Selecting the Proper Tactile Metal Dome

Selecting the Proper Tactile Metal Dome

By Phil Heft, Business Development Manager, Memcon North America

By Phil Heft, Business Development Manager,
Memcon North America.

Tactile metal domes, made of stainless steel, are momentary switch contacts that become normally open tactile switches when used in conjunction with a printed, flex or rigid circuit.

Benefits of Metal Domes

  • Many actuation forces are available to meet application needs
  • Integration into smaller and thinner packages is possible
  • Consistent tactile response
  • Increased product reliability
  • Lightweight and low-profile design
  • Reliable switches that withstand rugged commercial environments

Stainless Steel and Rust

Metal switch domes are stamped from a stainless steel alloy. A common misconception about stainless steel is rust does not easily form on its surface. However, what really happens is visible rust is prevented by the formation of chromium oxide “rust” on the surface of the stainless steel.

A small percentage of chromium is found in all stainless steel alloys. e chromium oxide is transparent and seals the surface so that iron particles do not form iron oxide. e chromium oxide is what makes stainless steel “stainless.”

Chromium oxide is non-conductive but results in resistance, which makes for an unsuitable switch surface. While stainless steel domes can be plated with nickel, which does a respectable job of preventing chromium oxide growth, gold plating prevents chromium oxide growth and electrically performs better as a switch surface.

Dome Shapes

Metal domes can be found in circular (round), four-leg, triangle, oblong or custom shapes. They may be incorporated into a wide range of applications including mobile phones, test equipment, industrial controls, diagnostic equipment and measurement equipment. The choice of a dome shape depends on the needs of the project.

Tactile Metal Domes

Metal domes. (Image courtesy of Memcon North America.)

Circular Metal Domes

Circular metal domes, also called round metal domes or circle tactile domes, have limited availability and can only be used in circuit boards of two or more layers or in a precision switch. They’re perfect for applications requiring a low-profile switch under a membrane overlay, or keypad. They also may be used in smaller spaces and have less trip force than four-leg domes. ough they have less life expectancy than four-leg domes, they have longer life cycles than triangle domes. Other benefits include their large offering, arrays, dimpled and non-dimpled options and nickel and gold plating.

Four-Leg Metal Domes

Four-leg metal domes are designed specifically for mounting on single- or double-sided circuits where traces of negative and positive can be designed under the dome on the same side of circuits. Four-leg metal domes have a better clicking feel than round and oblong domes. They are dimpled and non-dimpled, nickel- and gold-plated, can be taped and reeled, and have a longer life cycle than triangular domes. In fact, a four-leg metal dome's normal life cycle is greater than one million. Four-leg domes often are higher in the z-axis than other domes and have the highest trip force. They’re readily available with a large offering of arrays. Their only cons are their comparatively large size and how they must match in shape to the circuitry.

Triangle Metal Domes

Triangle metal domes, also known as tactile metal domes, are designed for mounting on single- or double-sided circuits where a via is run through the circuit board. Applications requiring a click feel but limited to narrow space, such as small handheld devices, are the best fit for triangle metal domes. Most triangle metal domes have two legs, or feet, that can be inserted into the hole of the circuit board, so they may be fixed on the circuit without any adhesive tape and are suitable for high temperatures. Soldering triangle metal domes legs on circuits is not recommended, as it reduces their life cycle and click feeling. Triangle domes can be dimpled and non-dimpled, have a nickel and gold plate and can have a high trip force. However, they have limited availability, are more expensive and require specific orientation.

Oblong Metal Domes

Oblong, or oval or rectangle, metal domes are switch contacts designed for mounting on circuitry and are perfect for applications that require a low-profile switch on an overly populated circuit board. They also may be used in long, narrow spaces that cannot accommodate other domes. ey can have one or two activation locations, are dimpled and non-dimpled, and have nickel and gold plating. In addition to requiring specific orientation, oblong domes have a limited available selection, and are more expensive.

Custom Metal Domes

Some manufacturers can design custom metal domes to meet application needs including custom size, shape, trip force, rebound/ return force or travel. Several membrane switch/circuit manufacturers offer dome arrays to reduce cost and speed up assembly times. Rather than placing each dome individually, operators place one large dome array in the application. is also may lead to quality improvements and reduce placement errors or double doming. Dome arrays also enhance the tactile feel of a rubber keypad, and by sticking the array directly on a printed circuit board (PCB), they can be the answer to creating a tactile switch effect. Dome arrays carry an additional cost over traditional placement methods.

Custom Metal Dome Arrays

Dome arrays (Image courtesy of Memcon North America.)

Venting Metal Dome PCB Switches

Venting is the movement of air in and out from under the metal dome, and it is crucial for proper functioning. Lack of venting can cause adverse effects on the switch’s feel and function. Manufacturers have a variety of ways to vent metal domes, including vent channels from dome to dome via a space layer, dome arrays that are top vented through the polyester material, and vent channels that go through the board.

Dimpled or Non-Dimpled Domes

Another feature available for metal domes is a little dimple located in the center of the sweet spot. e dimple, a small concave feature on top of the dome, can be as deep as 0.2 millimeters (0.008 inches).

Reasons for dimpled domes vary but historically the design was created to reduce the “tease effect” that occurs when a dome clicks but has not made contact with the circuitry. Today’s domes have made corrections to that initial flaw, but there may be designs where the dimple will prevent overtravel or ensure electrical connection in a dusty environment.

There are some drawbacks to dimples. During the manufacturing process, the shaping of the dimple puts added pressure and gives the dome additional points of failure, and during life cycle tests, dimpled domes generally do not last as long as non-dimpled domes. Non-dimpled domes and lower trip force domes are recommended for more difficult applications because their life cycles are longer.

Industry leader Memcon, an FP3: Functional Printing, Process and Products conference Platinum Sponsor, offers a broad selection of shapes, sizes and activation force for all domes with top-quality standards that ensure performance backed by experienced technical expertise. Memcon is a worldwide manufacturer and solution provider to the print and flex circuitry industry, providing a broad range of products including interconnects, domes, LEDs and adhesives.

SGIA JournalJanuary/February 2019 | Pages 53-54 | Visit SGIA at SGIA.org

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