Polyimide vs FR4: Which is best for Aerospace PCBs?

The human spirit is indomitable. Collectively, we have an undeniable ability to face and conquer what once seemed to be insurmountable obstacles. A good example of this is flight. We have come from marveling at birds for being able to travel from place to place aerially at will to avoiding them as we routinely traverse the skies together. For 2020, it is estimated there will be over 40 million flights worldwide carrying passengers and/or cargo. Yes, Wow! And this does not include military or flights into space. It does; however, accurately demonstrate our capacity for adapting to whatever is required to achieve a goal.

Flex circuit board and connector

Flexible PCB material

Although there are similar basic requirements for any type of flight, as we extend outward from the earth, there are additional considerations that must be addressed. This includes the electronic systems that drive, monitor and control flight vehicles. For space platforms, this means that we must develop PCBs that can operate in the extreme environmental conditions present for atmospheric levels at which spaceships transcend and satellite technology is deployed.  In doing so, a major issue is selecting the best circuit board material. Let’s take a look at why this is so important for aerospace PCBs and then compare two common options: polyimide versus FR4; to ascertain which is the better choice for your development.

Aerospace PCB Material Considerations and Preferences

The construction process for PCBAs is basically divided between two major stages: fabrication and assembly. The first choice to be made for fabrication is what materials to use. Unfortunately, this selection is not always given the attention that it deserves. The primary reason for this is that the default dielectric material type for many circuit design software programs, basic FR4, is acceptable for a wide range of applications.

Although there are times when acceptable may be good enough, there also times when good enough is not sufficient. For those situations that include aerospace PCB development, an alternative material type; such as polyimide, and a better material selection process is warranted that includes an understanding of PCB material properties and how they impact the manufacturing of your board. The fact that boards utilized in space systems must operate in hazardous environments, necessitates that special consideration is taken for the PCB material selection for aerospace electronics.  Primary issues that must be considered are as follows:

Considerations for Aerospace PCB Materials

  • Strength

Board strength is important because of the vibration and mechanical stress that may be encountered during space vehicle launch and flight.

  • Durability

Unlike almost all other PCB installations, the ability to repair and replace boards when required is virtually non-existent for satellite deployment.

  • Thermal resistance

The ability to continually operate in the face of thermal extremes and changes is one of the most important requirements for aerospace PCBAs.

  • Stability

Temperatures in space can be very hot or extremely cold depending upon the location from the sun and orientation to it. The ability to withstand these extremes is a basic requirement for any space vehicle system.

  • Cycling

In addition to extreme temperatures themselves, space vehicles and their systems are subject to and must be able to withstand repeated cycling between cold and hot.

  • Elasticity

Not all aerospace PCBs are required to be elastic or flexible; however, there are many that are, including multi-boards and their interconnections (as used in CubeSats, for example).

  • Chemical resistance

Internal areas of aerospace systems can be affected by the introduction of external chemical contamination. The most significant of these is space radiation effects. However, other types of contamination are possible, especially if there is moisture present. To avoid this, hermetically sealed packages are nitrogen purged and sealed to have as little moisture in the cavities as possible and conformal coating is used.

The most common materials used for aerospace PCBs are FR4 and polyimide. FR4 is a good general choice across most PCB applications, while polyimide has superior flexibility and thermal attributes that make it the best option for some deployments. Let’s take a look at these options from the perspective of what is most important for aerospace PCB materials, as listed above.

Comparison of Polyimide vs FR4 for Aerospace Boards

The manufacturing of aerospace PCBs is regulated by a number of standards. Additionally, aerospace boards must adhere to the specifications of IPC-6012DS Space And Military Avionics Applications Addendum To IPC-6012D Qualification And Performance Specification For Rigid Printed Boards and IPC-6013D Qualification and Performance Specification for Flexible/Rigid-Flexible Printed Boards. As both polyimide and basic FR4 generally satisfy the regulatory requirements for aerospace boards, it is necessary to look at defined considerations; such as those listed in the table below, in order to make an informed decision as to which to use for a specific application.

Choosing the Best Aerospace PCB Material for Your Application

Aerospace PCB 

Material Consideration

Polyimide FR4*
Strength

(Tensile strength)

231MPa 70 MPa
Durability Better Good
Thermal stability

(Operating range)

-200°C to 300°C -50°C to 110°C
Thermal cycling Better Good
Elasticity

(Young’s modulus)

4.0 GPa 24 GPa
Chemical resistance Good Good

* Entries in the table refer to basic FR4, which is the pseudo-standard board material type throughout the PCB industry. There are; however, other versions of FR4 that may exhibit more favorable properties for aerospace applications.

As shown in the table above, polyimide and FR4 do differ a bit for the aerospace PCB considerations targeted. One obvious variation is in elasticity or flexibility. Polyimide is a flexible material and is used for flex and rigid/flex boards, while FR4 is for rigid PCBs. Even so, polyimide is actually stronger than FR4. This characteristic lends itself to the better durability for polyimide. Polyimide is also preferable if extreme temperatures and thermal cycling are issues, while both material types perform well in resisting contamination.

The answer to the question of which is the best material for aerospace PCBs is it depends. For some applications, polyimide is the best choice. However, we must qualify this by noting that for some space vehicles, internal environmental conditions are highly regulated by thermal management systems (TMSs) that relax the requirements placed on PCBs and allow for the usage of FR4. Another consideration is board design. Flex boards also have fairly different design guidelines, such as avoiding vias usage in bend areas; curving traces as much as possible to increase lifetime durability and decrease risk of cracking the internal copper; cross-hatching copper pours at 45-60 degrees (some manufacturers might have different numbers) to increase bendability; layer count with respect to the bend radius; and other design choices.

Developing boards for different industries oftentimes means adapting your material selection process to ensure that the best choice is made. This adaptability is especially important when designing and building PCBs for the aerospace industry where the extreme environment must be taken into account.

Tempo's Custom Avionics for PCB Manufacturing Service
  • AS9100D and IPC J-STD-001E with Space Addendum certified manufacturing processes.
  • ISO-9001, IPC-600 and IPC-610 commitment to quality certifications.
  • Execute your full development cycle from proto to validation, NPI, and low volume production.
  • Accurate quote in less than a day.
  • DFX support, including DFM, DFA, and DFT from Day 1 of design.
  • Entire turnkey PCB manufacturing in as fast as 4 days.
  • Extreme space environment targeted manufacturing.
  • Use reputable components suppliers to ensure quality, security and traceability.
  • Performs multiple automated inspections during PCB assembly to ensure quality for prototyping.

At Tempo Automation, we are experienced in this area as we work with some of the top aerospace and defense manufacturers. We bring this expertise as well as our smart factory to ensure that your boards are built quickly and will satisfy the stringent requirements of the aerospace industry.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on polyimide versus FR4 or how to choose the best material for your design, contact us.

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