Component Placement and FLoor Planning
Introduction
What is Floorplanning?
Floorplanning is a critical stage in Printed Circuit Board (PCB) design. It involves deciding the optimal locations for components, connectors, and other elements on the board.
Why is it Important?
High-speed vs. Power Domains: Components must be placed according to their operational speeds and power requirements.
Cost and Real Estate: The area, especially around the CPU, is a premium asset, influencing manufacturing cost.
Component Optimization: Different components like sensors prefer specific placements to avoid noise and other interferences.
Floorplanning Insights
Floorplanning is the balancing act between high-speed and power domains. Starting with predetermined locations for connectors and mounting holes, engineers must consider cost factors such as dual-sided placements.
Premium real estate is typically near the microprocessor, reserved for components like memory, capacitors, and crystals that benefit from close proximity to the CPU.
Contrarily, sensors and analog devices like proximity sensors and cameras are often best situated farther away to avoid noise. Components with lower usage frequency, such as keyboard connectors, tend to occupy a middle ground.
Voltage domains become significant when components have varying power requirements, which should be grouped together for efficient power plane design.
Background
Mechanical Component Outline (MCO): Provided by the mechanical design team, the MCO sets the physical boundaries within which all components must fit.
Design Overview
Key considerations for floorplanning:
PCB Area: Must fit all components as outlined by the MCO.
Power Supply Isolation: Separate power planes and decoupling capacitor can isolate the noisy power supply from low-noise components.
ESD and Noise Filtering: ESD and noise filters should be placed at the source of the noise. For example, an ESD protection diode should be placed at the input of a connector to protect the PCB from electrostatic discharge.
Decoupling Capacitors: These should be located near the power pins of digital ICs for noise reduction.
Passive Components: Place them close to the ICs they serve to minimize parasitic effects.
Antenna Placement: Antennas should be isolated from noise sources such as the processor, system memory, and power supply.
Component Orientation: High-pin-count devices should be oriented in a way that aligns the trace routing breakout direction. This helps to minimize signal delays and noise.
Routing Strategy: Plan routes to minimize delays and optimize signal integrity.
Signal Integrity Analysis: Conduct this analysis to meet performance criteria and optimize design.
Example High Level Floor Plan for a Steaming Stick
Diagram Key Elements:
Board Outline: The board is rectangular, fitting within a 12x6 cm area to show a compact layout.
CPU (Red): Located centrally and made larger, this is the processing core of the device.
HDMI (Blue): Positioned on the far right, this is the HDMI output interface.
USB (Purple): Located at the bottom middle, this port serves as the power input.
RAM (Orange): Positioned to the left of Flash, this is the device's memory unit. This allows for easier connection to the HDMI plug.
Flash Memory (Green): Positioned above and to the right of RAM, this is for storing firmware and essential data.
Wi-Fi Module (Cyan): Located to the right of the antenna at the center-left, this module enables wireless communication.
Antenna (Pink): Positioned at the center-left, this is for enhancing wireless signal reception.
Power Regulators (Brown): Located next to the USB port at the bottom middle, these regulate the power supply to the various components.
This layout considers ease of routing and isolation of different functional domains, while also showcasing the challenges of working with tight layout constraints.
Summary and Conclusion
Floorplanning is pivotal for creating an effective PCB design that meets both performance and reliability criteria. Attention to high-speed and power domains, as well as component optimization, plays a crucial role in this process.
Here are some additional clarifications:
High speed domains: These are the areas of the PCB where high-speed signals are routed. These signals are more susceptible to noise and interference, so it is important to take special care when routing them. Some examples of high-speed signals include clock signals, data signals, and control signals.
Power domains: These are the areas of the PCB where power is distributed. It is important to isolate noisy power domains from low-noise domains to prevent noise from being transferred between them. Some examples of noisy power domains include the power supply rails for digital ICs and the power rails for analog ICs.
MCO: The mechanical component outline is a document that specifies the physical dimensions of the PCB. It is important to ensure that all components fit within the MCO to avoid design errors.
Routing strategy: The routing strategy is the plan for how the signals will be routed on the PCB. The routing strategy should take into account the requirements of the high speed and power domains, as well as the physical constraints of the PCB.
Signal integrity analysis: An evaluation process to ensure the PCB design satisfies performance requirements concerning signal reflection, delays and crosstalk noise. This analysis aids in identifying and resolving potential routing issues. Often tools like Sigrity from Cadence and built in signal integriy analysis from Alutum designer allows one to visualize signal integriy waveform. Example below is from Altium waveform Analusys window: Link here.
