Designers Notebook: Flexible and Rigid-Flex Circuit Design Principles, Part 5

Estimated reading time: 5 minutes

The outline profile of the flexible circuit is seldom uniform. It may have portions that maintain a uniform profile, but one of the primary advantages of the flexible design is that the outline can be sculpted to fit into very oblique shapes.

Outline Planning

Where rigid boards are commonly designed in a more uniform rectangle outline, flexible circuits or the flexible portions of the rigid-flex circuit may take any number of shapes. Because these circuits are initially fabricated in a panel format, maintaining a precise outline profile during singulation is paramount. The methods for singulation may employ dedicated die-punch processing, numerically controlled mechanical routing or laser profiling. To ensure the finished part will hold up to the various stages of assembly and use, the designer must specify attributes that will minimize physical damage in key locations—inside corners, narrow slots and slits or in areas accommodating connectors or component mounting.

Guidelines for specifying inside corner and slot features in flex are well-established. The designer must stipulate a radius at these locations that will lessen the potential for cracking or tearing of the flexible material. The minimum radius on inside or tangent corners of the flex film profile should be 1.5 mm (~0.06”). Manufacturers suggest, however, specifying larger radii in the corner or end-of-slot features will contribute to increasing a part’s reliability, such that it will be more resistant to tearing when twisted. It is recommended that slit features, on the other hand, be furnished with a 1.5 mm (~0.06”) diameter hole at each end of the slit.

Physical Reinforcement

A common method for minimizing tear and cracking of the material in the inside corner area is by retaining a narrow pattern of copper foil that follows the profile of the radii. Likewise, if the corner or slot will be subjected to excessive strain during use, retaining an additional thickness of the base dielectric in the area will further reinforce the site. Additional base material and even a thin rigid material backing may be needed to physically reinforce areas under components or to mechanically support connectors. Land pattern features on flex circuits for solder-attached connectors and surface mount devices are susceptible to peeling when excessive heat is applied. Heat applied during initial solder assembly may not be a concern, but heat applied during rework and repair can weaken copper adhesion, often resulting in pad lift. Pad features furnished for plated through-holes on flexible material offer greater strength because they will generally have an equally sized pad on the opposite surface.

When components are soldered onto the land features, thermal stress on the thin conductor-to-land interface can create damaging conditions. Physical stress in these areas can lead to eventual copper fatigue and cracking. Providing fillet features at the lead-to-pad or lead-to-land interface will help to minimize stress cracking of the copper foil at the transition area. Furthermore, to prevent pad lift of surface mount components during repair, the SMT land pattern geometry should be extended by 0.25 mm (0.010”) in each direction to enable cover-coat or cover-layer material to overlap the lands to provide reinforcement.

Accommodating Bends and Folds

For maximum dynamic flex life and reliability, conductors in the bend area should be routed perpendicular to, and evenly spaced across the bend area, and the designer should not specify additional plating in areas requiring bending or flexibility. Likewise, conductors should maintain a uniform width in the bend area when feasible. The neutral bend axis, where possible, should be located at the center of the conductor pattern. Furthermore, when two metal flexible circuits are required, the circuit path on each side must be offset from one another to prevent stress in the copper foil. In regard to specifying fold and bend areas on the flexible circuit fabrication and assembly documentation, IPC-2223 recommends the following:

• Bend/fold line location dimensions on the fabrication document shall be depicted as a reference
• The assembly document should show the dimensions of the finished folded configuration as a reference only
• Bend/fold lines should be depicted as centerlines and noted as ‘bend line corresponds to center of the arc formed when folded’

To ensure repeatability of the bend or fold location it is common practice to pre-form the flexible circuit using a forming tool. This process will likely be implemented just prior to installation of the flexible circuit in the end product. In regard to bending or folding flexible circuits, the designer must consider the copper thickness and its ductility and understand that once the bend or fold is made it cannot be returned to its original state without creating micro-cracks in the copper foil. Three common methods of permanently forming circuits are: cold forming, thermal forming, and form-in-place.

1. Cold forming will likely be performed with some method of fixturing just prior to the installation process. It’s important that the radii in the bend area be specified on the assembly documentation. For 90° bend in single metal flex, for example, fabricators recommend that the minimum bend radius of the flexible circuit be 10x the overall composite materials thickness.
2. Thermal forming of the flexible circuit is employed when a more precise angle and location is specified. When thermal forming is implemented the overall material set (base material, adhesive and cover layer) will ideally be from the same dielectric family. The dedicated fixture developed for thermal forming will ensure that each unit will furnish a uniform bend location, angle and radii.
3. Form-in-place is generally recommended for single metal layer flexible circuits. For 45° to 90° bends in the single metal flex, the bend radii should be no less than 10x the overall composite materials thickness. For other applications the circuit may be formed to follow the surfaces of the product.  

The IPC standard also points out that, “Whenever a two-layer flex circuit is bent, the copper material at the bend location will experience a variety of stresses. The materials on the inside of the bend will be subjected to compression forces, and the materials on the outside of the bend will be subjected to stretching or tension forces. The greater the thickness of the flex circuit, the greater these forces will become. If any of these forces exceed the limits of the flex materials, the result will be wrinkles, delamination of the cover-layer material, and/or cracks and tears.”

Vern Solberg is an independent technical consultant based in Saratoga, California specializing in SMT and microelectronics design and manufacturing technology. To reach Solberg, click here.