Guide

2-Layer PCBs: A Beginner’s Guide to 2 Layer Printed Circuit Boards

Double Layer PCB

Introduction to 2-layer PCBs

The 2-layer PCB is a basic PCB structure built on an FR4 material with copper on both sides etched to parametric patterns to get the desired structure of the component placement as designed.

A printed circuit board is a compact version of connecting various components of a circuit without using wires and the need for mechanical structures to support building a circuit. Printed Circuit board gives not only mechanical support but ambient electrical parameters based on manufacturability.

Key Components in 2-layer PCBs

Electronic Component and Symbols: Major Electronic components like Resistors, capacitors, inductors, diodes, transistors, and ICs have their symbol.

Footprint & Packages:  Footprint and Packages are nothing but the size of the electrical connecting point on the board for every component. Majorly there are two classifications for them

  1.  THT (Through hole Technology): Components are through hole packages and applicable for all electronic component classification. The electrical points are accessible in every layer that a PCB possesses. For Integrated Circuits SIP & DIP are major packages used.
  2. SMD (Surface Mount Devices): Surface Mount Devices have electrical conductivity space only on the top layer of the Printer circuit board as the name depicts. IC packages for Surface mount PCB consist of a variety of packages like SOP, BGA, SOT-23, QFN, QFP, and SOT223 are the few used widely across. Passive components like resistors and capacitors have packages like 01005, 0201, 0402, 0603,0805 … to 4440 or more

Lines /Connections:  Connections indicate the electrical connection on the Printed Circuit board. For better understanding and to keep lines easy, we use netlabels or annotation to indicate the purpose of the connection.

Power Sources and Ground: Power sources and Ground indicate the power supply concretions that power up the circuitry according to the topology generally all these connections are intended to have high current traces so they are wide and have polygon pours.

Component Placement for 2-layer Circuit Board: Component placement depends on various parameters like Functionality, Electrical parameters, power, thermal, and connection flow. The components are generally grouped together based on the logical functions by which they are designed. Components are placed in a way to have minimal routing distance based on priority and feasibility which also includes high-level signal integrity parameters.

Routing: The trace lengths have to be kept minimal for High-speed signal lines to reduce decaying of signals and interferences. Using wider traces for high-current lines minimizes the wire resistance and voltage drop. Traces are the most common term to coin electrical connectivity. Traces are routed on the copper layers of the PCB. Copper layers are etched to the desired shape to give proper conductivity and isolation with respect to adjacent traces.

Basic Stack up of 2-layer PCB

PCB stack-up is the material built for standard two sided PCB, the cross-section of the layers used is generally called a stack.

Stack-up of 2-Layer PCB

The stack-up of the 2-layer circuit board is shown in the above image. The inner part of the 2-layer PCB is built using an FR4 core as substrate material which is the standard 2-layer PCB core, which is 1.5 mm thick as depicted in the image.

The term Top layer and Bottom layer is nothing but the copper layer which is present on top of it. The copper layers are etched. The thickness of the copper layer can be 0.5 Oz or 1Oz, 2Oz or a little higher but it is a little expensive. The thickness of the copper depicts the current carrying capability, where the heavier the copper more the conductivity. There is another layer called Prepreg in the top and bottom layer which is again an insulating material diving proper isolation between adjacent copper layers.

Advantages & Disadvantages of 2-layer PCB

Advantages of 2-layer PCB 

  1. The manufacturing cost of 2-layer PCB is much less when compared to multi-layer PCB
  2. The size is reduced when compared to single-layer PCBs

Disadvantages of 2-layer PCB

  1. High-density designs cannot be incorporated
  2. Manufacturing needs more work when compared to single-sided PCB
  3. The board takes up large space in case of more subsystems in the circuit

Applications of 2-layer PCB

  1. Power supply designs
  2. LED lighting PCB
  3. Hard drives
  4. Vending machines
  5. Traffic lights
  6. Power conversion circuits
  7. Simple consumer electronics

Parameters to be Considered in 2-layer Circuit Board Routing

Few essential points below need to be followed in 2-layer PCB routing. Unlike multi-layer, we don’t have any specified stack up for two-layer PCB. The PCB needs to be compact and simple.

  1. The inter-layer connectivity of a 2-layer circuit board is achieved through via. Via is a hole drilled on a Printed circuit board and plated with metal on the inner layer of it. This not only helps to connect the components on the other layer but also helps in the cross-over of traces.
  2. The schematic of a circuit contains all the circuit connections and the circuit design is imported to the PCB file in the project.
  3. Ensuring the return path is clear and functionally better for the circuitry and return or ground path is larger and less resistive if not it might cause power and signal integrity issues on the 2-layer PCB.
  4. Decoupling capacitors are placed close to the Power pins of the IC to stabilize power rails and filter out high-frequency noise
  5. Grounding also has to be isolated and connected from one point to another, it can be classified as digital ground or analog ground, and in a few cases, it is vital to use separate power ground.
  6. The grounding techniques can also be classified as star grounding, Ground grid, and ground stitching used based on the needs of the circuits
  7. If the power or ground planes have many cuts or holes then it might lead to integrity issues.
  8. Mechanical considerations play a vital role in the connector, casing, box build, and grade.
  9. Power distribution should also be taken care of as minimizing voltage drops, noise, and fluctuations is essential
  10. Silkscreen is nothing but Text written on top of any printed circuit boards to mark the name or designator of a component, Test point functionality, Connector name, orientation, first pin of Integrated circuits, Cathode pin of diode, etc.

Setting Rules in 2-layer PCB Routing

Setting design rules is the primary step before any board design is started. The rules consist of various rule parameters including Clearance, Short circuit, Unrouted Net, Un poured polygon, Creepage distance, routing width, topology, priority, layers, via style, routing corners,  differential pair routing, SMT, Mask parameters including paste mask expansion, solder mask expansion, Plane connect, plane clearance, polygon connect, net antenna, silk to silk clearance, silk to solder and also various rules that can be customized including length matching in high-speed category. Design guidelines are also followed as per IPC standards at an earlier stage. A few rules and options are listed below for understanding.

Set Rules

Set Rules

The rules are set based on the Printed circuit board manufacturer’s capability and also the complexity of the board.

Routing Topology

Routing Topology

The above image says the routing topology options available, where the best practices are followed ensuring no sharp traces or polygons are used on the board.

Trace Width

Trace Width

The trace width of the components can be set according to requirements and feasibility.

Rules in a 2-layer PCB board can be customized to many levels as many ECAD tools support the design by providing many options. As we conclude and understand every circuit functionality, signal, power, EMI/EMC factors need to be considered while designing any PCB.

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