What is FR4?

FR4 is a standard base material used for manufacturing printed circuit boards (PCBs), widely adopted in electronics due to its excellent electrical and mechanical properties. Below are some essential facts about FR4.

Properties of FR4:

🛡 Insulation: FR4 provides excellent electrical insulation, enabling multilayer PCB designs without short circuits.

🌡️ Temperature Resistance: The material withstands high temperatures, making it suitable for circuits that generate heat.

🧱 Mechanical Strength: FR4 is a durable composite that resists warping, cracking, and deformation.

💧 Moisture Resistance: Its chemical composition makes FR4 resistant to humidity and water absorption.

⚗️ Chemical Resistance: It resists corrosion and degradation from most chemicals used in manufacturing processes.

FR stands for “fire resistant.”

FR4 = Epoxy Resin + Glass Fiber Cloth

FR4 consists of layers of glass fiber cloth laminated with epoxy resin. It is RoHS/WEEE compliant and contains no harmful brominated compounds. Different applications require different FR4 variants with specific properties.

🌡️ Tg (Glass Transition Temperature) defines the temperature at which the glass-fiber structure begins to soften. The higher the Tg, the higher the MOT (Maximum Operating Temperature), enabling higher current capacity and better thermal stability.

Material	Tg	MOT
FR4 - Standard Tg	130°C	110°C
FR4 - Medium Tg	150°C	130°C
FR4 - High Tg	170°C	150°C
Polyimide - Super High Tg	250°C	230°C

Example: A Tg of 130°C corresponds to a maximum continuous operating temperature of approximately 110°C.

While the theoretical lifespan of a PCB is unlimited, in practice it depends on the quality of components and soldering. Temperature, humidity, and vibration significantly influence long-term reliability.

1. Available FR4 Base Materials for Single and Double-Layer PCBs

FR4 Thickness	18 µm (DS)	35 µm (DS)	70 µm (DS)	35 µm (ES)	70 µm (ES)
FR4 - 0.50 mm
FR4 - 0.80 mm
FR4 - 1.00 mm
FR4 - 1.50 mm
FR4 - 2.00 mm

2. Multilayer PCBs – Structure and Properties

2.1 Core: The core of a multilayer PCB is the central structural layer that provides electrical stability and rigidity. It typically consists of glass fiber-reinforced material (GFR) bonded with epoxy resin, and copper foils laminated on both sides to create conductive layers.

2.2 Prepreg: Prepreg forms insulating layers between copper laminates.

• Insulation: Ensures electrical separation between conductive layers.
• Thickness: Available in various grades for precise stack-up control.
• Types: FR-4 Prepreg (standard) and Polyimide Prepreg (high thermal resistance).
• Resin Content: Standard, Medium, or High – chosen based on heat and mechanical requirements.

2.3 Copper Foil for Multilayers: Copper foils are available in various thicknesses, typically measured in (oz/ft²) or micrometers (µm).
Outer Layers: 1–3 oz/ft² (35–105 µm).
Inner Layers: 0.5–1 oz/ft² (17–35 µm).

Surface finishes may include HAL (Hot Air Leveling), ENIG (Electroless Nickel Immersion Gold), or OSP (Organic Solderability Preservative) to enhance solderability and prevent corrosion.

3. Available Materials for Multilayer PCBs (Prepregs, Cores, and Foils)

Core Thickness and Copper Layers:

Core Thickness	Copper Thickness (Both Sides)
100µm	18µm/18µm; 35µm/35µm
150µm	18µm/18µm; 35µm/35µm; 70µm/70µm; 105µm/105µm
200µm	18µm/18µm; 35µm/35µm; 70µm/70µm; 105µm/105µm
300µm	18µm/18µm; 35µm/35µm; 70µm/70µm; 105µm/105µm; 140µm/140µm
400µm	18µm/18µm; 35µm/35µm; 70µm/70µm; 105µm/105µm; 140µm/140µm; 210µm/210µm

Prepreg Thickness:

Type	Thickness
1080	≈ 78µm
2116	≈ 120µm
7628	≈ 180µm

Final Thicknesses, Designation, and Structure

The table below shows which material thicknesses should be selected to achieve a conventional overall PCB thickness.

Layers	Final PCB Thickness	Base	Copper
4	0.8 mm	0.30 mm	35µm
4	1.0 mm	0.30 mm	35µm
4	1.2 mm	0.36 mm	35µm
4	1.6 mm	0.71 mm	35µm
4	2.0 mm	0.71 mm	35µm
6	1.6 mm	0.36 mm	35µm