An essential element of a flex or rigid-flex printed circuit board (PCB) design is verification that the construction will meet your mechanical bend requirements. Exceeding the minimum flex bend radius requirements creates the opportunity to exceed the physical properties of the copper circuitry resulting in failed parts and long term reliability concerns.
When a flexible circuit board is bent, compressive forces are created at the inside of the bend and tensile forces are created at the outside.
flex-bend-capability.png
If these forces exceed the copper’s ductility limits, the traces will work harden - become embrittled, and lead to the formation of cracks.
Bend requirements fall into three main categories as defined by IPC 2223C Design Standard for Flex Printed Boards, all of which have its own requirements and limitations.
Flex to Install
Dynamic Flex
One Time Crease
For each category, the minimum flex & rigid-flex bend capabilities are calculated based on the deformation that the copper is subjected to and is related as a multiple of the finished flex thickness. The layer count is also a factor. If the flex circuit is 3-layers or greater, bonded designs are require for greater minimum bend radiuses.
Flex To Install
The flex circuit is bent to a required shape and installed in the final assembly. In this scenario the flex circuit is not subjected to any further significant bend requirements.
Minimum bend radius:
1-2 layers = 6X
3 plus layers = 12X or greater
Dynamic Flex
The flex circuit is subjected to constant infinite repetitive bending as part of the operation of the final assembly.
Limited to 1 - 2 layer designs:
1 layer preferred
Allows copper to reside on neutral axis of bend radius
Minimum Bend radius:
Approximately 100X
One Time Crease
The flex circuit is bent/creased to the point of a zero bend radius and installed in the final assembly and not subjected to any further significant bend requirements. Creased fold is not to be opened.
Limited to 1–2 layer designs only:
1-layer preferred
Allows copper to reside on neutral axis of bend radius
Thin flex materials and copper weights required
PSA is typically added adjacent to crease area adhere the flex to itself and prevent any further manipulation of the creased circuit.
Take note that a lot of designs fall between the definitions of Flex to install and Dynamic Flex.
Semi Dynamic
The flex circuit is subjected to a number of bend cycles throughout its lifespan to allow for servicing or a predefined number of operational cycles. In this scenario, the bend radius and number of cycles should be carefully reviewed. Reduced flex thickness is also preferred to allow for sufficient safety margin and long term reliability.
Impact Of Electrical Design Requirements
Certain electrical design requirements result in a thicker flex design and in turn limit the minimum bend capabilities of the flex circuit.
Controlled Impedance:
Thicker flex core required to allow for correct dielectric spacing between signal layers and reference planes to meet specific impedance requirements.
Current Carrying:
May require thicker copper to meet current carrying requirements.
Wider trace widths preferred, if available room allows, as opposed to thicker copper.
Thicker copper requires thicker coverlay adhesive layers, to ensure full circuitry encapsulation, and further increases flex thickness.
EMI / RF Shielding:
Requires additional external layers to provide required level of shielding.
Layers may be either copper, silver ink or shield films.
Flex PCB Material Selection
A standard design practice is to minimize the thickness of the flex circuit as much as practical while meeting the electrical requirements of the design and not incurring unnecessary added material costs.
As a result the most common materials are as follows in order of preference:
Copper Weight: ½oz , 1oz
Flex Core Thickness: 1 mil, 2 mil, 3 mil
Coverlay Thickness: 1 mil, ½ mil
Flex cores are also available in two different construction types: Adhesive and Adhesiveless. Refers to method used to attach the copper to the polyimide core.
Adhesive utilize a layer of flexible adhesive to bond the copper to the polyimide core
Adhesiveless has the copper boned directly to the polyimide core
Higher cost material
Flex Circuit Constructions
Higher layer count designs may need to utilize an Un-bonded Air Gap construction. This construction technique configures the layers as separate independent pairs as opposed to a singular fully bonded unit. The thinner individual pairs provide a higher degree or flexibility.
Summary
Flex PCB bend requirements and the ability of the flex materials and construction to meet those specifications is an important element of an effective design. Whether in a “Flex to Install” or a “Dynamic Flex” application, pushing flex materials beyond their physical property limits will result in failures and long term reliability issues.
For the majority of applications the industry has developed a wide range of design options, materials and construction methods that can be selectively applied to a design to cost effectively and reliably meet the bend requirements of flexible circuit boards.
When a flexible circuit board is bent, compressive forces are created at the inside of the bend and tensile forces are created at the outside.
flex-bend-capability.png
If these forces exceed the copper’s ductility limits, the traces will work harden - become embrittled, and lead to the formation of cracks.
Bend requirements fall into three main categories as defined by IPC 2223C Design Standard for Flex Printed Boards, all of which have its own requirements and limitations.
Flex to Install
Dynamic Flex
One Time Crease
For each category, the minimum flex & rigid-flex bend capabilities are calculated based on the deformation that the copper is subjected to and is related as a multiple of the finished flex thickness. The layer count is also a factor. If the flex circuit is 3-layers or greater, bonded designs are require for greater minimum bend radiuses.
Flex To Install
The flex circuit is bent to a required shape and installed in the final assembly. In this scenario the flex circuit is not subjected to any further significant bend requirements.
Minimum bend radius:
1-2 layers = 6X
3 plus layers = 12X or greater
Dynamic Flex
The flex circuit is subjected to constant infinite repetitive bending as part of the operation of the final assembly.
Limited to 1 - 2 layer designs:
1 layer preferred
Allows copper to reside on neutral axis of bend radius
Minimum Bend radius:
Approximately 100X
One Time Crease
The flex circuit is bent/creased to the point of a zero bend radius and installed in the final assembly and not subjected to any further significant bend requirements. Creased fold is not to be opened.
Limited to 1–2 layer designs only:
1-layer preferred
Allows copper to reside on neutral axis of bend radius
Thin flex materials and copper weights required
PSA is typically added adjacent to crease area adhere the flex to itself and prevent any further manipulation of the creased circuit.
Take note that a lot of designs fall between the definitions of Flex to install and Dynamic Flex.
Semi Dynamic
The flex circuit is subjected to a number of bend cycles throughout its lifespan to allow for servicing or a predefined number of operational cycles. In this scenario, the bend radius and number of cycles should be carefully reviewed. Reduced flex thickness is also preferred to allow for sufficient safety margin and long term reliability.
Impact Of Electrical Design Requirements
Certain electrical design requirements result in a thicker flex design and in turn limit the minimum bend capabilities of the flex circuit.
Controlled Impedance:
Thicker flex core required to allow for correct dielectric spacing between signal layers and reference planes to meet specific impedance requirements.
Current Carrying:
May require thicker copper to meet current carrying requirements.
Wider trace widths preferred, if available room allows, as opposed to thicker copper.
Thicker copper requires thicker coverlay adhesive layers, to ensure full circuitry encapsulation, and further increases flex thickness.
EMI / RF Shielding:
Requires additional external layers to provide required level of shielding.
Layers may be either copper, silver ink or shield films.
Flex PCB Material Selection
A standard design practice is to minimize the thickness of the flex circuit as much as practical while meeting the electrical requirements of the design and not incurring unnecessary added material costs.
As a result the most common materials are as follows in order of preference:
Copper Weight: ½oz , 1oz
Flex Core Thickness: 1 mil, 2 mil, 3 mil
Coverlay Thickness: 1 mil, ½ mil
Flex cores are also available in two different construction types: Adhesive and Adhesiveless. Refers to method used to attach the copper to the polyimide core.
Adhesive utilize a layer of flexible adhesive to bond the copper to the polyimide core
Adhesiveless has the copper boned directly to the polyimide core
Higher cost material
Flex Circuit Constructions
Higher layer count designs may need to utilize an Un-bonded Air Gap construction. This construction technique configures the layers as separate independent pairs as opposed to a singular fully bonded unit. The thinner individual pairs provide a higher degree or flexibility.
Summary
Flex PCB bend requirements and the ability of the flex materials and construction to meet those specifications is an important element of an effective design. Whether in a “Flex to Install” or a “Dynamic Flex” application, pushing flex materials beyond their physical property limits will result in failures and long term reliability issues.
For the majority of applications the industry has developed a wide range of design options, materials and construction methods that can be selectively applied to a design to cost effectively and reliably meet the bend requirements of flexible circuit boards.
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