How Do FR4 & Polyimide Materials Differ From Each Other?

Different materials are used for manufacturing printed circuit boards (PCBs). The type of the material varies, based on the application for which the PCBs are to be used. Each material has its own properties, beneficial features, pros, cons, and capabilities, which determine their compatibility with the applications. Among the many materials that are used today, the FR4 and polyimide materials are used on a large scale. They differ from each other in a number of aspects. Now, you must be thinking, what are these aspects? Well, the following post is focused on discussing various features, which differentiate the polyimide and FR4 materials.

To understand the difference between the two, let us discuss the properties and specifications of FR4 and polyimide individually.

Everything about the FR4 Material

FR4 stands for flame resistant. The laminates, which fall under the category of FR4, are approved by the National Electrical Manufacturers Association, USA (NEMA). The material is a composite and is made from glass-epoxy compounds. The PCBs, which demand good mechanical strength use FR4 material. It is the most commonly used material for single sided, double sided, and multilayer printed circuit boards. FR4 is considered to be the standard PCB material, for most applications. Some striking features of this material are:

  • Excellent Strength-To-Weight Ratio
  • Extremely Versatile
  • Radiation Resistant
  • Chemically Resistant
  • Dimensional Stability
  • Great Electrical Insulator

The material is further classified into four subclasses, depending upon its properties and applications. These subclasses, include those with normalized index, with higher glass transition temperature, halogen-free, and standard.

Let’s Discuss the Technical Properties & Specifications of FR4 Material

Talking technically, the following are some properties and specifications of FR4:

Thermal Conductivity
2.2 – 2.5  cal./h. cm °C
Dielectric Constant (at 1 GHz)
Dissipation Factor (at 1 GHz)
Arc Resistance
125 sec
Specific Gravity
1.8 – 1.9
Glass Transient Temperature (Tg)
135 °C
Rockwell Hardness (M Scale)

Standard Assembly, Lead-free Assembly

Things You Need to Know about Polyimide Material

Polyimide material is most commonly used in the flexible printed circuit boards. The polyimides that are used in the flex PCBs are generally imide monomer polyimides. The military or defense PCBs use polyimide as a substrate. The thermal conductivity of this material is far better than that of the FR4 material. The following beneficial features of this material, make it a perfect choice for PCBs in a wide range of applications:

  • Excellent Tensile Strength
  • Very Stable over a Wide Temperature Range
  • Exceptionally Resistant to Chemicals
  • Amazing Electrical Properties
  • Vast Operating Temperature Range from -200 °C to 300 °C
  • Extremely Durable
  • Excellent Heat Resistance

The Technical Properties & Specifications of Polyimide

The following specifications and technical properties of polyimide, makes it differ from FR4 considerably:

Thermal Conductivity 0.2 W/mK
Dielectric Constant (at 1 GHz) 4.2
Dissipation Factor (at 1 GHz) 4.0
Arc Resistance 143 sec
Specific Gravity 1.6 g/cm3
Glass Transient Temperature (Tg) 195 – 220 °C
Flammability HB Class
Electrical Strength 57.1 kV/mm

All the properties, and specifications make these FR4 PCB material & Polyimide material, completely different from each other. If you are confused about selecting the right material for your application requirements, then you can always take help from the experts, such as Rigiflex.