PCB Materials-The basic components of the PCB

The Substrate Material

The substrate is the base material of a PCB, providing mechanical support and electrical insulation for the circuit. The most common substrate materials used in PCB manufacturing are:

FR-4 (Flame Retardant 4)

FR-4 is the most widely used substrate material for PCBs. It is a composite material made of woven fiberglass cloth impregnated with an epoxy resin. FR-4 offers excellent mechanical strength, electrical insulation, and thermal stability. It is also flame-retardant, making it suitable for applications that require high reliability and safety.

CEM (Composite Epoxy Material)

CEM substrates are a cost-effective alternative to FR-4. They are composed of a combination of woven fiberglass and paper or other reinforcing materials, impregnated with epoxy resin. CEM substrates are categorized into different grades based on their composition and properties:

Grade	Composition	Properties
CEM-1	Paper core with woven fiberglass surfaces	Lower strength and thermal resistance than FR-4
CEM-2	Paper core with woven fiberglass surfaces	Similar to CEM-1 but with higher thermal resistance
CEM-3	Nonwoven fiberglass core with woven fiberglass surfaces	Higher strength and thermal resistance than CEM-1 and CEM-2

Polyimide

Polyimide substrates are used in high-temperature applications, such as aerospace and military electronics. They offer excellent thermal stability, chemical resistance, and mechanical strength. Polyimide PCBs can operate in temperatures up to 260°C, making them suitable for harsh environments.

Copper Foil

Copper foil is the conductive layer that forms the circuit traces on a PCB. It is laminated onto the substrate using heat and pressure. The thickness of the copper foil is measured in ounces per square foot (oz/ft²). Common thicknesses include:

• 1/2 oz/ft² (17.5 µm)
• 1 oz/ft² (35 µm)
• 2 oz/ft² (70 µm)

Thicker copper foils are used in high-current applications or when greater mechanical strength is required.

Solder Mask

The solder mask is a protective layer applied to the surface of the PCB, covering the copper traces while leaving the pads and other exposed areas uncovered. It serves several purposes:

• Prevents solder bridges from forming between closely spaced pads during soldering
• Protects the copper traces from oxidation and environmental damage
• Provides electrical insulation between adjacent traces
• Improves the aesthetics of the PCB by providing a uniform color (usually green, red, blue, or black)

Solder masks are typically made of a liquid photoimageable polymer that is applied to the PCB surface and then exposed to UV light through a photomask. The unexposed areas are then removed during development, leaving the desired pattern of openings for the pads and exposed copper.

Silkscreen

The silkscreen is a layer of text and symbols printed onto the surface of the PCB for identification and assembly purposes. It is typically white, but other colors can be used for improved visibility or aesthetics. The silkscreen layer provides information such as:

• Component outlines and reference designators
• Polarity markers for diodes, electrolytic capacitors, and ICs
• Test points and fiducial markers for automated assembly
• Company logos, product names, and revision numbers

The silkscreen is applied using a screen-printing process, where ink is forced through a fine mesh screen onto the PCB surface. The ink is then cured to create a permanent, durable marking.

Vias

Vias are small holes drilled through the PCB to interconnect copper layers on different levels. They allow signals to pass between layers, enabling more complex routing and denser PCB designs. There are several types of vias used in PCB manufacturing:

Through-hole Vias

Through-hole vias are the most common type, extending through the entire thickness of the PCB. They are typically plated with copper to provide electrical connectivity between layers. Through-hole vias are also used for mounting through-hole components.

Blind Vias

Blind vias are drilled from one surface of the PCB to an internal layer, but do not extend through the entire Board Thickness. They are used to interconnect inner layers to one of the Outer Layers, reducing the need for through-hole vias and saving board space.

Buried Vias

Buried vias are located entirely within the internal layers of the PCB, connecting two or more inner layers without extending to either surface. They are used in high-density designs to maximize routing space and minimize signal interference.

Microvias

Microvias are small-diameter vias (typically less than 150 µm) used in high-density interconnect (HDI) PCBs. They are laser-drilled and can be stacked to create interconnections between multiple layers, enabling even greater routing density.

Surface Finish

The surface finish is a protective layer applied to the exposed copper pads and other areas of the PCB to prevent oxidation and improve solderability. There are several common surface finishes used in PCB manufacturing:

Hot Air Solder Leveling (HASL)

HASL is the most widely used surface finish, where the PCB is dipped in a molten solder bath and then blown with hot air to remove excess solder. This creates a thin, uniform layer of solder on the pads, providing excellent solderability and protection against oxidation.

Electroless Nickel Immersion Gold (ENIG)

ENIG is a two-layer surface finish consisting of a nickel layer plated onto the copper, followed by a thin layer of gold. The nickel provides a barrier against copper diffusion, while the gold offers excellent solderability and corrosion resistance. ENIG is popular for its flat surface, which is ideal for fine-pitch components and wire bonding.

Immersion Silver (IAg)

IAg is a single-layer surface finish where a thin layer of silver is chemically deposited onto the copper pads. It offers good solderability and is a cost-effective alternative to ENIG. However, IAg can tarnish over time and may not be suitable for applications requiring long storage periods.

Immersion Tin (ISn)

ISn is another single-layer surface finish where a thin layer of tin is chemically deposited onto the copper pads. It provides good solderability and is a cost-effective alternative to HASL. However, ISn can form a thin oxide layer over time, which may impair solderability.

Controlled Impedance

Controlled impedance refers to the precise control of the characteristic impedance of PCB traces to ensure signal integrity in high-speed designs. By carefully managing the dimensions and spacing of the traces, as well as the properties of the substrate material, designers can achieve the desired impedance and minimize signal reflections and distortions.

The two main types of controlled impedance traces are:

Microstrip

Microstrip traces are routed on the outer layers of the PCB, with a reference plane (usually ground) on the layer directly beneath the trace. The characteristic impedance of a microstrip trace is determined by its width, thickness, and the distance to the reference plane.

Stripline

Stripline traces are routed on the inner layers of the PCB, with reference planes (usually ground) on the layers directly above and below the trace. The characteristic impedance of a stripline trace is determined by its width, thickness, and the distance between the reference planes.

Controlled impedance is essential in applications such as high-speed digital interfaces, RF circuits, and high-bandwidth analog signals. Proper impedance matching helps to ensure signal integrity, minimize crosstalk, and reduce electromagnetic interference (EMI).

Frequently Asked Questions (FAQ)

What is the difference between a single-sided and a double-sided PCB?

A Single-Sided PCB has copper traces on only one side of the substrate, while a double-sided PCB has copper traces on both sides. Double-sided PCBs offer more routing flexibility and higher component density than single-sided PCBs, but are more complex to manufacture.

What is the purpose of a ground plane in a PCB?

A ground plane is a large area of copper on a PCB layer that is connected to the ground potential. It serves several purposes, including providing a low-impedance return path for signals, reducing electromagnetic interference (EMI), and improving signal integrity by minimizing crosstalk between traces.

What are the advantages of using a Multilayer PCB?

Multilayer PCBs have several advantages over single-layer and double-layer PCBs:

• Higher routing density, allowing for more complex circuits in a smaller footprint
• Improved signal integrity due to the use of dedicated power and ground planes
• Better EMI shielding and reduced crosstalk between signals
• Increased mechanical strength and thermal dissipation

What is the difference between a plated through-hole (PTH) and a non-plated through-hole (NPTH)?

A plated through-hole (PTH) is a via or component hole that is plated with copper to provide electrical connectivity between layers. PTHs are used for mounting through-hole components and creating electrical connections between layers.

A non-plated through-hole (NPTH) is a hole that is not plated with copper. NPTHs are used for mounting purposes, such as for mechanical support or for attaching the PCB to an enclosure. They do not provide electrical connectivity between layers.

What is the purpose of a solder mask expansion in a PCB design?

A solder mask expansion is an area around exposed copper features (such as pads or vias) where the solder mask opening is slightly larger than the copper feature itself. This expansion serves two main purposes:

• It ensures that the solder mask fully covers the edges of the copper feature, preventing solder from wicking down the sides and causing short circuits.
• It accommodates for any misalignment during the solder mask application process, ensuring that the copper features remain fully exposed for soldering.

Solder mask expansion is typically specified as a value between 0.05mm and 0.2mm, depending on the PCB design requirements and manufacturing capabilities.

In conclusion, PCBs are complex assemblies made up of various materials, each serving a critical role in the overall functionality and reliability of the board. By understanding the properties and purposes of these basic components, designers and engineers can create PCBs that meet the specific requirements of their applications while ensuring optimal performance and longevity.