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Tips for Beginners in Flexible Circuits Design —Part1

Flex Circuits Design Tips for Materials & Bendability

Introduction

The first key to any flexible circuits design project is a well-thought-out plan. An expertly designed flexible circuit board shines. because it’s lightweight, space-saving, heat-resistant, sturdy, and durable. These characteristics make it ideal for demanding applications. Such as aerospace, satellites, IoT, medical devices, and wearables.

When designing flexible circuits, focus on key aspects like the number of folds. bend radius, operating environment, mounting methods, assembly types, and circuit signal requirements. This helps determine material selection, layering structure, and processing methods for flexible circuits. GESFLEX will devote 2 articles to detailing considerations of flexible circuit design.In this article, we will focus on the raw material and bending considerations in the design of flexible circuits.

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Standard Process of Flexible Circuits Design

Material Properties in FPC Design

Heat Resistance:

Flexible circuit boards face high temperatures during soldering and other processes. Heat resistance becomes crucial. Polyimide is the top choice for high-heat applications.

Flexibility:

A material’s flexibility impacts how a circuit board folds and bends. Designers should ensure it can handle the necessary folds and bends without breaking. Consider the bend radius, as over-bending can cause cracks. This radius depends on the material used.

Thickness & Weight:

The thickness and weight of flexible circuits depend on the material. Thinner materials are lighter but may not be as durable. Find a balance between thickness and durability.

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Materials Guide in Flexible PCB Design

structure-of-flexible-circuit

Dielectrics

Comparison of properties of several commonly used substrates

 Polyester (with glue)Polyimide (with glue)Polyimide (glue-free)
 Flexibility (radius approx.2.0mm)generallygoodexcellent
ThermoformingCannono
MODULUS2800MPa~5500MPa2500MPa4000MPa
Tear strength800g500g500g
Peel strength (in air)1050N/M1750N/M1225N/M
Etching(>20%)excellentDifferencegood
UVPET: Difference
PEN:generally
goodexcellent
L certified maximum operating temperature85³~16585³~165105°~200
flame retardantVTM-0 (usedFRglue)VTM-0 (with FR glue)VTM-0
Dielectric constant (1MHz)3.43.53.3
Dielectric strength45KV/25μm3-5KV/25μm5KV/25μm
Insulation resistance10³0-cm10³Ω-cm 10³q-cm
welding5 seconds @246⁸~260 5 seconds @288° (requires pre-bakingbake) 5 seconds @288° (no need to pre-bake)
Through hole forminglimitedexcellentexcellent
Surface mount (IR reflow soldering) PEN – OKPET-NoGood~Excellentexcellent
Wire bondinnoCertain glues canexcellent
Chip (direct adhesion)DifferenceAverage to excellentexcellent

Conductor

Flexible circuits use conductor materials like copper foil, copper-nickel alloys, and conductive coatings. Copper foil comes in two types: Electrodeposited copper (ED) and Rolled-Annealed copper (RA).

Electrodeposited copper

Electrodeposited copper forms through electroplating, creating copper particles with a vertical grain structure. This leads to sharp edges during etching, which is great for fine lines. However, this structure can crack easily. making it unsuitable for frequent bending in flexible circuits design.

Rolled-Annealed copper

Rolled-Annealed copper is the most common in flexible circuits design. It has a horizontal grain structure, providing better flexibility and resistance. to cracking compared to electrodeposited copper. Since its surface is smooth, you need special treatment on the adhesive side for better bonding.

Adhesive

In flexible circuits design, choosing the right adhesive. to bond conductors and dielectric layers is crucial. It must ensure the FPC doesn’t delaminate. or cause excessive adhesive squeeze-out during processing. Common adhesives for flexible circuits include acrylic, modified epoxy, phenolic butyrals. reinforced adhesives, and pressure-sensitive adhesives.

Cover Layers

In flexible circuits design, cover layers made of dielectric film. and adhesive, or a coating on a flexible substrate. These layers protect the flexible circuits from dirt, moisture, and scratches. The common cover layers are cover films and solder masks.

Cover Films

Cover films combine dielectric film and adhesive. typically with a 25μm adhesive thickness. There are two main types:

  • Mide (PI): It’s highly flexible and withstands high temperatures, with thicknesses from 25μm to 125μm.
  • Polyester: This is cheaper and flexible but doesn’t handle high temperatures as well as polyimide. It comes in thicknesses of 25μm, 50μm, and 75μm.

Solder Mask

Solder masks protect conductors and add insulation. but are less flexible and thinner than cover films.

Cover Films vs. Solder Masks

  • Cover Films

Pros: They are flexible, thicker, and provide high insulation strength.

Cons: Adhesive overflow during pressing can be a problem, which doesn’t suit small pads. Creating windows for soldering can be complex, requiring additional molds or laser cutting. They’re generally more expensive.

  • Solder Masks

Pros: They are thinner and great for slim designs that don’t require much flexibility. They come in various colors and can be printed. Adhesive overflow isn’t an issue, so they’re suitable for fine-pitch pads. Generally cheaper.

Cons: Thinner means less insulation strength compared to polyimide cover films. Solder masks are less flexible and not ideal for dynamic applications requiring over 10,000 bends.

Stiffeners in Flexible Circuits Design

Stiffeners, or reinforcement boards, support flexible circuits in applications where soldering is common. They can be made from polyimide (PI), polyester film. fiberglass, polymer materials, steel, or aluminum.

  • PI or Polyester Film: These are the usual stiffeners, with a standard thickness of 125μm (5 mil), adding moderate rigidity.
  • Fiberglass: This is tougher than PI or polyester, ideal for areas needing more strength. The thickness varies from 125μm (5 mil) to 3.175mm (125 mil). Processing FR4 is more complex than PI.
  • Polymers: These absorb little moisture and resist high temperatures, useful in high-heat environments.
  • Steel and Aluminum Sheets: They offer excellent support and dissipate heat well.
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Foldable Features in Flexible Circuits Design

The flexibility of flexible printed circuits allows designers to work within compact packages. Understanding two key aspects of bendability is crucial: the number of bends and the degree of bending. Before designing, it’s important to know the number of bends, bend ratio, and bend radius for your flexible circuit design.

Number of Bends

The number of bends determines whether a circuit board operates in a static or dynamic environment. Static boards are designed to bend fewer than 100 times over their lifespan, usually for mounting. Dynamic boards endure thousands of bends, ideal for applications with little room for failure.

Bend Radius

The bend radius, also called the minimum bend radius of a flexible area, indicates how much the flexible section of the circuit can bend. This ensures the circuit’s structural integrity despite repeated bending. Generally, the standard minimum bend radius is 10 times the thickness. For one or two layers, the bend radius can be 6 times the final layer thickness; for three or more layers, the bend radius can be 12 times.

In dynamic bending applications, placing copper along the neutral axis of the bend radius is crucial. This neutral axis is where there’s neither compression nor tension. The bend radius for dynamic flexible PCB designs can be 100 times the final layer thickness, offering flexibility and durability.

Calculation of bending radius for single-sided flexible circuits

Here’s how to calculate the minimum bend radius:

 

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

In this formula:

  • R represents the minimum bend radius in micrometers (μm).
  • c is the copper thickness in micrometers (μm).
  • D is the cover film thickness in micrometers (μm).
  • EB stands for copper elongation as a percentage.

Different copper types stretch to different degrees. Rolled copper has a maximum elongation of 16%, while electrodeposited copper tops out at 11%. The same copper type can vary based on usage. In single-bend cases, set the elongation to its critical point—16% for rolled copper. In bend-mounted designs, IPC_MF-150 suggests a 10% minimum for rolled copper. For dynamic flexible applications, use 0.3%, and for head applications, use 0.1%.

Example:

For a flexible board with 50μm of polyimide, 25μm of adhesive, and 35μm of copper:

D = 75um,c = 35um

The total thickness of the board is 185μm.

For a single bend with 16% elongation, you get a radius of 16.9μm, or 𝑅/𝑇=0.09R/T=0.09. For bend-mounting with 10% elongation, the radius comes out to 0.08μm, or 𝑅/𝑇=0.45R/T=0.45. In dynamic bending with 0.3% elongation, you get a radius of 5.74μm, or 𝑅/𝑇=31R/T=31.

Calculation of bending radius for double-sided panels

The minimum bending radius can be calculated by the following formula:

R = (d/2+c)[(100-EB)/EB]-D

where:

  • R = minimum bending radius in um
  • C = copper skin thickness in um
  • D = cover film thickness in um
  • EB = copper skin deformation, measured as a percentage
  • d = thickness of interlayer medium in um

Example:

Substrate thickness: 50um polyimide, 225um adhesive, 235um copper

then d = 100um, c = 35um

Cover film thickness: 25um polyimide, 50um adhesive

then D = 75um

Total thickness of flexible sheet T = 2D+d+2C = 320um

Substitute in the formula above:

For one-time bending, EB = 16% R = 0.371um or R/T = 1.16

Bending installation, EB = 10% R = 0.690um or R/T = 2.15

Dynamic bending, EB = 0.3% R = 28.17um or R/T = 88

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Conclusion

In the next article, we’ll dive into how to optimize common issues in flexible circuit design. such as layout, routing, stack up, and EMI shielding. These insights will help you align your design. with flexible circuit manufacturing to save money and speed up your project. Stay tuned to discover the best practices for achieving success in your flexible circuits design.

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