routed.
To reduce the noise and crosstalk brought on by high-speed signals, it is recommended to design multilayer boards with ground and power layers as inner layers. The arrangement of these ground and power levels is such that the ground layer is directly beneath the signal layer, which serves as a reference layer and return channel for the differential signals on the top layer. To lower the impedance, the power layer is positioned after the ground layer.
To give each signal its own ground plane, copper must be poured in a split plane concept. By doing this, the HDI PCB design will function better without interference from nearby signals and the noise produced by various signals and components. Below are some of the most often used multilayer PCB design rules.
4. How to Choose Layer Count:
While complicated boards utilizing FPGAs, SoCs, or RF applications frequently require 6 to 8 or more layers, simple designs may just require 2 to 4 layers. Additional ground planes and carefully considered layer configurations are typically required for high-speed circuits and EMI-sensitive designs
Every layer in the stackup ought to serve a distinct purpose. While ground planes offer low-impedance return pathways that are essential for signal integrity, particularly for high-speed communications, signal layers are utilized for routing traces. Multiple power planes may be needed in dense designs since power planes are utilized to deliver voltage throughout the board. To reduce noise and electromagnetic interference, signal layers should ideally be close to ground planes. Analog and digital spaces must be carefully separated in mixed signal architectures.
5. Layer Stack-up Designs:
Noise can be minimized by selecting the layer stackup in accordance with a design's specifications. For example, a signal propagates faster if its path is placed over a ground plane. Additionally, by adding a power plane to the stack-up, fewer tracks are needed, enabling the usage of vias to supply VCC or 5V.
6. 4 Layer PCB Stack-Up:
4-Layer Stackup 1 (Standard for General Purpose):
Layer 1: Signal (Top)
Layer 2: Ground (GND) Plane
Layer 3: Power (VCC) Plane
Layer 4: Signal (Bottom)
4-Layer Stackup 2 (Signal Integrity Focus):
Layer 1: Signal
Layer 2: Ground
Layer 3: Ground/Power (Split if needed)
Layer 4: Signal
7. 6 Layer PCB Stack-Up:
6-Layer Stackup 1 (High-Speed Signals):
Layer 1: Signal
Layer 2: Ground
Layer 3: Signal
Layer 4: Signal
Layer 5: Power
Layer 6: Signal
6-Layer Stackup 2 (Power Integrity Focus):
Layer 1: Signal
Layer 2: Ground
Layer 3: Power
Layer 4: Ground
Layer 5: Signal
Layer 6: Signal
8. 8 Layer PCB Stack-Up:
8-Layer Stackup 1 (EMI Control, High-Speed Designs):
Layer 1: Signal
Layer 2: Ground
Layer 3: Signal
Layer 4: Power
Layer 5: Ground
Layer 6: Signal
Layer 7: Ground
Layer 8: Signal
8-Layer Stackup 2 (Dense Power Distribution):
Layer 1: Signal
Layer 2: Ground
Layer 3: Signal
Layer 4: Power
Layer 5: Power
Layer 6: Signal
Layer 7: Ground
Layer 8: Signal
Conclusion:
The application of the PCB and circuit determine which stack-up is best, ranging from 2-layers to 10-layer HDI board. High speed stackups are more expensive because we need to consider a lot of things such as EMI and signal integrity. The PCB may become unsuitable for high-speed applications if the stackup quality is not designed keeping these considerations. Working with a reputable manufacturer such as JLCPCB is essential. As one of the leading PCB manufacturers, JLCPCB offers state-of-the-art production lines that ensure even the most complex high-speed designs are manufactured reliably and cost-effectively. This not only guarantees smooth production but also enables engineers to bring cutting-edge and compact technologies to market faster.
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