PCB, short for Printed Circuit Board, is the fundamental platform to carry electronic components to achieve corresponding functions. Based on substrate material, PCB is fabricated in conformity with PCB design files with connection achieved between board layers, board and components. The leading function of PCB lies in its relay transmission role with full contribution to the electrical connection between all parts surrounding the circuit board. Therefore, PCB is usually regarded as the core of electronics products.
PCB should be manufactured strictly in accordance with PCB design files including Gerber files, NC drill files, stencil design files etc. all of which together will finally lead to real PCBs. This article will provide a quick guide to PCB layout to PCB design beginners, covering key issues concerning PCB design and layout. It’s hoped that this article will be a bandage for electronic beginner engineers.
What is PCB Layout?
PCB layout mainly contains component placement on board, routing, trace width, trace spacing etc. Since PCB board is applied in nearly all electronic products, PCBs have been widely applied in fields such as consumer electronics, information, telecommunication, medical care or even aerospace. PCB layout plays an essential role in affecting their expected functions and performance.
PCB Layout Basics
In the process of drawing schematics with PCB design software, it’s essential to master abbreviations of electronics since the beginning three letters are usually used to stand for a terms. For example, RES stands for resistor; CAP stands for capacitor; IND stands for inductor. Thus, it’s of great necessity to master some electronic terms: voltage, current, Ohm, Volt, Ampere, Watt, circuit, circuit element, resistance, resistor, inductance, inductor, capacitance, capacitor, Ohm’s law, Kirchhoff’s law, Kirchhoff’s voltage law (KVL), Kirchhoff’s current law (KCL), loop, network, passive two-terminal network, active two-terminal network.
Unavoidable Issues to Consider in PCB Layout
• Minimum Distance
A PCB design should feature a frame and the minimum distance between frame line and component pin should be at least 2mm and it is rational to set it to be 5mm.
• Component Placement
Basically, when it comes to a circuit system containing digital circuit and analog circuit, they should be separated in order to make systems systematically coupled into circuit belonging to the same category. Additionally, components should be placed according to signal flowing direction, functions and modules.
Input signal processing unit and output signal driving components should be placed near board side in order to make input/output signal lines as short as possible and reduce input/output interference.
In terms of component placement direction, components can only be placed vertically or horizontally. If a relatively high potential difference is available between components, the distance between components should be large enough to stop discharging.
As far as a circuit board with mid density is concerned, the distance between components with low power should be considered based on soldering. When wave soldering is selected, the distance between components can be in the range from 50mil to 100mil.
Power and Ground Line Design in PCB Layout
It’s not a difficult task for PCB design engineers to understand the cause for the noise generation between ground lines and power lines. Even if PCB layout is excellently carried out, the interference due to insufficient consideration on power and ground lines arrangement will still reduce product performance, or even lead to total failure. Therefore, it’s PCB layout engineer’s job to reduce noise interference as much as possible so as to guarantee product’s quality with the methods below:
a. Large-area copper layer is used as ground lines and all the unused parts should be connected with ground, which can be used as ground lines. In terms of multi-layer PCB, power and ground lines should be arranged in different layers respectively.
b. Decoupling capacitor should be added between power and ground lines.
c. The width of ground lines and power lines should be set as much as possible. It’s best to make ground lines wider than power lines. The width arrangement of ground lines, power lines and signal lines should be: ground lines > power lines > signal lines.
d. Wide ground conducting lines should be used to make a loop on a PCB with digital circuit.
Three Tips to Reduce EMI in PCB Layout
EMC (Electromagnetic Compatibility) maintenance is a must be in PCB layout. The implementation of EMC aims to reduce EMI (Electromagnetic Interference) as much as possible. To decrease EMI, the following three elements should be focused: electromagnetic interference source, coupling path and victim.
To achieve EMC, measures should start from the above elements. First, interference source, coupling path and sensitive devices should be analyzed and effective measures should be summarized and made to stop interference source, eliminate or reduce interference coupling, decrease the response made by sensitive devices on interference or increase electromagnetic sensitivity level.
In order to restrict interference caused by human and testify the validity of the applied technical measures, organizational measures should be also made. Thus, a set of complete regulations and standards should be made and followed with spectrum reasonably distributed. Additionally, the application of spectrum should be controlled and managed and working mode should be determined according to frequency, working time and antenna direction. Electromagnetic environment should be analyzed and placement should be selected with EMC administration carried out.
• Electromagnetic Interference Source
EMI source refers to any type (natural or radiated by electrical device) of electromagnetic energy that will do harm to people or devices in the same environment or will bring forward EMI damage to other devices, subsystem or the whole system, leading to performance degrading or failure.
• Coupling Path
Coupling path refers to the access or media used to transmit EMI.
Victim refers to the human or systems damaged by EMI, including components, equipment, subsystem or systems suffering from performance downgrading or failure.