What Is PCB(Printed Circuit Board) ? PCB Basics



Introduction

Printed circuit board, also known as PCB, is the electrical connection provider of the electronic components. It has been developed for more than 100 years; its key point is  about layout design. The main advantage of using circuit board is to greatly reduce wiring and assembly errors, improve the level of automation and productivity.

Since printed circuit boards are not general end products, there is a slight confusion in the definition of it. For example, the motherboard for personal computers is called the motherboard and cannot be called the circuit board directly, although there is a circuit board in the motherboard. They are not the same, so the relationship between the two cannot be said to be the same when evaluating the industry. Another example: because integrated circuit parts are mounted on a circuit board, the news media call it IC board, but in essence it is not equal to a printed circuit board. We commonly refer to PCB as bare board which components are not on it.

PCB Board

Catalog

Classification

Form

Features

Advantages

Four Aspects

PCB Material

Common and Various

Metallic Coating

Definition and Components

Line Design Software App

CAD, Pads, Protel, FreePCB, CAM350

Basic Manufacturing

Additive

Layer Method

Panel Method

Pattern Method

Complete Addition Method

Partial Addition Method

ALIVH

B2it(Buried Bump Interconnection Technology)

Design You Own PCBs

Function Testing

Efficient Testing Requirement

Technical Level

Testing Points

Automatic Exploration

Vendor's design

Boundary Scan

UUT

Design

Components and Wiring

Layout

Effective Layout Significance

Functional Differentiation

Functional zoning

Thermal Magnetic Balance

Surface

Spacing

Direction

Wiring

Wires: Width, Length, Gap, Path

Aperture and Pad

Grounding Design

Video

How A PCB Works

Recycle

Metal Waste

Classification

According to the number of PCB layers can be divided into single panel, double panel, four-layer board, six-layer board and multilayer board.

Form

The current circuit boards consist mainly of the following

The line and Pattern: line is a tool for conducting between the original parts. In the design, the large copper surface will be designed as the grounding and power supply layer. The wiring route is made at the same time as the pattern.

Dielectric is used to maintain insulation between lines and layers.

Through hole / via: it can make the lines above the two layers switch on each other, the larger ones are used to set components, and the non-through holes (nPTH) are usually used as surface mounting positioning and fixing screws for assembly.

Solder resistant / Solder Mask: not all copper surfaces needed solder parts with sin, so the non-tin soldering area, will print something to separate tin (usually epoxy resin), to avoid short circuit. According to different processes, this can be divided into green oil, red oil and blue oil to distinguish different functional areas.

Legend / Marking/ Silk screen: it is not necessary. Its main function is to mark the name and position of each part on the circuit board for easy maintenance and identification after assembly.

Surface Finish: because copper surface is easy to oxidize in the general environment, leading to fail to solder tin ( or solder poor), therefore, it will make the copper surface protection which needing to solder sin. The methods of protection include HASL, ENIG, Immersion Silver, tin, Immersion Tin, and OSP, which are generally called surface treatment, having their own advantages and disadvantages.

Features

Printed Circuit Board

Bare panels (no parts on it) are also known as "Printed Wiring Board (PWB)". The board itself is made of insulating, non-bending material. The thin wire material that can be seen on the surface is copper foil, which is originally covered on the whole board, but is etched away in the manufacturing process, and the remaining part becomes a small net lines. These lines are called conductor pattern or wiring, and are used to provide electrical connections to parts on the PCB.

The color of PCB is usually green or brown, which is the color of solder mask. It is an insulating protective layer that protects copper wire, prevents short circuit caused by wave welding, and saves solder consumption. Also A silk screen will be printed on the solder mask layer. Text and symbols (most are white) are usually printed on them to indicate the location of each part on the board. Silk screen is also called legend.

Integrated circuits, transistors, diodes, passive components (such as resistors, capacitors, connectors, etc.) and a variety of other electronic components are installed when the final product made. By connecting wires, electronic signals can be connected and their functions can be work.

Advantages

(1) Because of the reproducibility and consistency of the graphics, the errors in wiring and assembly are reduced, and the maintenance, debugging and checking time of the equipment are saved;

(2) The design can be standardized and interchangeable;

(3) High density of wiring, small size, light weight, which is beneficial to the miniaturization of electronic equipment;

(4) It is beneficial to mechanization and automatic production, increasing labor productivity and reducing the cost of electronic equipment;

Material

Common material of PCB boards are electric boards, glass fiberboards, and various types of plastic boards. PCB manufacturers generally use an insulating portion consists of glass fiber, non-fabric, and resin, then pressed with epoxy resin and copper foil to form a prepreg.

Metallic Coating 

The metal coating is the place where the substrate line is welded to the electronic component. In addition, metal solderability, contact, resistance and so on which will directly affect the effectiveness of the component. And the cost of production is directly affected by different metals. 

The commonly used metallic coatings are: copper, tin (the thickness is usually 5 to 15 μm), lead-tin alloy (or tin-copper alloy, that is solder, the thickness 5 to 25 μm, about 63% is tin), gold (usually plated on the interface), silver (usually plated on the interface, or as a whole is silver alloy).

Line Design Software App

Simple layout design can be realized by hand, but complex circuit design usually needs to be realized by computer aided design (CAD), and famous design software includes CAD, Pads (that is PowerPCB), Altium designer (that is Protel), FreePCB, CAM350 and so on.


Basic Manufacturing

The manufacturing methods of PCB can be divided into two categories: subtractive and additive method. At present, the etching copper foil method of the subtractive is mainly used in mass industrial production.

blank circuit board (a circuit board with a complete piece of metal foil), the rest of which is the required circuit board. 

Screen printing: a screen mask is made from a pre-designed circuit diagram. The unneeded circuit on the screen is covered with wax or waterproof material. After that,  the screen mask is placed on a blank circuit board, putting protective agent on the screen to avoid be corroded. Finally, put the circuit board into the corrosion solution, the part not covered by the protective agent will be corroded away, and the rest is what we need, cleaning the protective agent away.

Photosensitive board: a pre-designed circuit diagram is made on a transparent film mask (the simplest way is to print the film with a printer), and in the same way, the required part should be printed in an opaque color. Then apply photosensitive pigment to the blank circuit board, put the prepared film mask on the circuit board blazing for a few minutes, remove the mask and display the pattern on the circuit board with developer. Finally, the circuit will be made with the last step as the same as screen printing method.

PCB

Engraving: use a milling machine or laser engraving machine to remove unnecessary parts of a blank line directly.

Additive

The additive, is a pre-coated copper substrate coated with a photoresistor (D/F), exposed to ultraviolet light and exposed where it is needed. Then using electroplating to thicken the copper of the formal circuit line to the required specification, and plating a layer of anti-corrosion thin tin, and finally remove the photoresist (this process is called film removal), and then etch the copper foil layer under the photoresist.

Layer Method

The lamination method is one of the methods of making multilayer printed circuit board. The outer layer is made after the inner layer is wrapped, and the outer layer is treated by the subtractive or additive method. The sequential layer method can be used to get the multi-layer printed circuit board with multiple layers by repeating the action of the stacking method.

1. Making inner layer

2. Laminated formation( bonding different layers)

3. Layer completion ( Outer metal-containing foil film by subtractive method, mixing with the additive method)

4. Drilling

Panel Method

1. Whole PCB electroplating

2. Add a barrier layer where the surface is to be retained

3. Etching

4. Removal of barrier layer

Pattern Method

1. Add a barrier layer to the area where the surface is not required

2. Electroplating requires with thickness 

3. Removal of barrier layer

4. Etching into unnecessary foil film to disappear

Complete Addition Method

1. Add a barrier layer where there is no conductor

2. Circuit consisting of no electrolytic copper

Partial Addition Method

1. Covered with electrolytic copper PCB

2. Add a barrier layer where there is no conductor

3. Electrolytic copper plating

4. Removal of barrier layer

5. No electrolytic copper disappeared until etched under the original barrier layer.

ALIVH

ALIVH(Any Layer Interstitial Via Hole,Any Layer IVA), this is using aramid fiber fabric as the substrate.

1. Prepreg: dip the fabric in an epoxy resin to form a “Adhesive sheet”.

2. Laser drilling.

3. Filling the hole with conductive paste.

4. Attaching copper foil to the outer layer.

5. Making Circuit pattern by etching on copper foi.

6. Gluing the copper foil on the semi-finished product after the second step.

7. Laminated formation

8. Repeating steps 5 to 7 until completed.

B2it(Buried Bump Interconnection Technology)

1. First make a double panel or multilayer board.

2. Printing silver paste as cone on copper foil.

3. To place the adhesive on a silver paste, making silver cone to penetrate the adhesive.

4. Attaching the previous adhesive to the board of the first step.

5. Etching the copper foil of the adhesive into a circuit pattern.

6. Repeat the second to fourth steps until it completed.

PCB Etching

Designing Your Own PCBs

How do you go about designing your own PCB? The ins and outs of PCB design are way too in depth to get into here, but if you really want to get started, here are some pointers:

1. Find a CAD package: there are a lot of low-cost or free options out there on the market for PCB design. Things to consider when choosing a package:

Community support: are there a lot of people using the package? The more people using it, the more likely you are to find ready-made libraries with the parts you need.

Ease-of-use: if it's painful to use it, you won't.

Capability: some programs place limitations on your design- number of layers, number of components, size of board, etc. Most of them allow you to pay for a license to upgrade their capability.

Portability: some free programs do not allow you to export or convert your designs, locking you in to one supplier only. Maybe that’s a fair price to pay for convenience and price, maybe not.

2. Look at other people’s layouts to see what they have done. Open Source Hardware makes this easier than ever.

3. More practice.

4. Maintain low expectations. Your first board design will have lots of problems. Your 20th board design will have fewer, but will still have some. You’ll never get rid of them all.

5. Schematics are important. Trying to design a board without a good schematic in place first is an exercise in futility.

Finally, a few words on the utility of designing your own circuit boards. If you plan on making more than one or two of a given project, the payback on designing a board is pretty good- point-to-point wiring circuits on a protoboard is a hassle, and they tend to be less robust than purpose-designed boards. 


Function Testing

More intensive PCB, with higher bus speed and analog RF circuits pose unprecedented challenges to the testing, where efficient testing requires careful design, thoughtful testing methods and appropriate tools which can provide credible test results.

Technical Level

In high-density UUT, if calibration or diagnosis is required, a manual work is likely to be required. This is because the machine is limited and the test require faster (the UUT can collect data quickly with a probe instead of feedback the information to the edge connector), in this case that the operator is required to probe the test points on the UUT to make sure the test points are clearly marked.

Testing Issues include:

Is the probe bigger than the test point?

Is the probe in danger of shorting several test points and damaging UUT?

Is there a shock hazard to the operator?

Can each operator find out the test point quickly and check it out? 

Are test points large and easy to identify?

How long does it take the operator to press the probe on the test point to get an accurate reading?

If the time is too long, there will be some trouble in the small test area, for example, the operator's hand will slide, so it is recommended to expand the test area to avoid this problem.

After considering the above problems, the test engineer should re-evaluate the type of the test probe, modify the test file to better identify the location of the test point, or even change the requirements for the operator.

Automatic Exploration

In some cases the use of automated probes may be required, such as when PCB is difficult to detect manually, or when the test speed is significantly reduced due to the technical limitations of the operator, under this case which an automated approach should be considered.

Automatic probe can eliminate human error, reduce the possibility of short circuit at several test points, and speed up test operation. However, it should be noted that there may be some limitations to automated probes, depending on the vendor's design, including:

A size of UUT

Number of synchronous probes

How close are the two test points?

Positioning accuracy of the testing probe

Can the system detect UUT on both sides?

How fast does the probe move to the next test point?

What is the actual interval required for the probe system? (it is generally larger than an offline functional test system.)

Automatic detection usually does not touch test points with probe and is generally slower than the production line, so two steps may be required: if the detector is used only for diagnosis, the traditional function test system can be used in the production line, and the detector should be put on the side of the production line as the diagnostic system. If the purpose of the detector is using the UUT to calibrate, it is necessary to use multiple systems, which is still much faster than manual operation.

Another key issue is how to integrate the test system into the production line. Is there still room on the production line? Can the system be connected to the conveyor belt? Fortunately, many new detection systems are compatible with the SMEMA standard, so they work in an online environment.

Boundary Scan

This technology should have been discussed before the product design phase because it requires specialized components to perform the task. In UUT with a digital circuit, devices with IEEE1194 (boundary scan) support can be purchased, so that most diagnostic problems can be solved with little or no detection. But Boundary scanning will reduce the overall functionality of the UUT because it increases the area of each compatible device (4 to 5 pins per chip and some wires). It is aim to improve the diagnosis when choosing this technology. In addition, it is reminded that boundary scans can be used to Flash memory and PLD devices on UUT to program, which further increases the reason for choosing the test method.

Design

In the design of printed circuit board, the layout of components and wiring of circuit connection are two key aspects.

Layout

PCB Layout

Layout, is to put the circuit device in the printed circuit board wiring area. Whether the layout is reasonable or not only affects the wiring work behind, but also has an important effect on the performance of the whole circuit board. After ensuring the function and performance of the circuit, to meet the requirements of process, detection and maintenance, the components should be uniform, neat, compact cloth placed on the PCB to minimize the lead and connection between the components, to obtain uniform assembly density.

The position, input and output signals of each functional circuit unit should arranged according to the circuit flow; the high and low level parts are not crossed as far as possible, and the signal transmission route is the shortest.

Functional Differentiation

The position of components should be grouped according to power-supply voltage, digital and analog circuits, speed, current and so on, so as to avoid interference with each other.

When the digital circuit and analog circuit are installed on the circuit board, the ground wire and the power supply system of the two circuits should separated completely, what’s more, the digital circuit and the analog circuit arranged in different layers if the conditions permit. When the fast, medium and low speed logic circuits need to be arranged on the circuit board, they should be placed close to the connector, while the memory should be placed far away from the connector. In this way, the common impedance coupling, radiation and crosstalk are reduced. Clock circuit and high frequency circuit are the main source of disturbance emitter, must be arranged separately, away from the sensitive circuit.

Thermal Magnetic Balance

The heating parts and the heat sensitive parts are as far away as possible, the influence of electromagnetic compatibility should be considered.

Manufacturability

(1) Surface

The mounting parts are installed on one side as far as possible and simplifies the assembly process.

(2) Spacing

The minimum distance between components is determined according to the shape of components and other related properties. At present, the distance between components is generally not less than 0.2mm~0.3mm, the distance between components and PCB edge should be more than 2mm.

(3) Direction

The direction and density of the elements should be favorable to the convection of the air. Considering the assembly process, the component direction is as consistent as possible.


Wiring

1. Wires

(1) Width

The minimum width of the printed wire is determined by the adhesive strength between the conductor and the insulating substrate and the current value flowing through them. Printed wire can be as wide as possible, especially power lines and ground wires, as wide as possible under the condition of plate surface, even if the area is tight, generally not less than 1mm. In particular, ground wires, even if they are not allowed to be widened locally, it is necessary to widen somewhere permitted to reduce the resistance of the whole ground wire system. For example, the conductors longer than 80mm, even if the current is small, it should be widened to reduce the influence of conductor voltage drop on the circuit.

(2) Length

To minimize the length of the wiring, the shorter the wiring, the less interference and crosstalk, and the lower the parasitic reactance and the less radiation. Especially the FET gate, transistor base and high frequency circuit should pay more attention to short wiring.

(3) Gap

The distance between adjacent conductors should meet the requirements of electrical safety. Crosstalk and voltage breakdown are the main electrical issues affecting the wiring spacing. For ease of operation and production, the spacing should be as wide as possible, and the minimum spacing should be appropriate to the applied voltage. This voltage includes operating voltage, additional fluctuation voltage, overvoltage, and peak voltage for other reasons. When there is a current voltage in the circuit, the spacing should be wider for safety reasons.

(4) Path

The width of the signal path from driver to load should be constant. Changing the path width changes the path impedance (resistance, inductance, and capacitance), resulting in reflection and line impedance imbalance. Therefore, it is best to keep the width of the path unchanged. In addition, for the wiring corner, it is best to avoid right and sharp angles, which should generally be greater than 90 °. The inner edge of the right path can produce a concentrated electric field, which produces noise coupled to the adjacent path, and the 45 °path is superior to the right angle and acute angle paths. When two conductors meet at an acute angle, the acute angle should be changed to a circle.

2. Aperture and Pad  

PCB's Aperture and Pad

The aperture of components should be better matched with the diameter of the lead, in other words, the diameter of the installation hole should slightly larger 0.15~0.3mm than that of the lead diameter of the component. In general, the aperture of DIL packaging pins and most small components is 0.8mm, and the pad diameter is about 2 mm. For large pad aperture, in order to get better adhesion ability, the ratio of the aperture and the diameter of pad is about 2 for epoxy glass plate and 2.5~3 for phenol cardboard.

Perforation, generally used in multilayer PCB, the minimum available diameter of which is related to the thickness of the plate, and the ratio of the plate thickness and aperture is usually 6:1. when having perforation, 1~4nH inductance and 0.3~0. 8pF capacitance are generated by a high speed signal. Therefore, when laying high-speed signal channels, the holes should be kept to an absolute minimum. For high-speed parallel lines (such as address and data lines), if layer changes are inevitable, it is necessary to make sure that the number of holes in each signal line is the same, and the number of holes should be minimized. A printed conductor protection ring or protective line should be set up when necessary to prevent oscillate and improve circuit performance.

3. Grounding Design

Unreasonable grounding design will affect the printed circuit board, fail to reach the design target, and even can not work. The ground wire is the reference of the potential in the circuit and the common current channel. Ground potential value is zero theoretically, but in fact, because of the existence of conductor impedance, the potential everywhere of ground wire is not all zero. As long as the ground wire has a certain length, its potential may not in zero everywhere. The ground wire is not only a necessary common circuit channel, it also a channel for interference.

One point grounding is the basic principle of eliminating grounding interference. The ground wire of all circuits and devices must be connected to a unified grounding point, which is used as the circuit and the zero potential reference point of the equipment. One point grounding is divided into common ground wire series grounding and independent wires parallel grounding.

The common ground-wire series grounding is simple. The grounding lead of each circuit is relatively short, and its resistance is relatively small. This kind of grounding method is often used in the earthing of the equipment cabinet. Theindependent wires parallel grounding has one ground point which is defined as the ground reference point. The other points that need to be grounded are directly connected to this point, and the earth potential of each circuit is related only to the ground current base impedance of the circuit, will not affected by other circuits.

The Following Points Should Be Noted in Specific Wiring:

(1). The line length is as short as possible in order to minimize the lead inductance. In low frequency circuits, multipoint grounding is avoided because the ground current of all circuits flows through a common grounding impedance or grounding plane.

(2). Common ground wires should be arranged as far as possible on the edge of the printed circuit board. As much copper foil as possible should be retained on the circuit board as the ground wire, which can enhance shielding ability.

(3). The double-layer plate can use the ground surface, the purpose of which is to provide a low-impedance ground wire.

(4). In multi-layer printed circuit board, grounding layer can be set, and it is designed as a mesh. The spacing of the earth grid can not be too large because one of the main functions of the earth wire is to provide the signal return path. If the spacing of the grid is large signal-loop area will be formed, which will cause radiation and sensitivity problems. In addition, if the signal reflux path is a small loop area, other ground lines will not take into effect.

(5). The earth surface can minimize the radiation loop.


How Do PCBs Work

Recycle

PCB manufacturing technology is a very complex, comprehensive processing technology. Especially in the process of wet processing, a large amount of water is needed, so there are many kinds of heavy metal wastewater and organic wastewater discharged. The composition is complex, and the treatment is difficult. If the copper foil utilization ratio of printed circuit board is 30% and 40%, most of the copper content is in waste water. If the thickness of each copper foil is 35 microns based on 10, 000 square meters of double panels, the waste water contains about 4500 kilograms of copper, and there are many other heavy metals and precious metals. These are found in waste liquid and waste water, if the metal is discharged without treatment, it is not only a big waste but also pollutes the environment. Therefore, the treatment of waste water and the recycle of copper and other metals in the process of PCB production are of great significance and are indispensable parts in PCB production.

It is well known that the waste water in the production of printed circuit board is a large amount of copper, and a very small amount of lead, tin, gold, silver, fluorine, ammonia, organic compounds and organic complexes, etc.

As for the production of copper wastewater, the main processes are: copper sink, copper plating, copper electroplating, etching and various PCB pretreatment processes (chemical pretreatment, brush plate pretreatment, pozzolanic ash grinding plate pretreatment, etc.).

The copper-containing wastewater produced by the above processes can be divided into complex wastewater and non-complex wastewater according to its composition. In order to make the wastewater treatment meet the environment-protection standard, and the maximum allowable compound concentration of copper is 1mg/l (according to copper), but different wastewater treatment methods must be adopted for different copper containing wastewater.

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