What are PCBs made of?

Overview: As an electronic component commonly used in daily life, printed circuit boards are widely used in electronic products such as mobile phones, televisions, and refrigerators. Its functions include the following two points: I. To establish a reliable electrical link between electronic components in PCB. II. To designate a proper location for electronic components. Although electronic components are commonplace in our daily lives, it is not known what circuit boards are made of. In this paper, FS Technology will discuss the composition of PCB from two aspects of PCB structure and manufacturing materials.

What is the structure of the PCB board?

The structure of the PCB board

Printed Circuit Board (PCB), are the electronics boards that are made of electronic devices where the connection of the components is carried out using pads, lines, and tracks on a laminated sheet of copper. It is made up of layers of copper circuitry buried internally or externally on a flat laminated composite substrate. They also include composite material, fiberglass, and composite epoxy. 

PCB comes with various layers and designs. In the structure of multilayer PCB, advanced hardware like graphics cards and motherboards are used. One of the flat layers of PCB is for supporting all the soldered electronics components while another layer connects the components electronically. 

Various layers of PCB are joined together with the help of heat providing the compact shape of a single object. The following is a description of the PCB layers that make up the circuit board:

Substrate Layer:

It is the base layer of PCB and is composed of fiber-glass. The FR4 is the most common fiberglass used. Some of the PCBs also come with phenolics and epoxides which are less expensive and less durable compared to FR4. The nature of the substrate materials would define the flexibility of the PCB board. 

Copper Layer:

A thin layer of copper foil is present beside the substrate. Heat and Adhesive are used for laminating the foil of copper on the board. In double-sided PCB, both sides of the PCB are laminated with copper. The thickness of copper varied based on the layers of boards and can be defined in ounces per square foor. Each inch per square has 35 micrometers of thickness. 

Solder Mask Layer:

The solder mask layer is present above the copper layer. This layer is applied in the copper layer to insulate the copper layer to avoid the conduction if there occurs direct contact by conductive materials. Thus, we can say the solder mask protects all the circuits on the outer layer of the PCB. Some of the commonly used solder masks are green in color while there is also a red mask available on the market. 

Silkscreen Layer:

The silkscreen layer existed above the solder mask layer. It helps the user to add the number and symbols for the understanding of the PCB boards. This silkscreen label provides a clear indication of the function of each and every component and pin on the board. It is mostly used in white color but can be found in various other colors like grey, red, yellow, and black. 

The above content is our discussion of the basic elements of the structure of the PCB. In the actual production process, as the complexity of electronic applications increases, we need to use more stacked structures for wiring, which leads to increased manufacturing difficulty and increased prices. Next we will explore another important topic of what the PCB boards are made of—PCB manufacturing material.

What material is PCB made of?

Each PCB has stacks of insulating and copper foil layers and lamination to the substrate. Tracks on PCB were developed using chemical etching, separating conducting layers into directive straight lines. The tracks work like wire, connecting different electronic components on board. Below we will introduce you to the four substrate materials that make up circuit boards.

FR-4

This is one of the common PCB substrate material that provides thickness to the PCB, and rigid boards built using this material are called FR-4 PCBs. Essentially, it is a glass-reinforced epoxy laminate sheet. Water resistance and flame retardancy are two properties of the epoxy used. Besides, it is also good for strength-to-weight ratios. This material offered a higher tensile strength. It is common among PCBs with lower layer counts. 

Adding additives to epoxy resin in a blend can enhance its thermal, electrical, and flame survival performance, making it ideal for high layer count builds high thermal stress designs and high-speed circuit designs at a lower cost. 

PTFE (Teflon)

Teflon laminates and bonding material provides excellent properties of electrical thus it is used for high-speed and high-frequency applications as this plastic material doesn’t provide any resistance. The flexibility of PTFE is extremely high and thus it provides tight tolerances application. It also has the property of flame resistance, provides temperature stability, and is versatile in application. They can be coated into glass fabric whose mechanical properties can be increased using additives and special fillers. 

Metal

The materials like copper, iron, aluminum, etc. are used in PCBs. As a result of these materials, Surface Mount Technology (SMT) can be utilized for component integration. Mechanical durability is also provided by them. Therefore, the product life of insulated metal substrate PCB is much longer.

Flexible Laminates

Electronic designs can be folded using flexible laminates, with no loss of electrical continuity. Instead of glass fabric, they use plastic film for support. Whether they are folded for a one-time project such as flex to install, or for a continuous application like dynamic flex, they can be equally effective. Polyimide and LCP (liquid crystal polymer) are high-temperature materials that can be made into flexible laminates, or polyester and PEN are very low-cost materials. The thin laminates also require specialized equipment and processing, along with the expectation that PCB manufacturing yields will be low. Manufacturing flexible circuits can therefore require a uniquely skilled workforce, specialized machinery, and specialized equipment.

BT, cyanate ester, ceramics, and blended resin systems can also be used to design laminates and bonding materials with superior electrical and/or mechanical performance. In general, PCB manufacturers consider them expensive alternatives because of the low volumes and difficulties in manufacturing.

For a high-quality PCB, the laminate must meet the appropriate electrical, dielectric, mechanical, and thermal properties.

Summarize:

The above content is FS Technology’s explanation on “What are printed circuit boards made of”. In this article, we start from the internal structure of the PCB and end with the manufacturing materials of the PCB, comprehensively explaining what the PCB is made of. If you are not a practitioner in the PCB industry but are interested in these contents, you can click the link below to learn more basic PCB knowledge.

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