A Definition of Rubber Metal Dome Keypads
A hybrid of a flat-panel membrane and mechanical-switch keyboards, dome-switch keyboards bring two circuit board traces together under a rubber keypad. When a key is pressed, it collapses the dome, which connects the two circuit traces and completes the connection to enter the character. These keypads use metal dome switches or polyurethane formed domes, or polydomes. The metal dome switches are pieces of formed stainless steel that provide users with positive tactile feedback when compressed. These metal types of dome switches are very common, especially because they are reliable to more than 5 million cycles. Metal dome switches can be plated in nickel, silver, or gold.
Benefits of Rubber Metal Dome Keypads
While polyurethane formed domes tend to be less expensive than metal domes, the metal domes are preferred because of their crisp snap, rather than the “mushy” response often provided by collapsing polydomes. Users of rubber metal dome keypads immediately know that their action was received by the keypad because they can feel the response of the metal dome switch. Rubber metal dome keypads also have a higher life specification, which offsets their higher expense. The automotive industry especially opts to use rubber metal dome keypads, because integrated metal domes ensure long life combined with excellent haptics.
One major benefit of metal dome keypads is their low profile. Apple announced in early spring 2015 that it would be using a new stainless steel dome switch in the new MacBook to achieve key assembly about 40% thinner than previous designs. The new stainless steel dome switch “undergirding the butterfly mechanism provides a solid feel and positive feedback.”
Design Features of Rubber Metal Dome Keypads
Manufacturers and designers prefer rubber metal dome keypads because they improve the aesthetic look and feel of a switch. Designs may include a variety of effects from embossed and debossed features, printed text, painted icons, backlighting, and rocker or toggle actuators. Polyurethane, epoxy, and UV coatings are available to protect rubber metal dome keypads from environmental factors and abuse. 3-D rubber keypads also offer the reliability of a membrane or PCB-substrate switch while providing the three-dimensional look and tactile feel of a silicone rubber keypad. These keypads have several appropriate applications, including medical, industrial, and commercial products.
Rubber keypads also allow you to design the complete keypad and a key in any shape, size, or color. With metal domes, rubber keypads offer the desired tactile response. They offer a more uniform feel than traditional webbing-based design. Webbing-based designs need the web area to be very thin, which can lead to the tearing of the key from the rubber base. Metal dome designs, on the other hand, do not need extremely thin areas to create the tactile feedback, so they can be more robust.
Challenges Posed By Rubber Metal Dome Keypads
There is an issue of dome switch alignment with respect to keypads placed over the dome switches. Typically, keypad overlays have built-in actuators aligned to the center of the dome. Generally, the diameter of the actuators should be no larger than 25% of the dome diameter. Anything larger can infringe on the transition ring of the dome, particularly if it is misaligned. This affects its feel and function. When oversized actuators crush the transition rings, switch function can be “catastrophic,” which frequently results in switch failures.
As electronics become smaller, metal domes have had to be manufactured in sizes as small as 3mm in diameter. These smaller tactile domes also pose manufacturing challenges in relation to aligning them with keypad overlays. In these miniaturized electronics, even a slight misalignment presents potential problems.
A solution for rubber metal dome keypad misalignment is to form a reverse dimple in the center of the dome to serve as a tactile concentrator. The misalignment then can be offset as long as the keypad actuator is positioned over the protruding tactile concentrator dimple.