A publication of the National Electronics Manufacturing Center of Excellence June 2003

EMPF Director

Michael D. Frederickson
mfrederickson@aciusa.org


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Flex Connector
I
ncorporating flexible connectors into an electronic packaging configuration can save assembly time and cost as well as increase product reliability. It is important that certain design considerations be considered when developing the flexible connectors in order to maximize their effectiveness. We will examine several of these design considerations throughout this article and explain their relationship to effective flexible connector utilization.

End Usage:
Determining the end usage of the flex connector will drive all of the design decisions. Having a flex connector that will meet these requirements and provide a long term, reliable service life can only be achieved by selecting the correct materials and careful layout of the circuit interconnects.

Flex To Install applications require the flexible connectors to withstand flexing during the initial installation and /or subsequent service operations. An example of this application is the interconnection of stationary PCB's within an assembly.

Dynamic Flex applications require the flexible connectors to withstand continuous flexing for the number of cycles as required by the application involved. An example of this type of application may be found inside a robotic arm requiring several hundred thousand cycles throughout it's lifetime.

Additional end use considerations involve the operating temperature of the end product. If the product is required to operate above 105 C or is subject to specific UL criteria, the selection of material used to manufacture the flexible connector is critical.

The table below shows various operating characteristics to be considered when choosing a flex material.
The coverlayer is a combination of dielectric film and adhesive (coverfilm) or it can also be a dielectric coating (overcoats) used to insulate the conductive layers. When using coverfilm in a dynamic flex application, it is required to balance the circuit and coverfilm layers. Covercoats can be made photoimageable for fine pitch pad designs.

Bondply is simply a flexible adhesive film used to bond together multiple flexible layers. A special conductive adhesive, known as Anisotropic, can be used to electrically and mechanically connect vertically adjacent pads while maintaining isolation of the laterally adjacent conductors.
Flexible laminate materials (see Figure 1) are a combination of dielectric films and metallic foils. The conductive foils can be attached to the film by adhesive or can be direct deposited, known as adhesiveless.

  Polyester (with adhesive) Polyimide (with adhesives) Polyimide (adhesiveless)
Flexing Characteristics (R=2mm) Fair Good Excellent
Thermal Forming Yes No No
Peel Strength 1050 N/M 1750 N/M 1225 N/M
Tear Strength 800 g 500 g 500 g
Max. operating temperature (UL) 85 C - 160 C 85 C - 160 C 105 C - 200 C
Flam Retardancy VTM - 0 (with FR adhesive) VTM - 0 (with FR adhesive) VTM - 0
UV (withstand) Poor Good Excellent
Dielectric Constant (1 MHZ) 3.4 3.5 3.3
Dielectric Strength 4-5 Kv/25um 3-5 kv/25um 5 kv/25 um
Insulation Resistance 10³ Ù - cm 10³ Ù - cm 10³ Ù - cm
Soldering processes temperatures 5 sec @ 246 C - 260 C 5 sec @ 288 C (predry required) 10 sec @ 288 C (no predry)
Conductor plating critical issues must also be considered. If conductor thickness is critical use a pad only selective plating process. Nickel plating over flexible sections can present problems because of the brittleness of the nickel. If you are using tin lead plating, it should be fused to the conductor.

For optimal performance, the conductor layout (see Figure 2) in the bend area should meet the following requirements:

  • Uniform in width
  • Evenly spaced across the bend area
  • Perpendicular to the bend
  • No additional plating applied to the conductors
  • The number of layers should be minimized in the bend area
  • Vias and plated through holes should be avoided in the bend area
  • Conductors in double-sided circuits within the bend area should not be placed directly over each other.
This condition produces an "I beam" condition, reducing flexibility. The bend radius of a flex circuit should be as large as possible. The minimum bend radius is dependent on the overall thickness of the flex material.

In the following examples the material thicknesses used in the calculations are:

Laminate material: 50 um
Adhesive: 25 um
Copper: 35 um

Where:
R = min. bend radius in um
c = copper thickness in um
D = dielectric thickness in um
E? = amount of copper deformation in %
Use 16% for one time crease
Use 10% for Flex to install
Use 0.3% for Dynamic Flex
d = flexible clad dielectric thickness

The formula for calculating the minimum bend radius of a single sided flex circuit is:

R = (c/2)[(100-E?)/E?]-D

The formula for calculating the minimum bend radius of a double-sided flex circuit is:

R = (d/2+c) x [(100-E?)/E?] - D

Understanding the requirements and limitations of flexible connector circuits and their associated materials will yield designs that provide a reliable service life. If you would like additional information on flex circuits or any other subject related to electronic design and assembly, contact the EMPF Helpline @ 610-362-1320.


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