Mechanically Mounted & Fastened Keypads

Mechanically Mounted & Fastened Keypads

While the majority of keypad assemblies are simply adhered to the end product using rear pressure sensitive adhesive (PSA), in some cases the product design calls for mechanical mounting. Mechanically fastening the keypad assembly not only provides additional mounting support and rigidity, it also prevents the keypad from being removed or pried from the front (in many cases for security purposes) and allows our customers the flexibility to remove the keypad assemblies out in the field when required. 

The rubber keypad assembly shown below is used in an outdoor lockbox application and has the following design features: 

  • Mechanically fastened using 10 PEM studs that are integrated into a metal backer. 
  • Backlit keys using LEDs, light guide film and laser etching technology.
  • Environmentally sealed based on the design of the rubber which wraps around the entire assembly acting as a gasket.
  • Tactile keys using metal domes.
  • Cable assembly with female connector soldered and sealed to the PCB for connection to customer’s PCB. 
Mechanically Fastened Rubber Keypad
Rubber Keypad Mounted to Outdoor Lock Box
Mechanically Fastened Rubber Keypad
Rubber Keypad with PEM Studs for Mounting
Mechanically Fastened Rubber Keypad
Rear View of Rubber Keypad with PEM Studs

Elastomer Rubber Keypad Webbing Explained

Elastomer Rubber Keypad Webbing Explained

Over the years, we’ve written a number of posts regarding the benefits of using molded silicone in keypad assemblies. One question that frequently gets asked by many of our customers is what type of webbing should be utilized and integrated into the rubber keypad design. 

There are a number of factors that should be taken into consideration when answering this question and they really are dependent on the end product’s functionality requirements. For instance, does the particular product call for a keypad with buttons that are relatively difficult to actuate to avoid accidental activation of keys? Does the product require a keypad that has buttons that are easy to activate? Does the product call for buttons that make an audible tactile snap when pressed? Regardless of the requirement, we have a number of methods to customize your elastomer keypad to work and feel a number of different ways. The main design feature that is utilized is the webbing or flex wall.

An elastomer button with an active web design has a small web or flex wall at the base of the button that flexes when the button is pressed. The resistance of this flex wall gives a tactile response as the button is pressed. The tactile response lets the user of the switch know that the key as been actuated. This particular design requires a carbon or metallic pill or puck which is molded into the underside of the rubber button. This pill completes the circuit when the button is pressed by making connection with the contact designed into the board or circuit. More on carbon pill technology can be found here. 

In contrast, a dead web rubber keypad design has buttons that do not include any web at the base of the keys or very minimal webs. By not using a web or flex wall, there must be some method of providing tactile feedback to the user. In this design, we typically use a metal dome which is placed under the rubber key. The metal dome makes contact with the board or circuit and provides the tactile audible feedback when pressed. There are a multitude of metal dome options that can be integrated into the keypad with different sizes and actuation forces really allowing the customer to get the exact feel they desire. More on metal dome technology and options can be found here. 

Rubber Keypad Carbon Pills
Rubber Keypad with Flex Wall & Carbon Pills
Rubber Keypad
Rubber Keypad with Dead Walls

Sealing Methods for Rubber Keypads

Sealing Methods for Rubber Keypads

When designing a product, one of the most critical considerations is sealing requirements. A properly sealed product prevents unwanted substance intrusion into the device. While internal components (most importantly electronics) are typically protected by the main enclosure, it is equally important that the keypad or interface area is also properly designed and sealed. Any weak-points related to the HMI or HMI area, can potentially allow for ingress of dust, liquids, and other substances that can cause permanent damage to the product. 

The good news is that there are a few methods that can be designed into the HMI to completely prevent particulate ingress. Below are some of the most common types of sealing methods used in our keypad designs:

Perimeter or Frame Seal: One of the most common methods of sealing, this method involves designing a frame of adhesive that goes around the perimeter of the keypad preventing any liquid or chemical ingress. Because it requires a specific amount and type of adhesive for an effective seal, the available space on the bezel and under the overlay is an important consideration. It is also important that there are no components on the edge of the keypad in order to accommodate this frame of adhesive.

Compression Seal: When using a rubber keypad design, the elastomer can act as an excellent seal as it has great compression and gasket-like characteristics. The silicone rubber is molded to be compression sealed to the front of the part or compressed from the sides into a well where the keypad sits. The compressed rubber offers a high level of protection from liquids and chemicals.

Wrap Around Seal: When using a rubber keypad design, the silicone rubber can be designed so the rubber actually wraps around the edge of the part, sealing off the front and edges of a device. The rubber wraps around the keypad’s internal layers, keeping the circuitry safe from any particulate ingress.

Front Surface Seal: If there are any openings in the front of the product’s bezel or case (typically around the perimeter of the part), front surface sealing can be an effective option. A sealant compound is dispensed into any openings or gaps in order to prevent particulates or liquids from entering. This is typically a “last resort” type of option, as the sealing process can be labor intensive and the final outcome not as aesthetically pleasing with a potting compound around the perimeter of the keypad. 

Rear Surface Seal: Rear sealing involves sealing any openings behind the keypad, typically around tails or cables that come through the rear housing of the device.

Hot Bar Soldered Membrane Switch
Perimeter Seal
CSI Keyboards Aerospace Membrane Switch
Compression Seal
Molded Rubber Keypad with Light Block Inserts
Wrap Around Seal

Designing a Sealed & Backlit Rubber Keypad​

Designing a Sealed & Backlit Rubber Keypad

Designing a rubber keypad that is both backlit and completely sealed may seem like a daunting task, but that’s what the experts at CSI are here for!  There are essentially two main components of the keypad assembly that must be properly designed: the rubber and the circuit. To better speak to the process, we will use the sample keypad on the right side of this post.

 

Rubber Decorating Process:

  1. The rubber starts off as a clear/milky translucent color (the color of the base material). The black portions of the keypad are rubber light blocks that are molded into the rubber to prevent light from bleeding into other portions of the keypad.
  2. The rubber is then sprayed translucent white.
  3. The rubber is then sprayed opaque black and then laser etched down to the translucent white material (for the power button) and hte clear base material (for the LED indicators) in the areas that are backlit.
  4. The rubber is carefully designed so that the actuators on the back of the keys press into the metal dome switches efficiently. 
  5. The rubber is also designed so that it provides a seal. Silicone is typically used for gasketing in many products, so why not utilize it’s properties for the same reasons in your keypad?!
  6. The rubber is then sprayed with a UV resistant coating that protects the keypad from ultraviolet exposure, while also providing chemical resistance. 

 

Additional Sealing Features:

Copper Flex Circuitry, also known as polyimide Kapton circuitry, are used in the majority of CSI Keyboards’ keypad designs due to its excellent dielectric strength, thermal stability, chemical resistance and flexibility. Copper Flex membrane switch panels are produced using polyimide (Kapton) as the base material. Copper flex switches are manufactured by laminating a thin sheet of copper to a flexible film substrate. The copper is then chemically etched away, leaving the copper traces. An additional layer of polyimide is laminated to the circuit leaving the gold contacts exposed. Copper flex has become the superior choice over printed silver especially for outdoor applications. Copper flex circuitry construction designs offer a significant advantage over printed silver and a printed silver circuit can be replaced with a copper and polyimide construction with minimal additional cost.

Seal Frame is a perimeter frame of adhesive that protects your circuitry from any moisture ingress. It have proven to be as robust as other sealing methods such as perimeter temperature sealing and can be included in your design at minimal additional cost.

PU (polyurethane) Coating was applied, which protects the printed and molded colors on the rubber keypad, and also increases the longevity of the printing and graphics. This coating ensures a longer keypad life regardless of the environment. We also utilize a proprietary coating specially formulated for marine and outdoor applications which provides added protection against UV exposure.

Rear 3M 300LSE Adhesive was utilized, which is the top of the line adhesive specially formulated to provide high bond strength to surfaces. Our adhesives are resistant to humidity, UV, water, temperature, and chemicals.

EMI/RFI and ESD Protection is obtained using a metalized Mylar shield layer. A separate tail for the shielding layer was designed to connect to the housing or another mechanical piece already grounded. Another option could have been grounding to a trace on the interface panel and then routed to a grounded plane on the motherboard to carry the static charge away from any nearby conductive components.

Undecorated Rubber Keypad
Undecorated Rubber Keypad
Decorated Backlit Rubber Keypad
Metalized Mylar Shield for Rubber Keypad
Backlit Rubber Keypad

Utilizing Vias in Membrane Switch Circuitry

Utilizing Vias in Membrane Switch Circuitry

When designing a membrane switch on a flexible printed circuit, the circuit can often require either vias or a double-sided circuit design.

What are vias?

Vias are small holes, typically measuring approximately .008”, that form a pathway between two sides of the circuit material. Vias allow electrical connections to flow between both sides of the circuit substrate. The circuitry ink is essentially forced through these small holes which create the via connection. If you look at a two-sided circuit membrane, the holes will be located at the end of the routes (as shown in the picture to the right). 

Why are vias used? 

Vias are typically used due to the size and space constraints, where the membrane switch circuitry has a small footprint that is unable to accommodate the circuitry routing and traces. By utilizing vias, we are able to use both sides of the circuit substrate which essentially doubles the real estate without having to increase the size of the part. 

Membrane Switch Vias
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