Dome Switches with Flex Circuits

Posted November 24, 2021

Rush PCB Inc. makes several types of switches on flex circuits. One of the popular momentary switches is dome switches. They are useful as they provide tactile feedback when operated. Many applications need a dome switch along with a flexible circuit board.

For instance, these applications can range from a few simple switches on a handheld device to large panels with multiple dome switches acting as an interface. With the base of a flex circuit or rigid-flex circuits in complex designs, this allows a complete integration. In the rigid-flex design, no additional connectors or wires are necessary between the area with the dome switches and the rest of the circuit, thereby saving substantial amounts of assembly costs and at the same time, saving space and weight.

Gerber Data for Flex PCB with Dome Switches

There is no extra requirement for generating the Gerber data for the flex PCB and the dome switches. Only a few additional considerations are necessary—the footprints of the dome switch. Depending on the specific switch the designer is using, they can use the footprint provided by the supplier of the switch. However, the designer must also consider two things.

The designer must be careful in providing the coverlay openings. Dome switches require exposure of their entire footprint, rather than only the central part and the contacts independently. There are two reasons for this. Unlike with a soldermask, a polyimide coverlay with two separate openings is not feasible. Secondly, the coverlay opening edge near the center of the switch interferes with the operation of the switch, preventing a reliable actuation.

Another consideration for the designer is to provide a vent hole. This is necessary to allow the air entrapped between the dome switch and the flex circuit to escape. Without a vent hole present, the operation of the switch may cause a pressure build-up, leading to the separation of layers from the flex circuit. Rush PCB Inc. prefers the vent hole to be a non-plated through hole in a design with a single-layer flex board design, and a plated through hole in a two-layer design.

Sealing IP Rating for Dome Switch on Flex Circuit

The designer must also decide the manner of mounting the dome switch on the flex circuit and its sealing IP rating. The IP rating tells the user the extent to which the assembly prevents ingress of moisture, dirt, etc. Usually, this feature depends on the spacing the designer is allowing from the edge of the footprint of the switch to the outline of the flex circuit.

Rush PCB Inc. prefers to use a minimum spacing of 3 mm typically. This allows the overlay on the dome switch to seal it from the flex circuit, including the capability to accommodate additional sealing methods in the final enclosure. However, some applications may require more space.

Materials for Flex Circuits with Dome Switches

Dome switch specifications usually do not influence the choice of materials for flex circuits. Typically, the application defines the materials the designer must use for the flex circuit. In a typical flex circuit design, the area with the dome switches has a rigid-flex design, terminating in a flexible circuit that connects to the control PCB. Depending on the enclosure the design uses, the flex part may have to bend to fit. If there is a space restriction, the flex part may have to bend tightly or follow a convoluted path. This may require the flex circuit to be made of special materials to make it thinner than standard flex circuits and to meet extreme bend requirements with good reliability.

Another possibility is the flex circuit part can plug into a zero insertion force or ZIF connector on the control board. This requires the designer to add a polyimide stiffener on the flex circuit end to form ZIF contact fingers. Depending on whether the ZIF connector is of the SMT or PTH type and male or female, the connector may need an FR4 stiffener to support it. For meeting cosmetic requirements, the designer/fabricator may use a colored coverlay.

Surface Finish for Flex Circuits with Dome Switches

Surface finish for flex circuits for dome switches depends on the presence of other components to be soldered on the circuit, rather than on the type of dome switch the designer is using. Major considerations for the surface finish requirements are the degree of reliability the application demands, the number of cycles for the dome switches specified by the life of the product, and the contact resistance demanded by the electronics that detect the actuation of the dome switches. Rush PCB Inc. uses three standard surface finish options for flex circuits with dome switches:

ENIG Plating: This plating uses Electroless Nickel Immersion Gold with a 100-200 µin thick Nickel layer and a 3-5 µin thick Gold layer on top. ENIG surface finish is good for most ZIF connector requirements and allows good soldering for all components. As this surface finish offers low cycle life, it is not suitable for high-reliability applications.

Hard Gold: This plating uses a Gold layer with 30 µin thickness for less than 1 million cycle life, and 30-50 µin thickness for more. This surface finish offers low contact resistance, while meeting ZIF connector requirements. If the board requires soldering of other components also, the Gold thickness must be about 15 µin to prevent formation of brittle solder joints.

Nickel Plating: This plating is useful if there are no contact resistance requirements. A Nickel layer of thickness 50-200 µin is useful for less than 1 million life cycles, and for more, the thickness must be 200-500 µin. However, this surface finish is not suitable for ZIF connector requirements, and also does not allow soldering other components. Therefore, Rush PCB Inc rarely uses Nickel surface finish.

PSA and Stiffener Requirements for Flex Circuits with Dome Switches

Designers use pressure sensitive adhesives or PSA to attach flex circuits to the enclosure. They also use PSA behind the dome switch area to attach them to the panel. If the enclosure demands, the designer may have to use a stiffener in addition, to achieve a specific thickness. For flex designs that require SMT assembly, Rush PCB Inc uses PSA type 3M 9077 and for flex designs without any assembly, and we use PCA type 3M 467 when the board has SMDs. For other requirements, we use various adhesives.

Conclusion

Several industries use circuit boards with dome switches for many applications. The combination offers several advantages like low space requirements and high design flexibility.

Rush PCB Inc offers flex circuits with dome switches in a simple design process. Contact us with your requirements, as we need to evaluate several elements before incorporating them in the design to ensure a successful product. We offer dome switch projects as a complete project or a partial design for a user interface product.