Providing tactile feel is but one important function of a silicone keypad. The other is being able to serve as an electrical switch on the PCB.
The following sections discuss how to implement electric switching in a silicone keypad.
Before a key is pressed, it is in the fully "up" position. Here the conductive pill at the bottom of the key is well above the PCB trace. In this case, electricity cannot flow through the traces and so the switch is considered open.
When a user pressed a key they press the bottom of the key (the conductive pill) against the PCB traces. The conductive pill allows the flow of electricity from one trace, through the pill's conductive material, to the other trace. Here the switch is closed, electricity flows through the circuit until the key is released.
Conductive pills are made just like silicone keypads. They are composed of a mixture of silicone and conductive materials. The mixture is molded on 0.3mm thick sheets and the round pills are stamped out in standard sizes between 2.5mm to 8mm diameter
Standard Carbon Pills are made from a mixture of silicone rubber (an insulative material) and carbon particles (a conductive material). The silicone allows the pill it to be insert molded into the keypad. The carbon pills are suspended in the silicone mixture.
When the key is pressed, the carbon particles in the conductive pills are squeezed together, allowing for the flow of electricity from one side of the conductive pill to the other side.
There are several different types and sizes of pills, but all pills are molded into the keypad using the insert mold method.
There are several standard sizes of pills available, from 2.5mm to 8mm pills. Smaller pills (2.5 and 3.0) will have a slight decrease in performance. Avoid using two or more sizes of pill per keypad as this increases the number or process and thus increases cost. Customized pill solutions (oval pills, donut pills) can also be an option for unique applications
Each pill is insert molded into the main silicone keypad tool. The pills are pushed flat into pill pockets. If the pill pocket is too shallow, the pill may be flushed out of the pocket during the main keypad molding. Abatek recommends at least 0.3mm deep pill pockets. A maximum of 2mm should also be used.
The area at the bottom of a key contains critical features which are quite delicate in the tool. To prevent tool damage and assure correct loading of the pills, Abatek recommends at least a 1.5mm clearance between the pill pocket and all other features including key webs and anti-rocking pins.
Abatek supplies three main types of pills. They are: standard carbon pills, SC-L (low) pills and SC-M (medium) pills. The type of circuit and application typically dictates what type of pill is used. The main differentiation between pills is the total resistance load of each pill, and the maximum current it can handle.
The most commonly used pill for silicone Keypads. Carbon pills are typically used with microprocessors because of their 100 Ohm resistance and 12ms Voltage Drop
Developed exclusively by Abatek to replace gold pills, SC-L pills are commonly used on voltage or resistance coded switches where low resistance and fast switching are desired. SC-L pills offer all the dust exposure resistance of standard carbon pills with the performance of gold pills at a much lower cost.
SC-L pills are best used for applications where low contact resistance, dust exposure resistance and reliability are critical
Another type of pill developed exclusively by Abatek, the SC-M pill is typically used for high current applications where the switching current is also used to directly drive the component (such as housing lights).
As with all pills in the SC family, the SC-M is a direct replacement to Gold pills, for applications where low contact resistance, rotation actuation, dust exposure resistance, lubricants, small pill size, flexible PCB layouts and/or reliability are critical
Other types of pills include Gold pills and LR (low resistance) pills. These are no longer used by Abatek except for the very rarest of applications. LR pills have been replaced by the SC pill family and do not offer any performance or cost advantages. Gold pills have mainly been replaced by SC-L and SC-M pills. Unless the application calls for high current (300 mA) with super long actuations (1million+), Gold pills are unnecessarily expensive compared to better performing SC pills.
specification element
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For best performance, Abatek recommends using Gold over Nickel traces (thicknesses of Ni: 3 - 6 μm, Au: 70 - 120 nm) with a meander pattern as shown on let. The interlocking "fingers" allows maximum assured contact with the conductive pill.
Abatek has tested inferior trace materials, such as Copper+Carbon, Silver+Carbon and HAL tinned circuits. Not only is there a decrease in conductive pill performance, but the inferior trace materials are likely to wear off with multiple actuations and/or corrode with time.
Silicone keypads can be made clear so they can be backlit by a direct LED under the key. Careful design consideration must be taken to ensure that the position of the LED does not interfere with the conductive pill switching. Typically, the LED is constrained to being positioned centered to the key.
There are two options to help re-position the conductive pill or pills: first is using multiple contacts (for large keys, second are specialized donut conductive pills
If the key is large enough (9mm in length or more), then it is recommended to use two carbon pills, one each side of the LED. This will also help improve the actuation feel by reducing rocking (see anti-rocking). For square keys 9x9 mm or larger, four pills can be used, one at each corner of the key.
For small keys, a donut conductive pill can be used. These types of conductive pills have a hole in the middle to allow clearance for a centrally mounted LED. They come in various sizes and are available in Standard Carbon (Inner/Outer Diameter: 4/6, 6/8, custom) or SC-Pill type (custom sizes, a minimum 2.5mm thick wall is required).
The trace pattern for donut pills is very unique, but still follows the meander pattern of a standard pill trace. Material should be the same: Gold over Nickel traces (thicknesses of Ni: 3 - 6 μm, Au: 70 - 120 nm). The example at the left is for a one sided circuit, but a two sided board is also recommended.
