The weaknesses of the material itself limit the design of flexible printed circuits. You must overcome hardening and fatigue of copper foil, keep good flexibility, and prevent tearing of FPC substrate. In order to produce reliable flexible printed circuits, there are something you should do, and something you shouldn't. Let's talk about 7 tips.

 

1. Keep flexibility

 

When designing FPC, you need to determine the flexibility according to the actual use demand. It is simple, but important. If the flexible circuit is only intended to be folded during the assembly, and then installed in a fixed position, then we will have a lot of freedom in choosing the number of signal layers and copper type.

 

If the flex circuit is subject to constant movement, bending, or rotation, the number of layers should be reduced. You should choose adhesive-free materials.

 

When designing, we can use the following formula to determine the minimum allowable bending radius.

 

R = minimum bending radius, μm

 

c = thickness of copper foil, μm

 

D = dielectric thickness, μm

 

EB = deformation of copper foil, %

 

d = thickness of flexible copper dielectric

 

This formula is available for single-sided flexible board:

 

R=（c/2）[（100-EB）/EB]–D

This formula is available for double-sided flexible board:

 

R=（d/2+c）[（100-EB）/EB]–D

We choose EBs based on actual use, 16% for applications with little bending, 10% for flexible mounting, and 0.3% for dynamic flexible designs. Dynamic flexible designs refer to those that require constant bending and rotation during use, such as TFT panel connections in mobile DVD players.

 

2. FPC cutout

 

Grooves, slits and inner corners should have tear-proof openings or arcs with a minimum radius of 1.5mm between tangent lines. This can greatly reduce the possibility of tearing the flexible circuit at the corners.

 

For the same reason, holes with a diameter of 3mm or more should be placed at both ends of the grooves and slits in the flexible circuit to prevent tearing.

 

3. Reduce bending

 

It's recommend that the copper traces of flexible circuits are bent in a vertical direction. But sometimes this is not possible, so please try to reduce the bending amplitude and frequency, or use a tapered bend according to the mechanical design requirements.

 

4. Use curved routing

 

It is best to avoid using abrupt right-angle or 45° angle routing, and instead use an arc-angle routing mode, which can reduce the stress on the copper during the bending.

 

5. Don't change trace width suddenly

 

When the trace is connected to the pad, especially the arranged flexible circuit terminal, a weak point will be formed. The copper will easily age over time.

 

Unless you use a reinforcing plate, or there is no bending during the application, it is better to use a gradually narrowing wiring method. In flexible printed circuit boards, teardrop treatment can be applied to pads and vias.

 

6. Use polygonal copper

 

When placing power or ground plane on a flexible printed circuit board, you can use solid copper if you don't mind significantly reducing flexibility and possibly wrinkling the copper.

 

Generally speaking, it is best to use polygonal copper to maintain a high degree of flexibility. Traditional polygons will have excess copper reinforcement at 0°, 90° and 45° angles. A more optimized pattern is hexagonal approach.

 

7. Reinforce the pad

 

Due to the low viscosity adhesive, copper on the flexible circuit is more likely to detach from the PI substrate. Therefore, it is important to provide reinforcement for the exposed copper.

 

Plated through holes provide proper anchoring for the two flexible layers, so using vias is a very good reinforcement method.

 

Surface mount pads and non-plated through hole pads have no reinforcement measures themselves, so additional reinforcement is required to prevent detachment.

 

SMT pads are the most fragile. Flexible circuits can bend under rigid pins and pads. The pads on FPC boards must be larger than the pads on the typical rigid boards. This significantly reduces the mounting density of flexible circuit components, but the density of flexible circuits cannot be too high compared to rigid circuits.