“These keys don’t have the same snap as the sample switch. I though you were using the same metal domes?!” At one time or another, I’m sure most membrane-switch engineers have heard a customer say this.
Customers often fail to realize that all the other options they request increase the layers of the switch assembly, resulting in a different feel or tactile character than the sample, even though the same metal domes were used. Still, they expect the switch to possess all of the requested features without sacrificing the tactile feedback that confirms a connection has been made.There is usually is a solution, but it does not come without some added cost.
A metal or polyester dome might provide distinct tactile characteristics when it’s depressed on the surface of table. But as soon as you put it in a switch assembly, the tactile sensations it creates can practically disappear, especially in assemblies where backlighting has been added or small domes are used.
The sample switch in Figure 1 illustrates two things that can happen to prevent the dome from snapping well. First, the spacer hole size and the spacer top adhesive in this sample are restricting the upper layers from following the top of the metal dome as it is depressed. The problem is not unlike one that screen printers face when printing with a screen where there is too little free mesh area between the image and frame.
The second problem here is that as the thickness of the layers increase, the concentration of load to the top of the dome is dispersed, causing a higher return force and lower tactile ratio. Both factors can lead to poor mechanical performance and can also affect the electrical response of the switch. A dome that is collapsing from indirect pressures presents a greater risk for switch bounce or other electrical problems, such as switch tease or sensitivity. Let’s look at a couple ways of improving the snap of the metal dome without decreasing the thickness of the layers above it. Figure 2 illustrates an example where we change an existing dome-retainer layer into an actuator layer. Thin polyester or polycarbonate can be debossed with small-diameter bumps directly above the center of the metal dome. This way, when the key is pressed, the load is directly concentrated to the center of the metal dome. This method does add some cost to the assembly, but sometimes the extra expense for reduced headaches is worth it to both you and your customer.
In certain situations, you may also choose to increase the spacer hole size, as shown in Figure 3. This will allow the materials above the metal dome to move more freely and better transmit the true snap characteristics of the dome.
You can simulate the result of enlarging the spacer hole by first testing a metal dome by itself on a flat surface. Actuate the metal dome with your finger and sense its force and movement. Now place several layers of polyester (about 0.02-in.total) on top of the dome and depress it again. You’ll notice a slight reduction in tactile feedback from the dome, but not too much.
Now do the same experiment with a metal dome inside a spacer hole, and you will notice that with the 0.02 in. of material on top, the response is dramatically worse than when the dome was not in the spacer.
Increasing the size of the spacer hole does have its drawbacks. This approach provides on means of locating the metal dome or keeping if from moving while you are laminating the next layer during production. A thin tape can be used to maintain the position of each hand-placed dome, but this requires added labor. Using a large spacer hole is feasible only for those assemblies that have ample room around the keys or those with just a few keys.
A third method combines these approaches. You can combine the actuator from Figure 2 and the large spacer hole from Figure 3 to enhance the tactile “feel” of the dome even more. Allowing the materials and the metal dome to move freely is just as important as concentrating the load to the top of the dome. When very small domes (e.g., 6-mm in diameter) are placed in a spacer, they can lose their tactile characteristics very quickly, even with just one layer above them. An actuator layer will definitely help these small domes since it is very difficult to locate them without a spacer.
The same metal dome used in two different switch designs can perform very differenly. Consider the impact the switch layers will have on the dome’s performance. Also consider the effects of preload on them metal dome, the thickness of the spacer, venting (or lack of), and graphic embossing. All of these variables can lead to unexpected changes in a switch’s tactile response and unwanted surprises for your customers.