Strain Relief for Rigid-Flex Circuits

Rush PCB Inc makes rigid-flex circuits with high mechanical bend reliability. This is related to the area of the rigid-flex circuit where it transitions from the rigid area to the flexible part or from a stiffened area to the flexible part in a flex circuit, and the bend reliability of this area can have a significant impact on the success of the rigid-flex or flex circuit.

The change in the material thickness during the transition is a vertically linear change that concentrates the mechanical stress in this area that depends on the bend requirements. If this mechanical stress is not addressed, the flex circuit can suffer damage when it is bent into shape.

PCB manufacturers address this concern in most designs by adding a suitable strain relief. This prevents the flex layers from going beyond their physical bending capability. It is necessary to understand the type of strain relief required, the materials used, and the necessary design considerations.

What is Strain Relief?

The strain relief helps to protect the part under stress when bent by manipulating the transition point. This is easily seen in wiring applications where a cable must pass through an opening in the enclosure. If no protection is applied, the sharp, hard corner edge of the opening can damage the cable or its insulation when it is bent or pulled. There are many types of strain relief, like the pass-through bushings that protect against fraying at hard and sharp edges, or over molds that bend the cable in a smooth arc, thereby keeping it within its bend capabilities and preventing it from cracking or breaking.

Why is Strain Relief Necessary?

For flex circuits also, the same situation as that of the wiring applications can occur. The flex circuit layers can extend out beyond the component area stiffener or from within the rigid areas of a rigid-flex circuit. Add to this a bend requirement that is very close to the transition area, and a situation occurs where the flex layers are forced to bend sharply, rather than forming a smooth radius. Such a sharp crease will in all probability exceed the minimum bend capabilities of the flex layers.

The copper layers in the flex circuit will deform, and they will lose their ductility as they harden. If subsequently, the flex layers are straightened and again bent, chances are very high that the copper circuits are going to crack, forming intermittent or permanently open connections.

IPC 2223, a design standard for flexible circuits, in its Section 5.2.9, mentions another reason for applying a strain relief. Rigid-flex designs often use pre-preg to laminate the rigid and flex layers together. Sometimes a small amount of pre-preg may extrude beyond the rigid layers and come onto the surface of the flex layers. After curing, the protruding pre-preg takes the form of a sharp, hard ragged edge. If the flex circuit is left unprotected from this edge, it can cut or tear through the flex circuit when bent into position. Although strain reliefs are not necessary for all designs, all high-reliability Class 3 designs have them as a default requirement.

Materials for Strain Relief

Manufacturers make a strain relief on flex or rigid-flex circuit by adding beads of flexible material along the transition. When cured, this material forms a tapered fillet. A manual or pneumatically assisted syringe typically applies the bead, allowing a relatively precise amount of flexible material deposition along the vertical edge of the stiffener or rigid area. The material’s flexibility and tapered shape encapsulate the extended pre-preg, forcing the flex layers to bend in a smooth arc.

Manufacturers use a variety of materials as strain relief. Silicones and RTVs are the most common in the industry. Another common material is the two-part epoxy system of Ecobond 45/15 made by Henkel Loctite. It is possible to mix this epoxy in different ratios to change it into rigid, semi-rigid, or flexible material after full curing. To act as a strain relief, the epoxy must only be mixed in the flexible formula. The Ecobond can withstand reflow temperatures as an additional benefit. Therefore, flex circuit fabricators can apply it to the bare circuit board before assembly. When using other materials, it may be necessary to apply them after the assembly and reflow, if they cannot withstand reflow temperatures.

Design Considerations for Strain Relief

Strain relief applications need to follow a few design requirements. In a rigid-flex design, the height difference between the surface of the rigid area and the surface of the flex layers is important. There must be adequate space for the application of the strain relief, and the recommendation is a minimum of 0.01 inches. This also depends on the viscosity of the strain relief material. Suppose the strain relief application is too thick, after curing. In that case, it may extend beyond the surface of the rigid area, thereby creating difficulties for assembly by preventing the solder paste stencil from registering flush on the rigid area surface.

Another important concern is the flex section length connecting two rigid sections. Typically, the strain relief bead has a width between 1 and 2 mm. It runs out on the flex area surface, depending on the viscosity of the material. The minimum flex length that manufacturers can comfortably make is about 3 mm. Applying strain relief to both ends of a 3 to 4 mm wide flex circuit, will leave only a very small length of flex circuit not encapsulated by the strain relief—this may not be adequate to meet the bend requirements as per the design. In the worst-case scenario, the strain relief may encapsulate the entire flex area.

Manufacturers typically apply strain relief to both sides of the flex layer at the specific location of transition. However, if the design construction has offset or unbalanced flex layers, it is possible to apply for the strain relief only on one side of the flex layer. This is due to the insufficient height of the rigid area available on the other side.

Conclusion

According to Rush PCB, many designs require flexible strain reliefs to improve enhanced mechanical bend reliability and prevent damage to the flex circuit when bent in place.