Venting Membrane Switches: How is it Done?

Venting Membrane Switches: How is it Done?

Venting: 

When a key is pressed in a membrane switch, the air pressure within the switch cavity increases. In order for the switch to close properly, air within
a switch cavity must be displaced, equalizing the internal pressure. There are two standard venting methods that can solve this issue:

Internal Venting: 

Designing narrow channels between key location cutouts (in the spacer layer) allowing the air from one key to move to another key locatoin when that key is pressed. 

External Venting: 

Similar to internal venting, narrow channels between keys are cut into the spacer layer. These channels then exit through the sides, rear, or internal cut-outs of the membrane switch. External venting is not recommended for membrane switches exposed to harsh environments, as there is a greater risk of contamination. 

Standard Electrical Specifications for Membrane Switches

Standard Electrical Specifications for Membrane Switches

Due to the customized nature and the wide variety of membrane switch designs, it can be somewhat challenging to outline a general set of specifications that covers all membrane switches.  We’ve done our best to to listed below some basic electrical performance specifications.

  • Contact Material: silver, carbon, gold or nickel
  • Voltage: 30 volts DC
  • Rated Current/Voltage: 20mA@30 volts DC resistive load
  • Contact Bounce: < 20 milliseconds
  • Loop Resistance: The loop resistance of a switch is a function of trace width and length. In most applications the maximum loop resistance is less than 100 ohms.
  • Dielectric Strength: 5000V max on polyester material 
  • Open Circuit Resistance: 10 ohms
  • Capacitance: 20 picofarads
  • Maximum Switch Power: 1 watt
  • Design Configuration (illustrated below): Common Bus, XY Matrix, or Custom

Shielding Options for Membrane Switches

Shielding Options for Membrane Switches

Depending on the type of membrane switch design, the graphic overlay material or the molded rubber materials both have a relatively high dielectric strength and high volume resistivity. If required, a higher degree of electrical shielding can also be integrated into the membrane switch construction. The requirements can vary depending on the industry the product will be used in. 

Shields can be designed for the following:

  • ESD (Electrostatic Discharge)
  • EMI (Electromagnetic Interface)
  • RFI (Radio Frequency Interface)
 

CSI offers four shielding methods to protect membrane switches:

  • Foil: Laminated aluminum foil and polyester.
  • Transparent Film: Shielding required over windows (more costly).
  • Printed Screen: printed with silver conductive ink in a grid, bus-bar or full-coating format. Typically, the grid format is chosen because it is very reliable and does not use as much silver conductive ink as does the full-coating format.
  • EMI Mesh: The mesh is die-cut to the shape of the keypad and then assembled into the internal layers of the keypad assembly.
Membrane Switch with EMI Mesh

Shield Termination Methods:

  • Tab: The preferred method for reliability. Can be attached to a stud or stand-off on a back panel or metal enclosure.
  • Connector:  Shield layer can be terminated into a pin or pins on the circuit tail connector.
  • Wrap-Around: Shield layer can wrap completely around the membrane on all four sides to ground to a chassis. Although this method is very reliable, it is more costly than the other two methods due to the added labor and material necessary to execute. 

Silicone Rubber Explained – Part II

Silicone Rubber Explained - Part II

CSI Keyboards utilizes molded silicone in many of our keypad designs. Silicone is truly remarkable material. Without silicone’s properties, rubber keypads would not have moving keys, be able to close electrical switches or have self positioning features. 

Part II of our Silicone Rubber Series reveals many of silicone’s basic properties that allow it to be molded into a multi-functional keypad.

Silicone Raw Material

Raw silicone starts off with the consistency of clay. The raw silicone is first mixed together with a catalyst which assists in the molding processes. Different base raw silicones are mixed together in specific ratios and formulas to achieve a very specific silicone hardness.

Silicone Hardness / Durometer

Silicone rubber keypads can be made with different hardness (also known as durometer) ranging from 30 shore A to 80 shore A durometer. Rubber bands, for instance, have a durometer of 20 shore A, plastic is about 95 Shore A or higher. Raw silicone comes in base hardness of 30, 50, 70 and 80 Shore A. The standard durometer used for a molded rubber keypad is 60 Shore.

Two raw silicones can be mixed together to achieve a specific hardness (Example: a mixture with 50% silicone at 40 Shore A and 50% at 60 Shore A will result in a final material with a Durometer of 50 Shore). The hardness or durometer is based on the purpose of the rubber such as sealing requirements, tactile force requirements, insert molded keytops, molded light blocks and pull through tabs.

Silicone Color

Without pigment, silicone has a clear but slightly milky white color. Pigments can be added to the raw silicone mixture to make parts in virtually any color. Rollers are used to integrate the pigments into the raw silicone. If a silicone rubber keypad has multiple silicone colors, each color must be prepared separately.

Matching Silicone Color

The color of the silicone rubber keypad is controlled by the amount and type of pigments used in the molding process. CSI uses the Pantone system to match silicone colors, but we can also color match parts to plastic samples or color chips. Because of silicone’s unique texture, it is possible that the keypad will not color match in certain lighting conditions. This is called metamerism. In this case, the pigment formula can be slightly changed (example: made darker or more blue). Once the color is approved, the pigment formula is locked in assuring consistent color throughout production and life of the rubber keypad product.

Transparent & Tinted Silicone

Raw silicone without pigments will appear transparent (clear) or slightly milky white after molding. This is perfect for backlighting the keypad whether backlighting indicators, nomenclature or symbols. Using a tiny amount of pigment, the silicone can be made mostly transparent with a slight color tint. By varying the pigment color and amount, the tint can also be varied from highly transparent to virtually solid.

Choosing the Proper Adhesive for Your Membrane Switch Assembly

Choosing the Proper Adhesive for Your Membrane Switch Assembly

Choosing the proper adhesive for your membrane switch is one of the more critical decisions you can make during the design process. Achieving a solid bond between your membrane switch and the surface to which it is adhered is vitally important to reducing failures and ensuring the long term reliability of your product.

Your choice of adhesive essentially comes down to identifying the appropriate family of adhesives and the proper adhesive thickness. There are two families of adhesive used for these applications: “acrylic” adhesives and “modified acrylic” adhesives.

Acrylic Adhesives

Acrylic adhesives are a popular choice, and generally considered the industry standard, for membrane switch attachment applications. They provide outstanding adhesion to metal and high surface energy plastics. These adhesives provide some initial re-positionability for placement accuracy when bonding to plastics. They also perform well after exposure to humidity and hot/cold cycles. Typical metals to which acrylic adhesives bond well to are aluminum and steel. Compatible high surface energy plastics are usually ABS, acrylic, and polycarbonate, to name a few. Acrylic adhesives offer outstanding temperature and chemical resistance, as well as excellent shear strength to resist slippage and edge lifting.

A popular acrylic adhesive choice is 3M™ High Performance Acrylic Adhesive 200MP adhesive, which offers the following performance characteristics:

Humidity Resistance: High humidity has a minimal effect on adhesive performance. Bond strength shows no significant reduction after exposure for 7 days at 90⁰F (32⁰C) and 90% relative humidity.

UV Resistance: When properly applied, membrane switches are not adversely affected by outdoor exposure.
Water Resistance: Immersion in water has no appreciable effect on the bond strength. After 100 hours at room temperature, the high bond strength is maintained.

Temperature Cycling Resistance: High bond strength is maintained after cycling four times through: 4 hours at 158⁰F (70⁰C) 4 hours at -20⁰F (-29⁰C) 4 hours at 73⁰F (22⁰C)

Chemical Resistance: When properly applied, nameplate and decorative trim parts will hold securely after exposure to numerous chemicals including oil, mild acids and alkalis.

Bond Build-up: The bond strength of 3M™ 200MP increases as a function of time and temperature
Temperature/Heat Resistance: 3M™ 200MP is usable for short periods (minutes, hours) at temperatures up to 400⁰F (204⁰C) and for intermittent longer periods (days, weeks) up to 300⁰F (149⁰C).

Lower Temperature Service Limit: The glass transition temperature for 3M™ 200MP is -31°F (-35°C). Many applications survive below this temperature (factors affecting successful applications include: materials being bonded, dwell at RT before cold exposure, and stress below the TG [i.e.expansion/contraction stresses, impact]). Optimum conditions are: bonding high surface energy materials, longer time at RT before cold exposure, and little or no stress below the TG. The lowest service temperature is -40°F (-40°C).

Benefits of 3M Membrane Switch Adhesive

3M is a leading manufacturer of membrane switch adhesives, offering a number of benefits for membrane switch design engineers and product manufacturers. From excellent moisture and solvent resistance to exceptional resistance to high temperatures and a high shear strength to withstand the ongoing stress from repeated actuation, 3M membrane switch adhesives are a viable option for a great many membrane switch applications.

3M membrane switch adhesives may also be die-cut, allowing for the use of these adhesives in applications in which selective die-cutting is necessary or desirable. These benefits coupled with UV resistance and easy application make 3M membrane switch adhesives a leading choice among membrane switch designers, although other options do exist in the market. Available in sheets, custom sizes, and rolls, there are a multitude of options to choose from depending on your design and production requirements.

Modified Acrylic Adhesives

These adhesives provide high bond strength to most surfaces, including many low surface energy plastics such as polypropylene and powder coated paints. The modified acrylic adhesives also provide excellent adhesion to surfaces contaminated lightly with oil typically used with machine parts.

An often-used modified acrylic adhesive family is 3M™ Adhesive 300LSE. Typical performance characteristics are:

Bond Build-up: The bond strength of 3M™ Adhesive 300LSE increased as a function of time and temperature, and has very high initial adhesion.
Humidity Resistance: High humidity has a minimal effect on adhesive performance. No significant reduction in bond strength is observed after exposure for 7 days at 90°F (32°C) and 90% relative humidity.

U.V. Resistance: When properly membrane switches are not adversely affected by exposure.

Water Resistance: Immersion in water has no appreciable effect on the bond strength. After 100 hours at room temperature, the high bond strength is maintained.

Temperature Cycling Resistance: High bond strength is maintained after cycling four times through: 4 hours at 158°F (70°C) 4 hours at -20°F (-29°C) 4 hours at 73°F (22°C)

Chemical Resistance: When properly applied, membrane switches will hold securely after exposure to numerous chemicals including oil, mild acids and alkalis.

Temperature Resistance: 3M™ 300LSE adhesive is usable for short periods (minutes, hours) at temperatures up to 300°F (148°C) and for intermittent longer periods of time (days, weeks) up to 200°F (93°C).

Lower Service Temperature: -40°F (-40°C).

Choosing the Proper Adhesive Thickness

Once you have determined the appropriate adhesive family for your membrane switch adhesive, it is necessary to specify the thickness of the adhesive required to achieve a successful bond of the membrane switch to the mounting surface. When bonding the membrane switch to a smooth surface, it is generally acceptable to use a 2-mil (.002″) thick adhesive. If a texture is visible on the mounting surface, a 5 mil (.005″) thick adhesive would be suggested., in order to maximize the adhesive’s ability to flow around the textured surface profile, maximizing the surface area to which the adhesive can bond.

Silicone Rubber Explained – Part I

Silicone Rubber Explained - Part I

Silicone rubber is an elastomer (rubber-like material) composed of silicone (itself a polymer) containing silicon together with carbon, hydrogen, and oxygen. Silicone rubbers have become widely used in the keypad industry, and there are multiple formulations. Part I of our Silicone Rubber Series will detail these formulations.

Silicone rubbers are often one- or two-part polymers, and may contain fillers to improve properties. Typically the silicone rubber contains properties called “Organosiloxanes Polymer” which have originated from its unique molecular structure that can carry both inorganic and organic rubbers. Due to the Si-O bond of Silicone Rubber and its inorganic properties, Silicone Rubber is superior to ordinary organic rubbers in terms of heat resistance, chemical stability, electrical insulating, abrasion resistance, weather ability and ozone resistance etc. The silicone rubber compounds are used in various ways of application such as rubber keypads, industrial rolls, wires, thermal conductive pads, medical products, kitchenware, etc.

With these unique characteristics, silicone rubber has been widely used to replace petrochemical products in various industries like aerospace, munitions industry, automobile, construction, electric and electronics, medical and food processing industry. Recently, these scopes of silicone applications have been expanding at a great speed by the high demand of industries that require a more reliable elastomer.

HTV Silicone Rubber

Silicone Rubber is classified into HTV Silicone Rubber (High Temperature Vulcanization Silicone Rubber) and RTV Silicone Rubber (Room Temperature Vulcanization Silicone Rubber) by its curing temperature. Also, HTV Silicone Rubber is divided into Millable Type Silicone Rubber and Liquid Type Silicone Rubber by its degree of polymerization.

Millable Type Silicone Rubber (High Consistency Silicone Rubber)

Millable Type Silicone Rubber is composed mainly of Polyorgarnosilioxan (Silicone Polymer) and Silica with various additives to grant diversified characteristics. We call this stage of Silicone Rubber as “Base Compound”. This “Base Compound” is catalyzed, pigmented with a roll and cured by press molding and extrusion etc. Millable Type Silicone Rubber is also typically called as “HCR (High Consistency Silicone Rubber)”.

Liquid Silicone Rubber

LSR is Liquid Type and High Temperature Vulcanization Silicone Rubber. LSR differs from Millable Type Silicone Rubber and RTV (Room Temperature Vulcanization) by its degree of viscosity and curing temperature. LSR (Liquid Silicone Rubber) is perfect rubber material for automated injection molding due to its excellent liquidity. Also, LSR (Liquid Silicone Rubber) is ideal for complex molds, demanding design and tolerance because it can easily fill the most complex part of a mold.

Hot Bar Solder Lamination: What is It and Why Utilize It?

Hot Bar Solder Lamination: What is It and Why Utilize It?

Our customers sometimes require a membrane keypad with a connection that calls for flex circuits directly connected to rigid boards versus using connectors. Connecting a flexible circuit directly to a printed circuit board requires a manufacturing procedure called hot bar solder or heat seal lamination. There are many benefits for using a direct flex-to-board connection. 

The Main Benefits of Hot Bar Solder Lamination:

1. Saves vertical height in the design

2. Fast temperature ramp-up and cool-down 

3. Closed loop temperature control

4. Can be more reliable than fine pitch connectors

5. Accurate positioning of the parts

6. Multiple connections can be made simultaneously

7. Cost effective due to elimination of third component such as connector

ZIF Connectors on Membrane Switches: What Are They, What Do They Do?

ZIF Connectors on Membrane Switches: What are They & What do They do?

ZIF connectors have no physical connector on your membrane switch and rely on a receptacle connector that is available from numerous manufacturers. It is important to choose a ZIF connector that is suggested for membrane switches and to keep in mind that smaller pitches are available in polyimide flexible printed circuits only.

ZIF connectors are increasingly common in several applications, especially when connecting to a small display or keyboard. ZIF connectors meet a range of requirements including low profile, lightweight, secure, and removable connections. ZIF connectors are appropriate for more complex applications and allow for higher levels of integration. For ZIF connections, the connector system is not on the membrane tail, but on the PCB. The tail of the circuit is inserted into the ZIF connector to create the contact, and a stiffener is laminated under the tail to ensure stability and maintenance of the electrical contact. ZIF connectors may have anywhere from 2 to 30 positions on a single row, and distances of between 1mm and 2.54 mm are available.

Keep in mind that ZIF connections degrade with insertions, so the maximum number of insertions should not exceed 10 cycles. It’s also important to remember to take care when placing the tail die but in relation to the printed traces, since resistance in the circuit may increase in spacing of less than 1mm where the circuit traces are thinner.