One of the key design advantages of using a flexible circuit is that it can be bent, folded or twisted in just about any way imaginable. As part of a Marketing initiative, All Flex built several origami figures using flexible circuits to demonstrate this characteristic. The ability to flex creates an infinite number of options in interconnecting three dimensional packaging. But flexibility means different things in different applications, and an improper design can cause premature failure due to circuit trace damage during bending. Consider two broad applications for a flexible circuit:
- Flex to Install: This implies the flexible circuit needs to conform, bend, and/or twist only when installing or removing the flex. During product operation, the flex is not moving except perhaps due to some vibration. The vast majority of flexcircuit applications fall into this category.
- Dynamic Flex: This implies that the flexible circuit will be bending or twisting during its operation.
Determining whether a circuit is “flex to install” or a “dynamic flex” application is an important first consideration when selecting material sets and construction of a flexible circuit. Several combinations of materials and configurations will work fine for most flex to install applications. For dynamic flex applications, the choices start to narrow depending on the flex requirements. A flexible circuit mounted in the hinge of a car door will probably need to withstand a few thousand flexes during its life. A flexible circuit in a high speed printer can easily see millions of flexes. Within the classification of dynamic flex, an application requiring a few million flexes will have fewer design options than a flex that requires only a few thousand.
As the dynamic flex requirements become significant, the optimal design will trend to the following characteristics or restrictions.
- Copper Type: There are three basic types of copper used in flexible circuits and are identified in the industry by the way the material is fabricated.
- Electro Deposited (ED). ED copper is produced by electroplating from a copper solution onto a stainless steel drum. It is then rolled onto a spool creating a continuous reel of copper. Varying the speed of the drum rotation affects copper thickness.
- High Ductility Electro Deposited (HDED). Produced similarly to HD but with a heightened ability to deform under tensile stress. This is a better material vs. HD for bending applications.
- Rolled Annealed (RA). RA copper is produced by rolling thicker bands of copper under pressure to reduce thickness and create continuous reels. Sequential rolling operations creates thinner and thinner materials. This is the preferred choice for high flex applications as its elongated grain structure will allow for significant flexing.
- Grain Direction: RA copper has an elongated grain in the long dimension of the roll. This grain should be perpendicular to the fold plane for maximum circuit flexibility.
- Bend Radius: Design guides suggest a bend radius for dynamic applications should be 10X the thickness of the material. This is a safe guideline, especially when optimizing the design with the additional recommendations for extended flex life.
- Copper thickness: Thinner copper is generally better in dynamic flexing applications. Copper thickness of ½ ounce (17 microns) is a good selection for ultra-high flex applications.
- Neutral Axis: The area that bends dynamically during operation is ideally single sided copper. Locating the copper layer in the center of the circuit, with equal amounts of dielectric above and below the copper plane, places the conductors in the neutral axis. With this copper centric configuration, the copper is neither in compression nor tension when the circuit is bent.
There will likely be design tradeoffs for consideration in optimizing the overall flex life performance. Physical stresses induced by product design features might include some of the following examples:
- Any type of mechanical abrasion can cause premature flex life failure. Circuits may have stiffeners or patches added for wear resistance.
- Bending circuits close to solder joints or stiffeners creates mechanical stress and the possibility of premature failure.
- Sharp bends in traces should be radiused and intersection points between copper pads and traces should be filleted. These nuances are not issues with rigid PCB’s but can cause issues with continuously moving parts.
- Aligning traces above and below each other on a double sided circuit causes an” I beam” effect. Staggering the trace alignment helps minimize this issue. Most flexcircuit design guides have photos illustrating many of these “do’s and don’ts”.
Customers should lean on the circuit fabricator to provide a “flexized” design for customer approval prior to beginning circuit fabrication. This design will be optimized for reliability based on the experience of the manufacturer. Sharing end product usage requirements with the fabricator will help the applications engineer define the layout of a robust solution for dynamic flex applications.