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An Introduction to Printed Circuit Boards: What are Printed Circuit Boards

Printed Circuit Boards

An Introduction to Printed Circuit Boards: What are Printed Circuit Boards

We now exist in a world where our daily activities have been made more convenient and efficient. Take the example of the most carried and used handheld device—mobile phones. Everyone is dependent on gadgets for communication, transportation and information. Technology has greatly evolved to serve better functions with its intricacies hidden inside these nicely packaged gadgets. If you ever tried to crack open your electronic devices, you will find green boards where tiny components are mounted. These boards are called Printed Circuit Boards (PCB). What are PCBs and how are they made? We will dissect these common questions so we gain an understanding of the technology of printed circuit boards.

Table of contents

History of Printed Circuit Boards
Different Layers of PCB Consists
Classifications of Printed Circuit Boards
(1) Single Layer PCB
(2) Double Layer PCB
(3) Multi-Layer PCB
(4) Flexible PCB
(5) Rigid PCB
(6) Rigid-Flex PCB
Classifications of the components used in Printed Circuit Boards
(1) Active Components
(2) Passive Components
Two Methods to Mount Components on Printed Circuit Boards
The Testing Methods in Printed Circuit Boards Production
(1) In-Circuit Test (ICT) and Flying Probe Test
(2) Functional Test
(3) Visual Inspection
Summary

History of Printed Circuit Boards

Printed Circuit Boards serve as a mechanical support and a medium for electrical connectivity between circuits. The commercial use of PCBs dates back to the 1950s and has immensely progressed with the intent to expand its applications. The first relevant patent was contributed by Charles Ducas for his idea to attach metal deposits into an insulation material by utilizing a stencil. A breakthrough in PCB was then developed by Paul Eisler who suggested a copper clad insulation to use as base sheet for the fabrication of circuit boards. He introduced the etching process to form a circuit pattern onto the surface of the copper sheet. The PCB technology was further developed up to the present times to suffice the demand of consumer electronics.

Different Layers of PCB Consists

The layers of a PCB consist of an insulating layer, copper and solder masks. The insulating base material, also called substrate or laminate, acts as a platform on where the copper sits. This layer can be rigid or flexible. The most popular type is called FR4 which is an acronym for Fire Retardant Epoxy. It is a fiberglass-reinforced laminate that has high strength and good thermal and electrical properties.

The copper foil layer is the conductive medium which serves as a pathway for electrical signals. Copper has the benefits of electrical conductivity, mechanical properties and lower cost compared to other known metals. The copper layer provides the contact points for the components of the circuit board. The thickness of the copper layer has an impact on the flow charges and the subsequent etching process. Higher copper thickness is applicable for high-power applications.

The solder mask is what makes PCBs green. It is a polymer material that acts as an insulating and protective material. Through the solder mask, the function of a circuit is shielded from potential threats like foreign material, moisture and physical damage. It is literally called a mask since the exposed parts or openings are the areas where solder is required and where components are bonded.

Printed Circuit Board Layers

Printed Circuit Board Layers

Classifications of Printed Circuit Boards

As we now understand the basic stacks of the PCB, we will now proceed to the different PCB board types. There are several types of PCBs, which vary based on their specific characteristics:

1. Single Layer PCB

Single Layer is the most basic and least-cost type wherein the components are mounted on one side of base material. This type of PCB is often applied in simple electronic applications.

2. Double Layer PCB

A Double Layer PCB has conductive layers on two sides and is drilled with holes also termed as vias to connect the circuits. The circuit is situated on both sides of the substrate.

3. Multi-Layer PCB

Multi-Layer is a more complex type with several layers of PCBs with applications like data storage, military applications and GPS technology. It can be visualized as multiple PCBs sandwiched together with a higher density of components than other traditional types of circuit boards.

4. Flexible PCB

Flexible Circuits is a PCB type that can be bent to take advantage of space and weight in a device. This type of circuit is ideal for portable technology such as cameras and mobile phones. Polyimide material is the most widely used substrate for flexible PCB.

5. Rigid PCB

Rigid boards are PCBs that cannot be bent, which is the most commonly used circuit board. The material can be FR-4, CEM-1, CEM-3, aluminium, metal, ceramic., etc.

6. Rigid-Flex PCB

Rigid-Flex boards, as the name implies, are a combination of rigid and flexible circuits joined together to take advantage of volume and structural strength.

Classifications of The Components Used in Printed Circuit Boards

To get fully functional circuit boards, we need to assemble many kinds of electronic components on the PCB bare boards. The PCB circuit elements can be categorized into Active and Passive Components.

1. Active Components

The active components include microprocessors and logic gates that have outputs and inputs in the circuit. The active components have the ability to control signals by relying on an external power source.

2. Passive Components

The passive components include resistors, inductors and capacitors. The passive components do not require an input source to control electrical signals and to perform their functions. The design and layout of these components are dictated by the operating parameters of the PCB such as clock speed, power rating and thermal management.

Printed Circuit Board Testing

Printed Circuit Board Testing

Two Methods to Mount Components on Printed Circuit Boards

Generally speaking, the PCB mounting process has two types—Plated through-hole Technology (PTH) and Surface Mount Technology (SMT).

In the PTH method, the leads of the components are inserted through the holes and attached to the opposite side of the PCBs. The drilled hole is plated with copper to allow current to flow through.

The SMT process is the most commonly used assembly method wherein the component leads are mounted on the PCB pads. Solder paste is first applied through an SMT machine and components are picked and placed to bond the active and passive components. The boards are then conveyed into a reflow machine to cure the solder paste.

The Testing Methods in Printed Circuit Boards Production

The general fabrication process of PCBs includes substrate preparation, photolithography, etching, drilling, plating, solder mask and silkscreen printing, surface treatment and components soldering. Before shipment, these boards are inspected and tested to determine their functionality. There are several types of inspection, which include manual and automatic inspection depending on the capability of the manufacturer and the desired screenability. The following are the testing methods in the production of printed circuit boards.

1. In-Circuit Test (ICT) and Flying Probe Test

The main testing processes are In-Circuit Test (ICT) and Flying Probe Test. ICT makes use of a fixture to check the electrical connectivity between the components. The ICT is often advantageous in large-scale production. The Flying Probe Test has probes that are movable to be able to touch the contact points in the PCB. This test method is often used for small-scale or prototype runs.

2. Functional Test

Functional tests are also being done to simulate the functionality of the PCB by running signals into the circuitry and measuring its electrical characteristics.

3. Visual Inspection

Besides, visual inspection is also being performed to screen out defects on the PCBs.

Summary

Printed circuit boards are the basic core of electronic devices that we enjoy and use daily. We have tackled the fundamentals of the PCBs and we are prepared to go deeper into the complexities of this technology. It is also important to seek technical advice from professionals in this field before diving into your PCB project.

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