How to avoid trace fracturing on flexible circuits
Flexible circuits are ideal for applications that require bending and twisting. This flexibility gives designers options that are not available with the typical printed circuit board. This does not mean that a flexible copper trace will never crack, as like most metals, there are limits to the type of stress that copper can withstand. Most causes of the excess stress are due to poor design or improper material selection.
A well designed flexible circuit can meet a wide variety of flexing challenges. Flexible circuits are used in applications requiring dynamic bending (continued flexing while the product is being used e.g. a cable connected to the printhead of a printer) and in applications requiring the circuitry to be folded into small spaces within a multiplanar enclosure. The following are a few considerations to keep in mind to optimize the flexing, bending, and creasing performance of a flexible circuit.
In order to best understand the design issues for bending, folding and flexing, one needs to understand the physics of bending. This image shows a single sided flexible circuit bent around a tight radius. The inside of the bend tends to be compressed while the outside layer tends to extend. If the extension is too great, the copper layer will fracture.
For dynamic flex applications, it is best to use single sided (one layer of copper) circuits. This allows the copper to be located in the center of the construction with equivalent thicknesses of film dielectric on both sides of the copper layer. With this construction, the copper is neither in tension nor in compression during bending. This copper location is known in the industry as the “neutral axis”.
Thinner is better
A good rule of thumb for bending is that thinner is better. The thinner the layers, the smaller the bend radius and less stress on the outer layer. All Flex recommends thinner copper and thinner dielectric layers for applications requiring repeated bending. Applications engineers can help with material and design options.
I-beam construction occurs when traces on opposite sides of the dielectric lay directly over each other. This construction is much more rigid over a fold area with the extensional forces on the outside layer greatly increased because of the added thickness of the inside layer. To avoid this problem, traces on opposite sides should be staggered.
Sharp bending or folding
Many flexible circuits are folded as part of the packaging design. A properly constructed circuit can easily withstand a onetime fold or crease. Repeated folding and unfolding of creased circuits will certainly crack the traces and is not recommended under any circumstance. The following are design considerations that will reduce stress on the copper traces when folding or creasing is performed.
Radiused traces as shown will reduce copper stress along the fold axis and should be considered for all folded circuits.
The copper trace width needs to be uniform across the fold area as the inflection point of a width change will tend to focus the stress to an isolated point. It is recommended that trace width transition should be at least .030” from the fold area. The image below shows the fold location designated by “tick” marks. In this image the trace width stays uniform well beyond the fold area.
Solder or solder plated circuit traces
Soldering will create an intermetallic bond between the copper and solder. This bond is critical for electrical connection but the intermetallic layer is brittle and can only withstand gentle bending. The solder bulk layer also is rigid and will not bend. Soldered copper should not be bent or folded and needs to be kept away from inflection points. This keep out region includes plated thru holes for interlayer connection. The diagram below shows the recommended distance from the nearest bend inflection for a plated through hole.
Copper Type and Grain Direction
There are three basic coppers used in circuitry, electro deposited copper (ED), rolled annealed copper (RA) and high ductility electro deposited copper (HDED). RA copper should be used for dynamic flexing or for severe folding or bending, as the elongated grain structure of RA copper can withstand more stressing. Copper grain direction is also a key flex life variable. Circuits should be placed on the fabrication panel so the grain direction is perpendicular to the bend line.
Coverlay, Covercoat and Soldermask
Polyimide film is the recommended covering for areas that get bent or flexed. Equivalent thicknesses of this material on both sides of a single layer circuit locates the copper in the neutral axis. Screen printed covercoat or photo imageable solder mask are more brittle and are more likely to crack when folded. Cracked covercoat can “propagate” into the copper and cause fracturing.
For applications where there is coverlay on top and a stiffener on the bottom, the coverlay should overlap the end of the stiffener as shown to avoid stress points on the exposed copper.
Finally the overall electronic packaging design needs to be considered. Ideally the flexible circuit has some freedom to move within the package. During life cycle and qualification testing a part should be subjected to environmental and mechanical stress. It is recommended that even if the product is fully functional after environmental testing, the flexible circuit should be visually inspected for potential fracturing and unusual wear.