PCB Stackup is a comprehensive design of the arrangement sequence, functional allocation, and interlayer dielectric parameters of each conductive layer in a multi-layer printed circuit board. It is the basic framework of PCB design. Reasonable stacking design can significantly improve signal integrity (SI), power integrity (PI), electromagnetic compatibility (EMC), and reduce PCB manufacturing defects such as warping. The following is a science popularization from the aspects of basic concepts, core components, design principles, etc.:
Table of Contents
1、 Why do we need multi-layer PCB Stackup?
A single-layer PCB (single panel) has conductive copper foil on only one side, while a double-layer PCB (double-sided board) has copper foil on both sides and is connected through vias. But with the increasing complexity of circuits (such as high-speed signals, multiple power supplies, high-density components), single-layer/double-layer boards have obvious limitations:
-Serious signal interference (no shielding layer, strong high-speed signal radiation);
-The power/ground network is messy (large areas of copper foil are difficult to arrange, and the power supply stability is poor);
-Insufficient wiring space (due to high-density components causing difficulty in crossing wires).
Multilayer PCBs solve the above problems by adding inner layers (signal layer, power layer, ground layer):
-Using the ground plane (GND) as a signal reference plane to stabilize impedance and reduce radiation;
-Using a power supply layer (PWR) to provide a low impedance power supply path and reduce power ripple;
-Layered isolation of high-speed/low-speed, analog/digital signals to reduce crosstalk.
2、 Core components of PCB Stackup
The stacking of multi-layer PCBs is composed of alternating conductive layers and insulating dielectric layers, and the core conductive layers are divided into three categories according to their functions:
1. Signal Layer
-Function: Transmit electrical signals (such as digital signals, analog signals, high-speed differential signals, etc.).
-Category:
-Top Layer: The outermost layer of the PCB, often used to arrange component pads and some wiring;
-Bottom Layer: The lowest layer of the PCB, with the same functionality as the top layer;
-Inner Signal Layer: located in the middle of the PCB, suitable for high-speed signals (surrounded by reference planes, strong anti-interference).
2. Ground Plane
-Function:
-Provide a “reference zero” for the signal (signal return path, reducing impedance);
-Shielding interference (absorbing radiated signals, isolating different types of circuits);
-Heat dissipation (large area copper foil accelerates heat diffusion).
-Characteristics: Usually a complete copper foil layer (“flooring”), avoiding segmentation (unless it is necessary to isolate different ground, such as digital ground or analog ground).
3. Power Plan
-Function: Provide stable power supply for the circuit (replace scattered power supply wiring, reduce power supply network impedance).
-Characteristics: Similar to a grounding layer, usually a complete copper foil layer; If the circuit has multiple voltages (such as 3.3V, 5V, 12V), multiple independent power supply layers need to be designed (to supply power to corresponding components through vias).
Addendum: Insulation dielectric layer
-Located between the conductive layers, it serves as insulation and is typically made of materials such as FR-4 (glass fiber epoxy resin, low-cost), polyimide (high temperature resistant), etc.
-The thickness and dielectric constant (ε r) of the medium are key parameters that affect signal impedance (the thinner the medium, the lower the impedance) and signal transmission speed (the smaller the ε r, the faster the speed).
3、 Common stacked structures (classified by layers)
The number of layers is determined by the circuit requirements (the more layers, the higher the cost, but the stronger the performance). The following are typical structures:
1. 2-layer board (low-cost entry-level)
-Structure: Top layer (signal+partial power/ground)+Bottom layer (signal+partial power/ground)
-Features: No independent power supply/ground plane, requiring “large-area copper laying” to replace the ground plane; Suitable for simple circuits (such as toys, small appliances), high-speed signals are susceptible to interference.
2. 4-layer board (most commonly used)
-Classic structure: Top layer (signal) → Ground layer → Power layer → Bottom layer (signal)
-Advantages:
-There are independent grounding and power layers, and the signal reference is stable;
-The distance between the power layer and the ground layer is close (forming a “flat capacitor”), reducing power ripple;
-Applicable scenarios: general digital circuits (such as microcontroller systems), low-speed signals (≤ 100MHz).
3. 6-layer board (medium high speed circuit)
-Typical structure: Top layer (signal) → Ground layer 1 → Inner layer signal 1 → Power layer → Ground layer 2 → Bottom layer (signal)
-Advantages:
-Add one inner signal layer that can handle high-speed signals (such as USB 3.0, DDR);
-Double grounding layer enhanced shielding (isolating analog/digital signals);
-Applicable scenarios: circuits containing high-speed signals (100MHz~1GHz) and multiple power sources (such as industrial control boards).
4. Layers 8 and above (high-speed/complex circuits)
-Structure: The signal layer and ground layer are alternately arranged (such as “signal → ground → signal → power → power → signal → ground → signal”) to meet the requirements of multiple power sources and ultra high speed signals (such as RF signals above 10GHz).
-Applicable scenarios: server motherboards, communication equipment (5G base stations), high-end FPGA/CPU boards.
4、 The core principles of stacked design
A reasonable stacked design needs to balance “performance, cost, and manufacturability”, with the following core principles:
1. “Signal reference plane” nearest pairing
-The signal layer must be “closely attached” to a reference plane (ground layer or power layer): the signal return path is the shortest and the impedance is stable (such as a high-speed differential line that needs to run between two ground planes to form a “strip line” with controllable impedance).
-Example: In a 4-layer board, the top signal is closely attached to the ground layer, and the bottom signal is closely attached to the power layer (or ground layer).
2. The power layer and ground layer are ‘paired and close’
-The power layer and ground layer should be adjacent as much as possible (spacing ≤ 0.2mm), and the “flat capacitor” formed by the two should be used (capacitance ≈ ε r × S/d, S is the area, d is the spacing) to filter out high-frequency ripples in the power supply and reduce the power supply impedance.
3. High speed/sensitive signals’ go through the inner layer ‘
-High speed signals (such as clocks, differential pairs) and analog signals (such as sensor outputs) are susceptible to interference or radiation, and should be routed through inner layers (surrounded by upper and lower reference planes) to reduce crosstalk and EMI through planar shielding.
4. Layered symmetry to reduce warping
-The number of layers and materials should be symmetrical (such as the top and bottom copper thicknesses being the same, and the thickness of the dielectric layer being symmetrical) to avoid PCB warping caused by uneven thermal expansion and contraction (which can easily lead to pad displacement and through-hole fracture during manufacturing).
5. Isolate and stratify by signal type
-Digital signals (with high frequency noise) and analog signals (sensitive) need to be isolated by a ground layer (such as in a 6-layer board, where digital signals go to the top layer and analog signals go to the bottom layer, separated by a ground layer in the middle);
-The power layers with different voltages need to be separated (such as adding a ground layer to isolate between the 3.3V and 5V power layers to avoid crosstalk).
5、 Precautions for stacked design
1. Balance between cost and performance: The more layers there are, the exponential increase in cost (inner layer processing and material costs increase). Do not blindly increase the number of layers unless necessary (such as using 4 layers for simple circuits, without the need for 8 layers).
2. Impedance control: High speed signals (such as PCIe, Ethernet) require strict impedance control (usually 50 Ω single ended, 100 Ω differential), which is determined by line width, copper thickness, and dielectric thickness. The stacked design needs to confirm the dielectric parameters with the PCB manufacturer in advance.
3. Manufacturing feasibility: The thickness of the inner copper layer and the thickness of the dielectric should meet the manufacturer’s process capabilities (such as a minimum dielectric thickness of ≥ 0.1mm) to avoid the design being unable to produce.
Summarize
PCB Stackup is the “invisible skeleton” of circuits, which determines signal transmission, power stability, and anti-interference ability. When designing, the number of layers should be selected based on signal speed, power type, and cost budget, following the principles of “signal reference plane pairing, power ground proximity, symmetrical layout”, etc., in order to make the PCB both stable and reliable.
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