PCB Tech - Learn about flexible circuit design guidelines

There are many overlaps between the challenges faced by flexible circuit design and those faced by rigid PCB design, but there are also many differences. The basic properties of a flexible circuit that can bend and flex determines that it is more like a mechanical device than an electrical device. Therefore, flexible circuits have a series of unique requirements. Understanding the interaction between these requirements will help PCB designers to design reliable and cost-effective flexible circuit interconnection solutions on the premise of balancing electrical and mechanical functions.

Check the stress concentration characteristics of the design. The stress concentration characteristic is the only cause of mechanical failure of flexible circuits (ie, conductor cracking/broken, insulating material tearing, etc.). In order to avoid stress concentration points, the circuit structure should not be changed in the bending area or in the immediate vicinity. In the bending area, the width, thickness or placement direction of the conductor should not change, there should be no electroplating layer or coating, the covering layer or external insulating material should have no openings, and there should be no holes of any type in the bending area.

Bending ratio

Determine and evaluate the minimum bending ratio of the design. The bending ratio is the best index to evaluate whether the flexible circuit will have problems during use. The bending ratio is the bending radius-circuit thickness

Optimal bending ratio of structure

Conductor wiring

The conductor should pass through the bending area as much as possible and make the conductor perpendicular to the bending surface (Figure 1). Doing so can minimize the stress on the conductor when it is bent, thereby maximizing the service life of the circuit. You should always use curved curves instead of sharp angles to change the conductor direction. When it is not possible to change the conductor direction with a curved curve, it is best to use two 45° angles to change the conductor direction, and then only consider a 90° angle.

When the direction of the conductor cannot be changed with a curved curve, it is better to use two 45° angles to change the conductor direction than one 90° angle

It is best to place the small conductor inside the bending area. The ability of small conductors (<0.007") to withstand extrusion is better than the ability to withstand tension. Placing this type of conductor on the inside of the bending area can reduce or avoid tension. Do not stack conductors on a multilayer structure to avoid I-shaped Beam effect. Stacking conductors will inevitably increase the overall thickness of the circuit, thus reducing flexibility and the ability of the circuit to bend reliably.

conductor

The flexible circuit conductor is made using a photoetching process, that is, a whole piece of copper is used to start production. By adding a mask to the ideal conductive path, and then using chemical methods to remove the unnecessary copper, leaving the ideal circuit pattern, thereby forming a conductor. The etchant will dissolve the copper that has not been added with a mask, and will also etch away the edges of the conductor, causing "side etching".

As the thickness of the copper foil increases, the amount of side etching will also increase. Therefore, it is difficult for flexible circuit manufacturers to make very small conductors on very thick copper foils. There will also be differences in the etching process (mainly the strength of the etchant will vary with the copper content in the solution). Therefore, the designer must consider the processing tolerance of the trace width (and line spacing). In order to get the best etching yield, the width of the conductor should be at least 5 times the thickness.

It is recommended to set the conductor width to the widest possible. For example, if the design needs to squeeze a conductor with a width of 0.005" between the pads in the isolated area, once the conductor leaves the isolated area, the width should be increased by 0.010" to 0.012". This approach can improve the etching yield. This means that the total cost of the circuit will be reduced.

If the width of the conductor between the pads in the isolated area must be reduced, it should be adjusted to the original width after the conductor leaves the isolated area

Land fillet

It is better to insert a filling part at every position where the conductor enters the pad. Pad filling can reduce or eliminate potential stress concentration points.

Tearing releases the copper tear stop block, because such devices have proven to be ineffective in preventing tears from occurring or preventing cracks from spreading.

Design solutions that can reduce tearing

Via

Vias can connect all layers at the location of the vias. Blind vias can connect the outer layer and adjacent layers together, but do not run through the entire circuit. Buried vias will connect the inner layer, but will not extend to the outer layer. Blind vias and buried vias will increase the cost of the circuit, but will also increase the usable area of the PCB on the undrilled layer.

The two most common cover materials for SMT clearance openings are polyimide film and flexible solder mask. The methods of creating clearance openings on these two materials are completely different, so the design requirements are also very different. The clearance opening on the polyimide film can be formed by drilling, milling or punching. The shape and size of the clearance opening are limited by the shape of the circular drill or tool. Therefore, the SMT clearance opening on the polyimide film is either circular or elliptical. A set of clearance openings for multiple SMT pads is also a common method in flexible circuit design.

Flexible solder masks like conventional PCB solder masks are formed by photosensitive imaging, so any shape of opening may be obtained. The clearance opening of the solder mask should be slightly larger than that of the SMT pad to ensure that if there is an alignment deviation during the printing process, the solder mask will not be attached to the pad.

Controlled impedance and signal integrity

The operating speed of electronic equipment continues to increase, resulting in the characteristic impedance of all parts of the electronic assembly, including any flexible PCB or rigid PCB in the system, having matching impedance. Impedance mismatch will cause signal reflection and signal degradation at each mismatch point, resulting in erroneous signals, and ultimately causing the equipment to fail to operate normally.