I suppose that once upon a time, I could have been considered an athlete. I did play sports, football specifically, and I certainly spent enough time working out. As my primary objective when working out was to get bigger and stronger, lifting weights and repeating leg presses and situps dominated my gym sessions. I have always been impressed by athletes that seem to combine strength with flexibility, such as gymnasts. Experience tells me that their workouts must have included specialized training because my weight training alone did not yield a similar level of flexibility.
Just as athletes have to customize their training plan based on specific performance objectives, circuit board design has to be targeted toward the operational objective. Most electronic devices are comprised of rigid boards where material selection is geared toward the retention of structural integrity or mechanical strength. However, there is a growing demand for flex electronics and these devices require different design considerations and material choices to ensure they are manufactured to meet operational objectives. Let’s take a look at the different types of flex electronics boards and then define manufacturing considerations to be included in their design.
Types of Flex Electronics PCBs
All PCBs are subject to classification based on their end use, as stipulated in IPC-6011, General Performance Specification for Printed Boards. Additionally, according to IPC-6013D, Qualification and Performance Specification for Flexible/Rigid-Flexible Printed Boards, flex electronic PCBs are categorized as belonging to one of five classes based on the wiring type of their PCB stackup and whether or not plated through-holes (PTHs) or vias are present.
Flex Electronics PCB Classifications
- Class 1:
Single-sided flexible PCB with one conductive layer that may or may not use stiffeners.
- Class 2:
Double-sided flexible PCB with two conductive layers (top and bottom) and vias. Stiffeners may or may not be used.
- Class 3:
Multilayer flexible PCB with three or more conductive layers and vias. Stiffeners may or may not be used.
- Class 4:
Multilayer rigid and flexible PCB combinations with three or more conductive layers and vias.
- Class 5:
Flexible or rigid-flex PCB with two or more conductive layers and no vias.
It is also required that flex PCBs are accompanied by documentation with information about one of the following usage classifications:
- Use A - The board is able to withstand flexing during installation.
- Use B - The board is capable of continuous flexing for the specified number of cycles given in the procurement documentation.
- Use C - The board can operate in high-temperature environments (> 105°C).
- Use D - The board adheres to Underwriter’s Laboratory (UL) 94, Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances, and UL 796F, Standard for Flexible Materials Interconnect Constructions.
PCB Design for Flex Electronics Manufacturing
It is obviously important for your design package to include specifications for your flex electronics board’s wiring and usage type. For manufacturing, it is equally important whether or not your PCB requires stiffeners, meets the bending test requirements of IPC 6013-C, Qualification and Performance Specification for Flexible Printed Boards and conform to the mechanical design for your board.
The primary function of stiffeners is to ensure that the board is rigid at the points components will be attached. Therefore, they are required when:
- Components must be mounted in a flexible area of the board.
- Components will create stress in a flexible area of the board.
- Planarity of pads must be maintained.
- Pad stress due to flexing needs to be alleviated.
- Bending Requirements
To meet the performance specifications for flexible PCBs your board must undergo a stress test and adhere to the following specifications:
|Installation: Flex position||Min. Bend radius:||6X thickness (for 1-2 layers)
12X thickness (≥ 3 layers)
|Installation: Dynamic flex||Min. Bend radius:||100X thickness|
|Installation: One-time flex||Min. Bend radius:||6X - 100X thickness|
- Mechanical Design
Your board must be able to be mounted or deployed within the parameters of its enclosure. Depending upon your mechanical design this may require that your board withstand continuous flexing as for wearable devices or a fixed bent shape, such as in a hose or conduit.
DFM for HDI Printed Circuit Boards
Stiffeners, which are most often composed of FR-4 or Kapton material, are used to make your board more rigid for PCB assembly and can impact the fabrication time and cost for your boards. Therefore, if you can design your flex PCB such that stiffeners are not needed you can avoid the additional manufacturing time and costs.
|Tempo‘s Custom PCB Manufacturing Service
And to help you get started on the best path, we furnish information for your DFM and enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.
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 flex electronics or how to make the best design decisions for your flex PCBs, contact us.