Brief introduction to PCB materials

1. Copper foil
In the circuit board , Copper is the most important conductive material , The signals on the circuit board are basically etched from copper foil .
Copper thickness is a design index parameter that engineers need to consider when designing laminations , For high-speed circuit design , With the increase of frequency , It is also necessary to consider the skin effect of signal transmission , Because in high-speed circuit design , The roughness of copper foil will affect the signal transmission quality .

Electrolytic copper foil （Electrodeposited copper foil）
By electroplating , In the environment of copper sulfate plating solution , The distance between the cathode and anode of the huge plating bath is very small , Stainless steel cathode wheel rotates at high speed to impact the plating solution , Add high current （600ASF）, The continuous copper foil can be torn out on the smooth surface of the roller , After tumorization （ Increase the surface area ）、 Heat resistant layer treatment （ Isolate the explosion caused by amines ） And antioxidant treatment （ Rust proof and antifouling ） Later, it became commodity copper foil .
The side facing the roller is called the smooth surface （Drum Side）, The side facing the plating solution is called the rough surface （Matte Side）.

Calendered copper foil （rolled-wrought copper foil）
Crude copper is extracted from copper ore , After smelting 、 Electrolytic purification makes the purity reach 99.9%, And made into about 2mm Thick copper ingot . Take copper ingot as base metal , After pickling 、 Degreasing , Repeated many times in 800°C Carry out hot rolling under the above high temperature 、 Calender （ Long direction ） machining .

about AN-W Type calendered copper foil , After the above processing , High temperature annealing , Reach recrystallization softening , Then cold rolling , Such repeated processing to the required thickness . about LTA-W Type calendered copper foil , Only cold calendering .

When the above two types of calendered copper foil are made smaller than 0.1mm After the raw foil , Then roughen the surface 、 Heat resistant layer treatment 、 Anti oxidation treatment and a series of surface treatments .

Electrolytic copper foil is used in most circuit boards , Mainly hard board （Rigid Board）, And calendered copper foil is mainly used for bending 、 Flexible board with high tensile strength （Flexible） On .

According to the performance of copper foil, it can be divided into the following kinds ：

High temperature extensible copper foil （HTE: High temperature elongation electrodeposited）
When the multilayer printed circuit board is pressed, the heat will cause the copper foil to recrystallize , Therefore, copper foil is needed at high temperature （180°C） Throw it down to maintain the stability at room temperature . The main characteristics are dimensional stability , High flexibility and toughness , be chiefly used in FR-4 In the multilayer board of material .

Reverse treatment copper foil （RTF: Reverse treated copper foil）
The copper foil of the base plate faces inward , The hair face outward , This can ：
Change the good product rate ： Reduce short circuit , Because its adhesion surface rhombic line is very low , There will be no residual copper during etching
Reduce open circuit , Because the dry film can adhere very strong , Therefore, the probability of open circuit can be minimized
Shorten the process ： Speed up , The etching speed is faster , Browning treatment is faster
Line reliability ： Good insulation function between wires and layers , And has a high etching factor

Double sided copper foil （DST: double-side treatment copper foil）
Both smooth and rough surfaces are roughened , At present, the main purpose is to reduce costs , Roughening the smooth surface can eliminate the copper surface treatment and browning steps before laminating .

Ultra low diamond wire copper foil （VLP: Very low profile copper foil）
Silicification （Low profile） The diamond line roughness of traditional copper foil rough surface treatment , It is not conducive to the manufacture of thin lines , Therefore, we must try to reduce the height of the diamond line .VLP The smooth surface is also treated , Therefore, this problem is improved .

Ultra thin copper foil （UTF: Ultra thin copper foil）
Generally speaking, thin copper foil refers to 0.5 oz (17.5 micron) following ,3/8 oz The following is not easy to operate because it is too thin , Need another loader （Carrier） Can do all kinds of operations （ It is called compound copper foil）, Otherwise, it is easy to cause damage . Generally, there are two types of carriers , One is based on tradition ED Copper foil as carrier , Thick about 2.1mil; Another kind of carrier is aluminum foil , The thickness is about 3mil.

The most difficult problem for customer service of ultra-thin copper foil is “ pinhole ” or “ Sparse pore ”, Because the thickness is too thin , The sparse hole cannot be completely filled during electroplating . The solution is to reduce the current density , Make the crystal thinner . Fine lines , In especial 5mil The following requires ultra-thin copper foil , Reduce over etching and side etching during etching .

Standard electrolytic copper foil （STD）
commonly , Standard copper foil （STD） The roughness of is about 7-8μm, Reverse the copper foil （VLP） The roughness of is about 4-6μm, The roughness of copper foil with low roughness is about 3~4μm, The roughness of copper foil with ultra-low roughness is about 1.5-2μm.

2. Prepreg （PP） And core board （Core）
PCB The base material mainly includes semi cured sheet and core board , The core board is composed of semi cured sheet and copper foil .

PP Made of glass fiber cloth （Glass fabric）、 resin （Resin）、 Hardener （Dicyandiamide）、 Accelerating agent （Accelerator）、 solvent （Solvent）、 Fillers （Additive） form .
Semi cured sheet is what we usually call medium （Dielectric Layer）.

PP There is no copper foil on the surface , It is composed of semi-solid resin and glass fiber , comparison Core Be soft , It forms the so-called infiltration layer , stay PCB It mainly plays a filling role , Used to bond the core board Core.

Glass fiber cloth is divided into continuous type according to the manufacturing method of glass fiber （Continuous） And discontinuous （discontinuous）. Continuous type is used to weave glass cloth （fabric）, Use in FR4 Wait on the substrate , Discontinuous type can be made into sheet glass mat （Mat）, Use in CEM3 Wait on the substrate .

According to the composition , There are four grades of glass ：A Grade is high alkaline ,C Grade is chemical resistance ,E Level is for electronic purposes ,S Grade is high strength . The most widely used is electronic grade glass cloth （E-Glass）

The main features of glass fiber cloth include high strength 、 Resistance to heat and fire 、 Chemical resistance 、 Moisture-proof 、 Thermal properties and electrical resistance .

Glass fiber cloth is mainly filled inside the resin , Can increase PCB Of X-Y Mechanical strength of the direction . When choosing, we mainly consider the number and type of glass fiber cloth , And whether it is low loss glass fiber cloth .

Glass fiber cloth has two weaving directions , The horizontal one is generally called “ Weft ”, Longitudinal is generally called “ the warp ”. There are many types of fiberglass cloth , The number of warp and weft knitting of each model is also different .

Core plate ：
The base material is PCB The name of the factory , It's production PCB The basic materials , Also known as core board ,Core etc. , stay CCL（Copper Clad Laminate） Factory is also called copper clad laminate .Core It is to dip glass fiber cloth or other reinforcement materials with resin , A plate-shaped material made by hot pressing copper foil on one or both sides , Take on （PCB） conductive 、 insulation 、 Support three functions .

It can be divided into the following categories according to resin type and application type ：
Paper based phenolic board ： Include XPC、XXXPC、FR-1 And FR-2, It is composed of phenolic resin and fiber paper
CEM-1/CEM-3： The surface is made of glass cloth ,CEM-1 The inner core is fiber paper ,CEM-3 The inner core is glass felt
FR-4 Substrate ： It usually refers to the glass cloth base material containing epoxy resin
High performance substrate ： Other special / Non phenolic / Not FR-4 Resin like substrate , Also include Aramid、RCC And other newly developed materials
Halogen free substrate ： Base material using halogen-free resin system .
normal PCB What is commonly used is FR-4 Substrate .

3. Dielectric constant （Dk）
Dielectric constant is a physical property of dielectric materials , It represents the electrical properties of dielectrics or insulating materials , It is a dimensionless constant , It is directly related to the impedance of the transmission line and the signal in PCB Transmission speed in .
The main factor affecting the dielectric constant is the material composed of the medium itself , As for the FR4 Types of materials , The main reason that affects its dielectric constant is the type of resin 、 The type of fiberglass cloth and the content of resin .

Dielectric constant of common media

4. Dielectric loss angle （Df）
Df namely Dissipationfactor For short , The Chinese name is dielectric loss factor , Also called damping factor 、 Internal friction (internal dissipation) Or loss tangent (loss tangent), It is the tangent of the phase difference angle between the strain and the stress cycle of the material under the action of the alternating force field , It is also equal to the ratio of loss modulus to storage modulus of the material （ Generally speaking, it is the energy that has been lost in the insulating plate in the signal line , Ratio to the energy remaining in the line ）. Losses and Dk&Df Closely related to , The following is the approximate calculation formula of dielectric loss .

The larger the dielectric loss angle , The more energy loss per unit length of transmission line .

5. What is? FR4？

FR-4 It is the code of a fire-resistant material grade , It means that the resin material must be able to extinguish itself after burning , It's not a material name , It's a material grade , Therefore, the general circuit board used FR-4 There are many kinds of grade materials , But most of them are based on the so-called four functions (Tera-Function) Epoxy resin plus filler (Filler) And composites made of glass fibers .

FR-4 Epoxy glass cloth laminate , According to the use of different , The industry is generally called ：FR-4 Epoxy Glass Cloth、 Insulating board 、 Epoxy board 、 Epoxy resin board 、 Brominated epoxy resin board 、FR-4、 Fiberglass board 、 Fiberglass board 、FR-4 Reinforcing plate 、FPC Reinforcing plate 、 Flexible circuit board reinforcement board 、FR-4 Epoxy resin board 、 Flame retardant insulating board 、FR-4 Laminate 、 Epoxy board 、FR-4 Light plate 、FR-4 Fiberglass board 、 Epoxy glass cloth 、 Epoxy glass cloth laminate 、 Circuit board drilling pad . Main technical features and Applications ： Electrical insulation Can stabilize 、 Good flatness 、 A smooth surface 、 No pits 、 Thickness tolerance standard , Suitable for products with high performance electronic insulation requirements , Such as FPC Reinforcing plate 、PCB Drilling pad 、 Fiberglass mesons 、 Potentiometer carbon film printed fiberglass board 、 Precision planetary gear ( Wafer grinding ）、 Precision test plate 、 electrical （ Electrical appliances ） Equipment insulation brace clapboard 、 Insulating backing plate 、 Transformer insulation board 、 Motor insulation 、 Grind gears 、 Insulation board of electronic switch, etc .