PCB Manufacturing

PCB Board Materials: Building Blocks of Electronic Devices

PCB board materials

In the early 20th century, when the printed circuit board was invented, it was manufactured using materials like wooden planks, thin cardboard layers, Bakelite and Masonite. Initially, holes were drilled manually and wires were fixed on the boards. All the conductive traces were manually etched on an insulated material using copper foils. Although Bakelite had superior insulating properties, it was difficult to manufacture boards due to the PCB board material’s brittleness and moisture absorption capability. Late in the 1930s, the introduction of phenolic-resin-based laminates led to the improvement of mechanical stability and electrical insulation compared to other substrates.

In 1940, a major and significant development was made with the introduction of glass-reinforced epoxy laminates which is commonly known as Flame Retardant 4 (FR-4), which became very popular in the PCB industry and is still being used to this date. It offers excellent electrical insulation, mechanical stability and is resistant to moisture.

With the evolution of PCB technologies, different types of PCB board materials also emerged to meet the complexity of the circuit functionality. In today’s electronic world, a wide range of PCB materials with high-performance substrates, metal cores, flexible materials and many more plays a vital role in the development and miniaturization of products right from consumer electronic devices to aerospace, medical and defense applications.

Basic PCB material composition

PCB is made up of several layers wherein each layer has its construction method and the operation of the board. It includes prepregs, conductive layers, surface finish, and many more as per the product requirement. Now, let us see the PCB board materials used in PCB manufacturing one by one:

PCB materials

1. Substrate Materials

It is a base PCB board material that acts as an insulating material between two copper layers. The selection of substrate depends on factors like electrical performance, thermal conditions, type of environment, and the manufacturing processes involved.

Some of the frequently used substrates to manufacture pcb along with the prepreg PCB materials are listed below:

  • Flame retardant 4 (FR-4)

It is one of the predominant substrate PCB board materials. It is made up of woven glass fiber impregnated with epoxy-resin content. It is cost-effective, provides excellent insulation properties, and has high resistance to heat.

  • Polyimide (PI)

These are high-performance polymers used to make PCBs. They are also known as “Kapton” due to their thermal stability and resistance to chemicals. They are commonly used in aerospace, automobiles, and consumer electronics where high-temperature operation is required. It is also widely used in flexible PCBs due to its excellent flexibility property. It has good electrical insulation properties, thermal stability, chemical resistance, and excellent mechanical strength.

  • Metal-core PCB (MCPCB)

Boards that use high thermal conductive metals such as Aluminium or copper as a base layer, instead of traditional FR-4 or other epoxy PCB materials are called metal cores PCBs. Metal cores are used to draw the heat away from the circuit. Aluminum is the most commonly used metal due to its thermal conductivity and affordability. Copper also provides high thermal conductivity but the cost is higher compared to aluminum.

These metal core PCBs are used in applications where heat dissipation is more such as power electronic devices, high-power LEDs, medical devices, telecommunication, and automotive electronics.

2. Conductive material

Conductive PCB materials are nothing but the copper plated on the surface of the boards to create conductive traces & pads, which establish connections between multiple points. Below is a detailed explanation of conductive PCB materials used in PCB manufacturing:

  • Copper foil

Copper foil is one of the most used conductive PCB board materials manufacturing due to its electrical and thermal conductivity properties. The thickness of copper foil is selected based on the application where the board is operated. It is measured in ounces per square foot (Oz/ft²). These copper foils are laminated with prepreg in the inner layer as well as the outer to create multilayer PCBs.

Copper foil PCB material

  • Plated through holes (PTH)

These are holes that establish connections between different layers in a multilayer PCB. Once the hole is drilled, a thin layer of copper is electroplated inside the walls of the holes to create a connection between layers. The thickness of the plating depends on the design requirement which is usually in the range of 0.8 to 1 mil.

3. Surface Finish

Surface finish is done on the land pattern of the pads to avoid oxidation of copper which would result in poor solderability and malfunctioning of boards. There are different types of surface finish based on the application of the board. Let us get to know the composition and types of surface finish used in PCB fabrication one by one: –

  • ENIG

ENIG is short for Electroless Nickel Immersion Gold. There are 2 steps, firstly, a thin layer of Nickel (around 3-6 microns usually) will be applied on the pad first by electroless method, secondly, a thin layer of gold (around 0.025-0.1 microns usually)  will be applied onto the nickel layer by chemical method. ENIG has a very smooth surface, so it is widely used in multilayer PCBs and HDI circuit boards since some components like BGAs need very smooth surfaces.

  • HASL

It is a type of metallic surface finish that is typically made of tin-lead alloys. Lead-free HASL is similar to HASL but without lead content. It uses alloys like tin copper, tin nickel, and tin silver. The standard thickness of HASL coating varies from 1 mil to 2 mil. 

  • OSP

It is a surface finish coated on copper with a thin layer of organic material. It is a water-based chemical compound that contains organic materials like benzimidazoles, imidazole, and benzotriazoles.

  • Immersion silver

It is also a kind of metallic surface finish that uses silver material to coat the copper surface by using an immersion process. The main composition is silver in this finish.

4. Solder mask

Solder mask is a type of ink that is coated on the PCB surface to protect the copper traces from oxidation and environmental damage and to facilitate soldering during assembly. It comprises epoxy resin, a solvent, and a non-ionic surfactant. Solder masks possess several properties such as humidity, resistance, insulation, and high-temperature resistance. It is available in different colors like green, blue, black, white and red.

5. Silkscreen

This is the topmost layer of PCB used for the identification of components after the assembly process. Also, it can be used to provide information about the PCB project and the name of the company that designed it. This is a non-conductive epoxy ink that comes in different colors like white, black, yellow, blue, and red.

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