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PCB Materials and FR4 Stack-Up

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.

FR4 material temperature ratings by glass transition temperature and maximum operating temperature
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.

Available FR4 Base Materials for Single and Double-Layer PCBs

Available FR4 base materials by board thickness and copper thickness
FR4 Thickness 18 µm (DS) 35 µm (DS) 70 µm (DS) 35 µm (ES) 70 µm (ES)
FR4 - 0.50 mm Available Not available Not available Available Not available
FR4 - 0.80 mm Available Available Not available Available Not available
FR4 - 1.00 mm Available Not available Not available Available Not available
FR4 - 1.50 mm Available Available Available Available Available
FR4 - 2.00 mm Available Available Not available Available Not available
Available - Available from stock; other combinations available upon request.
DS = Double-sided
ES = Single-sided

Multilayer PCBs Structure and Properties

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.

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.

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.

Available Materials for Multilayer PCBs: Prepregs, Cores, and Foils

Core Thickness and Copper Layers

Core thickness and copper layer combinations for multilayer PCBs
Core Thickness Copper Thickness (Both Sides)
100µm18µm/18µm; 35µm/35µm
150µm18µm/18µm; 35µm/35µm; 70µm/70µm; 105µm/105µm
200µm18µm/18µm; 35µm/35µm; 70µm/70µm; 105µm/105µm
300µm18µm/18µm; 35µm/35µm; 70µm/70µm; 105µm/105µm; 140µm/140µm
400µm18µm/18µm; 35µm/35µm; 70µm/70µm; 105µm/105µm; 140µm/140µm; 210µm/210µm

Prepreg Thickness

Prepreg type and approximate thickness
Type Thickness
1080ca. 78µm
2116ca. 120µm
7628ca. 180µm

Final Thicknesses, Designation, and Structure

The table below shows which material thicknesses should be selected to achieve a conventional overall PCB thickness.
Final PCB thickness by layer count, base thickness, and copper thickness
Layers Final PCB Thickness Base Copper
40.8 mm0.30 mm35µm
41.0 mm0.30 mm35µm
41.2 mm0.36 mm35µm
41.6 mm0.71 mm35µm
42.0 mm0.71 mm35µm
61.6 mm0.36 mm35µm