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Dec 30 2019

Basic Knowledge of PCB


A printed circuit board (PCB) mechanically supports and electrically connects electrical or electronic components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. A PCB allows signals and power to be routed between physical devices.

What are PCBs? How Do PCBs Work?



Ⅰ PCB Basics

1.1 PCB Materials

Ⅱ PCB Product Characteristics

Ⅲ Common Sense of PCB Process

PCB Basics

1.1 PCB Materials

a. Copper Clad Laminate(referred to as CCL, or sheet material)

Tg: Glass Transition Temperature, which is the temperature at which glassy substances are transformed between glassy and highly elastic (usually softened). In the PCB industry, this glassy substance is generally referred to as resin or dielectric layer composed of resin and glass fiber cloth. Tg is an important technical index reflecting the heat resistance of printed circuit board substrates. Generally, the higher the Tg value of printed circuit boards, the better the heat resistance. The Tg of general FR-4 copper-clad epoxy glass laminates for SMT printed circuit boards is 130 ~ 140℃, which can meet the requirements when using Sn-Pn solder. For lead-free solders with higher melting points, the Tg of the substrate can not withstand the high temperature of welding if the Tg ≤ 150 ° C. In special cases (high temperature use), the Tg can be greater than 170 ° C. However, excessive Tg will cause the hardness of the substrate to increase and the material to become brittle. Therefore, we cannot simply pursue high Tg. We should comprehensively consider the performance of the board and choose a suitable printed board substrate with a higher Tg, which is one of the requirements for lead-free welding.

CTI: Comparative Tracking lndex (or comparative leakage index, tracking index). The highest voltage value that the surface of the material can withstand 50 drops of electrolyte (0.1% ammonium chloride aqueous solution) without the formation of leakage traces. The unit is V.

CTE: Coefficient of thermal expansion. Generally, the sheet material performance of a PCB is measured by the linear expansion coefficient, which is defined as the ratio of the increase in length to the original length under a unit temperature change, such as Z-CTE. The lower the CTE value, the better the dimensional stability and vice versa.

TD: Thermal decomposition temperature refers to the temperature at which the base material resin loses weight by 5%. It is a sign of delamination and performance degradation caused by the heat of the base material of printed circuit boards.

CAF: Conductive Anodic Filament of printed boards is the phenomenon of electrochemical insulation damage on insulating substrates. It refers to the state where dendritic metals are precipitated between wires under the action of an electric field after a voltage is applied to parallel circuits on printed circuit boards. Or conductive anodic filament(CAF) occurs along the glass fiber surface of the substrate, thereby reducing the insulation between the wires.

T288: It is a technical index that reflects the welding resistance of the printed circuit board substrate. It refers to the maximum time that the printed circuit board substrate can withstand the high temperature of welding at 288°C without blistering and delamination. The longer this time, the better it is for welding. For traditional Sn-Pb alloys with low welding temperature (220 ~ 230℃), when the thermal decomposition time of printed circuit board substrate is 260 ℃, T260≥30s can meet the requirements of SMT printed circuit board. For lead-free welding, the temperature is generally 250 ~ 260℃, the thermal decomposition temperature of the substrate of the printed circuit board will increase, and only T288≥300s at 288 ℃ can ensure that the substrate does not decompose and the performance is not damaged during welding.

DK: dielectric constant.

DF: Dissipation factor refers to the ratio of the energy that has been lost in the insulation sheet material of the signal line to the energy that still exists in the line.

OZ: oz is the abbreviation of the symbol ounce, which is British measurement unit and also unit of weight. 1 OZ means the thickness of copper with a weight of 1 OZ evenly spread over an area of 1 square foot (FT2). It is the average thickness of copper foil expressed by the weight per unit area. It is expressed by a formula, that is, 1OZ = 28.35g / FT2.


b. Copper Foil

ED Copper Foil: Electrodeposited copper, copper foil commonly used in PCB, cheap.

RA Copper Foil: Rolled annealed copper, copper foil commonly used in FPC.

Drum Side: smooth side of electrodeposited copper foil

Matt Side: rough side of electrodeposited copper foil

Copper: Elemental symbol Cu, atomic weight 63.5, density 8.89 g / cm3, and the electrochemical equivalent of Cu2 + is 1.186 g / Ah.


c. Prepreg: referred to as PP

Epoxy Resin: an organic polymer compound containing two or more epoxy groups in the resin molecule, which is a resin component used in prepregs that are currently commonly used.

DICY: Dicyandiamide, a common hardener

R.C: resin content

R.F: resin flow

G.T: gel time

V.C: volatile content

Harden: Under certain conditions (high temperature, high pressure or light), the epoxy resin and the hardener undergo cross-linking polymerization to form a polymer with a three-dimensional mesh structure.


d. Ink

Viscosity: Viscosity refers to the relative movement between adjacent fluid layers when fluid is flowing, and frictional resistance will be generated between the two fluid layers. Unit: Pascal. Seconds (pa.s).

Hardness: The hardness of the ink after pre-baking is 2B, the hardness of the ink after exposure is 2H, and the hardness of the ink after finishing is 6H. 

Thixotropic: a property that the ink is gelatinous when it is left to stand, but its viscosity changes when it is touched. It is a physical property of a liquid, that is, its viscosity decreases under agitation, and it will return to its original viscosity soon after standing. By stirring, the thixotropic effect lasts for a long time and it is enough to reconstitute its internal structure. To achieve high-quality screen printing, the thixotropic of the ink is very important. In particular, during the squeegee process, the ink is agitated to make it liquid. This action speeds up the speed of ink passing through the mesh, and promotes the original ink with separate wires to be evenly integrated. Once the squeegee stops moving, the ink returns to a stationary state, and its viscosity quickly returns to the original required data.


e. Dry Film

· Structure of dry film

Figure 1. Structure of Dry Film

Figure 1. Structure of Dry Film

· Dry film consists of three parts and ingredients:

   — Supporting film(Polyester)

   — Photo-resist dry film

   — Covering film(Polyethylene)

· Main ingredients

① Binder(film-forming resin) ②Monomer ③Photo-initiator ④Plasticiser ⑤Adhesion promoter ⑥Thermal polymerization inhibitor ⑦Dye ⑧Solvent

· The types of dry film are divided into three types according to the different methods of developing and removing the dry film: solvent-based dry film, water-soluble dry film, and peel-off dry film; according to the purpose of dry film, it is divided into: dry resist film, masking dry film and solder mask dry film.

· Photosensitivity: It refers to the amount of light energy required for the photoresist to react to form a polymer with a certain resistance under the irradiation of ultraviolet light. In the case of constant light source intensity and light distance, the sensing speed is expressed as the length of exposure time. Short exposure time means that the sensing speed is fast.

· Resolution: refers to the number of lines (or intervals) that can be formed by the dry film resist within a distance of 1mm. The resolution can also be expressed by the absolute size of the lines (or intervals).


f. Net Yarn

Net density:

— T number \ mesh number: refers to the number of meshes within 1 cm.


g. Drill bit

· geometry structure name of drill bit

Figure 2. Geometry Structure Name of Drill Bit

Figure 2. Geometry Structure Name of Drill Bit

· Point Angle

The point angle is composed of two narrow and long first point angle surfaces and two triangular hook-shaped second point angle surfaces. These four sides meet at the point angle, forming two short edges called chisel edges at the center of the joint. This is the place that the sheet material first touches. This chisel edge is first positioned under pressure and rotation to drill into the stack. A protruding square strip on each of the two outer sides of the first point angle surface is called a margin. This margin tends to spiral upward along with the drill body part, which is the contact part between the drill pin and the hole wall. The right angle at the intersection of the margin and the edge lip is very important to the quality of the hole wall. The point angle has a long edge between the first and second point angle surfaces. The point where the two long edges and the two chisel edges meet in the middle is the point angle. The angle formed by the two long edges is called the point angle. When the drilling paper is made of phenolic resin substrate, the drill point angle is about 90° ~ 110° due to less resistance. When drilling paper is FR4 glass fiber board, the point angle should be slightly blunt at 115° ~ 135°. The most common one is 130°. The angle between the first point angle surface and the horizontal plane of the long edge is about 15°, which is called primary face angle. The second point angle is about 30°, and the angle formed by the chisel edge and the edge lip is called a cheisel edge angle. 

· Types of drill bit

Figure 3. Types of Drill Bit

Figure 3. Types of Drill Bit


PCB Product Characteristics

1) Impedance

· The sum of resistance and reactance (capacitance, inductive reactance) on a vector. Common impedance types are characteristic impedance and differential impedance.

2) Warpage

· Maximum bow (Figure a) and twist (Figure b) of printed circuit boards using surface mount components should be less than or equal to 0.75%

Figure 4

3) RoHS

RoHS, the abbreviation of restriction of hazardous substances, is the restriction of the use of certain hazardous substances in electrical and electronic equipment. RoHS lists a total of six harmful substances, including: lead(Pb), cadmium(Cd), mercury(Hg), hexavalent chromium( Cr6+), polybrominated diphenyl ether(PBDE), polybrominated biphenyl(PBB).

4) Backlight

It is an enlarged visual inspection method to check the integrity of the copper wall of the through hole. The method is to carefully thin the substrate outside the hole wall from a certain direction, and then use the principle of resin translucency to shoot light from the back. If the quality of the chemical copper hole wall is intact and there is no any holes or pinholes, the copper layer must be able to block light and be dark in the microscope. Once there are holes in the copper wall, light spots must appear and be observed , and can be enlarged as photographic evidence. The ground sample is about 4-6mm wide.

Figure 5. Backlight Standard Diageam

Figure 5. Backlight Standard Diageam

5) Anode Phosphor Copper Ball

· Purity requirements


























· Features

— A black (or brown-black) film forms on the surface of phosphor copper after power is applied.

— Black (or brown-black) film is Cu3P, also called phosphor copper anode film.

· Role of phosphor copper anode film

— The anode film itself can catalyze and accelerate the (Cu+-E → Cu2+) reaction, thereby reducing the accumulation of CU+.

— After the anode film is formed, it can inhibit the continuous generation of Cu+.

— The electrical conductivity of the anode film is 1.5X104-1 cm-1, which has metal conductivity.

— Phosphor copper is less anodized than pure copper (1A / DM2 P0.04-0.065%, phosphor copper is less anodized than oxygen-free copper, 50MV-80MV) and will not cause anode passivation.

— Anode film will greatly reduce the phenomenon of tiny grains falling off the anode.

— Anode film prevents the copper anode from dissolving too quickly.

6) Method for Estimating Surface Area of Electroplated Copper Anode

· Method for estimating surface area of round titanium basket copper anode: pDLF/2

p = 3.14 D = diameter of titanium basket L = length of titanium basket F = factor

· Method for estimating surface area of square titanium basket copper anode: 1.33LWF

L = length of titanium basket W = width of titanium basket F = factor

· F is related to the diameter of the copper ball:

Diameter = 12 mm    F = 2.2

Diameter = 15 mm    F = 2.0

Diameter = 25 mm    F = 1.7

Diameter = 28 mm    F = 1.6

Diameter = 38 mm    F = 1.2

7) ICD Issues

Internal Connection Defects


 Common Sense of PCB Process

a. Etching Factor

The index used to consider the amount of etching lateral erosion, because the amount of lateral erosion will be different for different copper thickness, so the etching factor is different from the total copper thickness.

Calculation method:

Figure 6

b. Lateral Erosion

The etching of the side wall of the wire under the resist pattern is called lateral erosion, and the degree of lateral erosion is expressed by the width of the side etching:

Figure 7

· lateral erosion is related to the type, composition and etching process and equipment of the etching solution.


c. Pool Effect

During the etching process, the circuit board passes through the etching machine horizontally. Due to the gravity acting on it, the fresh medicine is blocked by the old one and cannot effectively react with the copper surface.


d. Different Stages of Resins

A-stage resin: Some thermosetting resins are liquid in the early stages of manufacture or liquid when heated, and can still dissolve in some liquids at this time.

B-stage resin: Some thermosetting resins can soften when heated in the middle stage of the reaction, but they cannot be completely dissolved or melted. At this time, they can swell or partially dissolve when contacted with some solvents.

C-stage resin: The later stage of the reaction of some thermosetting resins, when it is practically insoluble or infusible.


e. Font Color of Substrate

Red font: flame retardant grade, other fonts: non flame retardant grade.


f. Others

MSDS: Material Safety Data Sheet provides a variety of information on safety, health and environmental protection for chemical substances and products, and can provide information on basic knowledge, protective measures and emergency actions of chemicals. In some countries, MSDS is also called SDS, and SDS terminology is used in ISO 11014.

SGS: Societe Generale de Surveillance S.A. Founded in 1887, it is currently the world's largest and oldest non-governmental third party engaged in product quality control and technical certification of multinational companies. Headquartered in Geneva, it has 251 branches around the world, 256 professional laboratories and 27,000 professional and technical personnel, and carries out product quality inspection, monitoring and assurance activities in 142 countries.


IPC: The Institute for International and Packaging Electronic Circuits

ISO: International Standards Organization

MIL: Military Standard

JPC: Japan Printed Circuit Association

COV: Coefficient of variation

FR4: Abbreviation for Flame Retardant Type 4. It is the name of a flame-resistant printed circuit board material composed of a composite material of glass fiber and epoxy resin. It is the most widely used printed circuit board.


g. pH Value

Also known as the hydrogen ion concentration index and pH value, it is a scale of the hydrogen ion activity in a solution, which is a measure of the acidity and alkalinity of a solution in the usual sense. The concept was proposed by Danish biochemist Søren Peter Lauritz Sørensen in 1909. P stands for German Potenz, which means strength or concentration, and H stands for hydrogen ion (H +). Sometimes pH is also written in Latin as pondus hydrogenii.

Under normal conditions (25°C, about 298K), when pH <7, the solution is acidic, when pH> 7, the solution is alkaline, and when pH = 7, the solution is neutral.


h. Hull Cell Test

Hull designed Hull cell in 1939. The Hull cell test only requires a small amount of plating solution. After a short time test, the plating effect of the plating solution can be obtained in a wide range of current density. Because this test is sensitive to the composition and operating conditions of the plating solution, it is commonly used to determine the concentration and pH value of each component of the plating solution, and to determine the current density range for obtaining a good coating. The Hull cell has become an indispensable tool for electroplating research and electroplating process control.

Hull cells are usually made of insulating materials such as plexiglass or rigid polyvinyl chloride. The bottom surface is trapezoidal, and the cathode and anode are placed on two sides that are not parallel. There are five types of capacity: 250ml, 267ml, 320ml, 534ml, and 1000ml. Generally, 250ml plating solution is often added to a 267ml test cell, which is convenient for converting the additives into how many grams per liter.

Figure 8. Dimensions of Hull Cell

Figure 8. Dimensions of Hull Cell

i. Current Density A / dm2

A/dm2 — how many amperes per square decimeter area, 1A / dm2 — This is the current density of electroplating, which means that the current passes through the plating area of the workpiece per square decimeter is 1A and dm means decimeter. Generally, we use a 267ml Hull cell, place the test piece at the cathode and immerse it in the test solution. The area of the test piece is approximately 1dm2.



Calculation method:

Figure 9

Figure 10..jpg

k. Replacement Reaction

A replacement reaction is a chemical reaction in which a simple substance reacts with a compound to form another simple substance and a compound. Any replacement reaction is a metathesis reaction, including a reaction of metal and metal salt, the reaction of metal and acid, etc.; the replacement reaction must be a redox reaction, and the redox reaction is not necessarily a replacement reaction; the replacement reaction occurs according to the active list of metals.


l. EDS:Energy Dispersive X-ray Spectroscopy

The surface of the sample being tested is irradiated with a condensed electron beam. Due to the interaction between the electron beam and the sample, various electrons or X-rays, photons, and other information are generated. Then, this information is collected and processed in different ways to display various characteristics of the sample (morphology, microstructure, composition, crystal plane, etc.)


m. SEM:Scanning Electron Microscope

An electron beam with a diameter of 20 mm to 30 mm emitted from the cathode of the electron gun is accelerated by the voltage between the cathode and the anode, and is directed toward the lens barrel, and is condensed by the condenser lens and the objective lens, and is reduced into an electron probe with a diameter of about several nanometers. Under the action of the scanning coil on the upper part of the objective lens, the electron probe scans the surface of the sample in a raster pattern and excites a variety of electronic signals. These electronic signals are detected by corresponding detectors, amplified, converted into voltage signals, and finally sent to the grid of the picture tube and modulate the brightness of the picture tube. The electron beam in the picture tube is also raster scanned on the phosphor screen, and this scanning movement is strictly synchronized with the scanning movement of the electron beam on the sample surface, so that a scanning electron image corresponding to the contrast and the intensity of the received signal is obtained. The image reflects the topographical features of the sample surface.



Bonding is a wiring method in the chip production process. It is generally used to connect the chip's internal circuit with gold or aluminum wires to the package pins or gold-plated copper foil on the circuit board before packaging. Ultrasonic waves from an ultrasonic generator (generally 40-140KHz), which generates high-frequency vibration through the transducer, are transmitted to the splitter through the horn. When the splitter is in contact with the lead wire and the welded part, under the action of pressure and vibration, the surfaces of the metal to be welded rub against each other, the oxide film is destroyed, and plastic deformation occurs, causing the two pure metal surfaces to closely contact each other to achieve the combination of atomic distance, and finally form a strong mechanical connection. After bonding (ie, after the circuit is connected to the pins), the chip is packaged with black glue.


o. The Giovanni Effect

It means that due to the potential difference between two metals, an electric current is generated through the medium, and then an electrochemical reaction occurs, and the anode with a high potential is oxidized.


p. Vacuum Degree; Degree of Vacuum

The degree of thinness of the gas under vacuum is usually expressed by "high vacuum" and "low vacuum", high vacuum indicates "good" vacuum, and low vacuum indicates "poor" vacuum. If the pressure in the device under test is lower than atmospheric pressure, a vacuum gauge is required for pressure measurement. The value read from the vacuum gauge is called the degree of vacuum. The degree of vacuum is a value indicating that the actual value of the system pressure is lower than the atmospheric pressure, that is: the degree of vacuum = atmospheric pressure-absolute pressure

There are usually two ways to identify the degree of vacuum:

   — First, it is marked with "absolute pressure" and "absolute vacuum degree" (that is, how much pressure is higher than "theoretical vacuum"); in actual cases, the absolute pressure value of the vacuum pump is between 0 and 101.325KPa. The absolute pressure value needs to be measured with an absolute pressure meter. The initial value of the gauge (absolute vacuum gauge) for measuring the vacuum degree at 20°C and the place where the altitude = 0 is 101.325KPa (ie, a standard atmospheric pressure).

   — The second is to use "relative pressure" and "relative vacuum degree" (that is, how much pressure is lower than "atmospheric pressure") to identify. "Relative vacuum degree" refers to the difference between the pressure of the measured object and the atmospheric pressure at the measurement site and is measured with an ordinary vacuum gauge. In the absence of vacuum (that is, at atmospheric pressure), the initial value of the gauge is 0. When measuring vacuum, its value is between 0 and -101.325KPa (usually expressed as a negative number).

Commonly used vacuum units are Pa, Kpa, Mpa, atmospheric pressure, kilogram (Kgf / cm2), mmHg, mbar, bar, PSI, etc. The approximate conversion relationship is as follows:

   — 1MPa = 1000KPa

   — 1KPa = 1000Pa

   — 1 atmospheric pressure = 100KPa = 0.1MPa

   — 1 atmospheric pressure = 1 kg (Kgf / cm2) = 760mmHg

   — 1 atmospheric pressure = 14.5 PSI

   — 1KPa = 10mbar

   — 1bar = 1000mbar

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