PCB laminate Materials
Printed Circuit Boards (PCBs) are an essential element of every electronic device. So what type of PCB materials you choose will significantly influence your circuit's fabrication, performance, and durability. Printed circuit board materials provide a supportive foundation on which traces and components are assembled.
PCB Laminates, sometimes called copper clad laminates, are composed of sheets of prepreg, that are laminated together with heat and pressure, with sheets of copper foil on either side. Once the resin hardens, PCB laminates are like a plastic composite, with sheets of copper foil on both sides.
The choice of printed circuit board substrate should be considered in terms of electrical performance, reliability, processing technology requirements, and economic indicators. There are many substrates used in PCBs, mainly in two categories: organic and inorganic. Organic substrates are made of reinforced materials such as glass fiber, impregnated with resin binders, dried and covered with copper foil, and then made by high temperature and high pressure. This type of substrate is also called copper clad laminates (CCL), inorganic substrates are mainly ceramic plates and enamel coated steel substrates.
Printed circuit boards are divided into rigid printed circuit boards, flexible printed circuit boards and rigid-flexible printed circuit boards according to the rigidity and flexibility of the dielectric materials used to make the substrate. Rigid printed circuit board refers to a printed circuit board laminated with copper foil on the surface of a substrate that is not easy to bend. It requires flatness, has a certain mechanical strength, and can play a supporting role. Flexible printed circuit board refers to a printed circuit board made of copper foil laminated on the surface of a flexible substrate. It has good heat dissipation, ultra-thin, and can be bent, folded, wound, and can be moved and stretched arbitrarily in three-dimensional space.So it can form a three-dimensional circuit board.
The rigid printed circuit board and the flexible printed circuit board are combined to form a rigid-flexible printed circuit board, which is mainly used for the electrical connection of the rigid printed circuit board and the flexible printed circuit board.
Printed circuit boards are divided into single-sided, double-sided and multi-layer boards according to the number of layers of copper clad. Single-sided board refers to a printed circuit board on which only one surface of an insulating substrate is covered with conductive patterns. Double-sided board refers to a printed circuit board with conductive patterns on both sides of an insulating substrate, that is, conductors on both sides of the circuit pole are connected through pads and vias. Multilayer board refers to a printed circuit board formed by alternately bonding a layer of copper foil and an insulating substrate. If it is a four-layer copper foil, it is called a 4layer board. If it is covered with copper foil on six sides, it is called a 6layer board. The electrical interconnection between the board layers is through pads, through holes, blind holes and buried holes. Wait till now. Most motherboards are 4-10 layers structure, currently PCBMay can achieve 40 layers.
There are multiple types of materials used in PCB development, and each material has unique characteristics. The majority of PCB materials used are enhanced epoxy resin substrate, glass fiber, and copper foil bound epoxy resin substrate. The material selection depends on several factors such as application, specifications, environmental conditions, and cost. Before proceeding to the details on selecting printed circuit board material, one should know about the available types of PCB materials and how they differ.
Printed Circuit Board
Printed Circuit Board Material Types
The purpose of all printed circuit boards is the same, laying a foundation for the circuit. But printed circuit board materials extensively impact the circuit reliability and durability. This is because every board material has its own unique characteristics.
FR4 is the world’s most commonly used material for printed circuit board substrates. ‘FR4’ refers to a class of materials that are designed according to the standards of NEMA LI 1-1998. FR4 materials have good thermal, electrical, and mechanical strength. Moreover, their weight characteristics make them ideal for most electronic applications.
FR4 laminates and prepregs are typically made with epoxy resin and glass. Although FR-4 material based printed circuit boards are developed for circuits requiring a maximum of fourteen layers, they are commonly used for double layer printed circuit boards.
The thermal efficiency of FR-4 material is limited, but with a combination of additives, its thermal efficiency, power specifications, and UL flame ratings can be enhanced. Furthermore, such combinations can also enhance the board’s ability to withstand mechanical stress. FR4 laminates and prepregs are suitable for bulk production with consistent performance. The material develops a high standard printed circuit board’s core qualities, maintaining an effective balance of cost, reliability, durability, performance, and production capacity.
Polyimide laminates have greater temperature efficiency and reliability than FR4 materials. However, polyimides cost more than FR4 but provide a better survival rate in harsh environmental conditions. They are highly robust, and their low expansion rate during thermal cycling makes them a good match for multiple layer designs.
Teflon laminate materials give excellent electrical properties to the PCB, making them ideal for high-speed circuits. However, Teflon materials are costlier than polyimide. Teflon material, in combination with additives for mechanical strength, can be dressed on the glass. The manufacturing of Teflon printed circuit boards involves complex procedures and highly technical equipment. The production demand for Teflon printed circuit boards is lower than conventional types of printed circuit boards.
The flexible laminates are compact, which enables the circuit to fold without affecting electrical continuity quickly. They have no supporting glass fabric; instead, they are constructed on plastic film. Flexible laminate printed circuit boards are most suitable for one-time installation purposes as they can continuously fold without affecting the circuit’s life.
Higher temperature materials such as polyimide and fluid crystal polymers are commonly used for sensitive circuits. Due to the thinness of the flexible laminates, the development of flexible printed circuit boards requires specialized equipment and processes.
Flexible PCB material
Apart from the above-mentioned materials, there are many other laminates and bonding materials available in the market. These materials include cyanate ester, ceramics, and blended networks that integrate resins to get electrical and mechanical characteristics. Since their production volumes are much smaller than FR4, and the production is comparatively complicated, they are generally considered costly alternatives for printed circuit board development.
Since now we know about the printed circuit board materials used on a commercial scale, let’s find out how manufacturers select a particular type of material.
Properties to Consider When Choosing a PCB Material
Printed circuit board material selection is an extensively technical process. The designers have to consider certain specifications such as thermal, electrical, power, and heat ratings. Moreover, the chemical and mechanical characteristics of materials must be kept in mind while selecting one.
- Glass Transition Temperature (Tg): The substrate is sensitive to a specific temperature range, above which the substrate softens and hardens to its initial state once cooled. The Tg represents the temperature range in which this transformation occurs. Tg is expressed in degrees Celsius.
- Decomposition Temperature (Td): Printed circuit board substrate decomposes when uncovered to temperatures above a certain threshold. The Printed circuit board loses five or more percent of its total weight. As soon as the material is separated from its glass transition temperature, the Tg transformation reverses. The temperature of decomposition (Td) stands for the threshold above which the decomposition occurs. The most straightforward approach is to use a material whose decomposition temperature is higher than the printed circuit board’s operating temperature range.
- Coefficient of Thermal Expansion (CTE): The expansion rate of a printed circuit board is called CTE. This expansion occurs when a substrate is exposed to temperatures above its CTE, the material expands. This expansion is measured in parts a million (ppm). In general, a substrate has a higher CTE than a copper layer. Often, this disparity creates problems with the connections when the heat is applied. Because of the twisted glass restrictions around PCB content, CTE varies from 10 to 20 ppm along the X and Y-axis. Even if the threshold exceeds the Tg, CTE stays the same.
- Thermal Conductivity (k): Thermal conductivity stands for the heat-conductive properties of a printed circuit board. The thermal conductivity of a material is directly linked with the heat transfer capability of the circuit board. When the K level is is low, the heat transfer level remains the same. The copper layer carries heat faster than the dielectric heat in a platform. The thermal conductivity rate in Kelvin (K) is calculated in watts per meter.
- Dielectric constant (Er): The continuity of a dielectric constant is determined by impedance and signal integrity. The dielectric constant of a typical PCB material ranges between 3.5 and 5.5 (Er). The Er amount of a substance depends on the frequency and generally falls as the frequency increases. On certain PCB materials, the relative permittivity (Dk) changes less comparatively on the others. For safety of high-frequency applications, a stable dielectric constant must be maintained over a broad spectrum of frequencies.
- Dielectric Loss Tangent (Tan δ): If the tangent of loss of a printed circuit board is low, the material loses less strength. The dielectric loss (Tan β) tangent of materials used in printed circuit boards is commonly between 0.02 and 0.001. The first figure refers to the materials most widely used. The latter figure refers to high-end materials. With frequency, Tan μ rises. The loss tangent is usually small when we talk about digital circuits. Any application where the frequency level increases up to 1 GHz will have expectations for this law. For analog signals, loss tangent is extra critical.
- Volume Resistivity (ρ): The volume resistance (ρ) of dielectric material is an isolating material for electricity. The high-resistive printed circuit board material is the least possible to ease electrical loading. The dielectric’s resistivity can be calculated using Ohm meters (Ω-m) – and ohm centimeters (Ω-cm) units. The material on a circuit board must, like all-dielectric insulators, have high resistivity, ideally within the range between 103 to 1010 mega-ohms. External factors such as heat, cold, and humidity can affect the resistivity of the material.
- Surface Resistivity (ρS): Every dielectric material has a surface resistance of insulation and electricity, which is called a surface resistivity (ρS). About ρ level, the ρS must have a high value between 103 to 109 megaohms per square, ideally. As with ρ, high temperature and moisture may affect the surface resistivity of a material.
- Electrical Strength: A dielectric material’s potential to withstand an electrical breakdown is known as its electrical strength. The electric strength of a printed circuit board is calculated in Volts/mil. The electrical strength of commonly used printed circuit board materials lies between 800 V/mile and 1500 V/mil.
PCB assembly Electrical
- Flammability: Printed circuit boards are prone to severe burning if the material is highly inflammable. The UL 94, a standard for the safety of flammability, helps identify a material’s nature if ignited. According to the defined standards of flammability, a good material must not stay ignited for any more than ten seconds once the source of flame is removed. Otherwise, the material does not match the UL 94 standards and thus has high flammability nature.
- Moisture Absorption: A dielectric’s ability to withstand moisture exposures elucidates its moisture absorption ability, especially when immersed in a fluid. The most common printed circuit board materials have moisture absorption values ranging from 0.01% to 0.20%. The moisture absorption of the substrate also influences the dielectric’s electrical and thermal properties.
- Methylene chloride resistance: The chemical resistance of a printed circuit board material is called methylene chloride resistance (MCR), which tests dielectric resistance, most specifically against methylene chloride absorption. Dielectric strength usually has an MCR of 0.01% to 0.20%.
- Peel Strength: The bonding in a printed circuit board between the dielectric and the copper layers is defined as peel strength.
- Flexural strength: The dielectric material’s ability to withstand physical stress without fracture is known as flexural strength. It is measured in kilograms per square meter. The force is applied at the center to check the printed device board’s flexure strength while the ends are maintained. Dielectric strength is often calculated via the tensile module, which establishes a printed circuit board material’s stress/strain ratio and how good it can withstand stress in different angles. The traction module is also called the Young modulus, used by various manufacturers to measure a printed circuit board’s ability to withstand strain in place of flexural strength.
- Density: The density of dielectric material is calculated in grams per cubic centimeter. Furthermore, the PCB density can also be measured in pounds per cubic inch.
- Time to Delamination: The time dielectric material takes before delamination is called the time to delamination. It elucidates how long the printed circuit board layers can remain exposed to temperatures above a certain threshold before they unravel from one another. The material can delaminate if it is exposed to thermal shock or moisture beyond its time to delamination.
Factors to be Considered When Selecting a Printed Circuit Board Material
When selecting a printed circuit board material, quality is the most critical factor. Wherever the printed circuit board has to serve after assembling, it must function in compliance with the desired specifications. But it is possible only if the designed board has the right material placed on; so it functions correctly if exposed to physical stress.
Given the variety of printed circuit board materials, the designers can always choose the highest quality materials. Many designers use gold or solder tabs while designing. But the gold tabs are a little costly. It is impractical to select expensive material in many cases since the overall cost has to be considered. The cost should be kept in mind while choosing the printed circuit board material.
As already explained, the properties of the printed circuit board vary depending upon the type of material. Some materials are prone to intensive burning if exposed to flame for a short time, whereas other materials can resist burning. Similarly, the thermal conditions of materials vary, and their heat dissipation capabilities affect their functioning. The board must be able to withstand high currents, which normally requires wider traces and thick copper.
Evidently, selecting a suitable PCB material is not as simple as it looks; one must consider all of the above factors before selecting a suitable type.
Following are some of the most commonly used printed circuit board materials.
Here there’re many laminate material datasheets,they’re useful and helpful for you,please see it:
|SUPPLIER||PCB LAMINATE||TYPE||MATERIAL DATASHEET||TG||TD||DK(1MHZ)||DK(1GHZ)||DK(10GHZ)|
|Hitachi||MCL-BE- 67G||Modified Epoxy||DOWNLOAD||140||340||4.9||4.4||–|
|N4000-13EP SI||Modified Epoxy||DOWNLOAD||210-240||350||–||3.4||3.2|
|Stablcor||ST325||–||DOWNLOAD||Thermal conductivity:75w/m.k(with 1oz copper)|
|ST10||–||DOWNLOAD||Thermal conductivity:325w/m.k(with 1oz copper)|
|bergquist||ht-04503||–||DOWNLOAD||Thermal conductivity:2.2w/m.k(with 1oz copper)|
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