Introduction to membrane switch industry

A membrane switch is a control system that integrates button functions, indicator components, and instrument panels. It is aesthetically pleasing, lightweight, and has excellent conductivity. It is waterproof, dustproof, and resistant to electrostatic interference, making it widely used in fields such as electronic communication, medical equipment, industrial control, instrumentation, automotive industry, and smart home. The following is a brief introduction to the production process of membrane switches.

1. Engineering design

Several common structures of membrane switches

The membrane switch structure is divided into five layers: panel layer, surface adhesive layer, upper circuit, interlayer (isolation layer), lower circuit, and back adhesive. The panel layer generally uses high-toughness PET or PC transparent colorless sheets with a thickness of less than 0.25mm, printed with patterns and text, whose main function is to serve as a label and button. The surface adhesive generally uses strong double-sided adhesive with a thickness of 0.05---0.15mm to closely adhere the panel to the upper circuit. Both the upper and lower circuits use 0.125mm thick (in some cases, 0.075 and 0.1mm thick) polyester film (PET) with excellent toughness, screen-printed with conductive silver paste and conductive carbon paste, and dried and aged to achieve conductivity. For circuit outlet connections with a spacing of less than 1mm, the gold finger connection socket is prone to puncturing the silver paste, causing defects. It is recommended that the entire circuit adopts FPC with strong wear resistance to eliminate defects. The interlayer serves as an insulating and bonding layer between the upper and lower circuits, generally using double-sided adhesive with a thickness of 0.1--0.25mm and a PET substrate. For the bottommost back adhesive, it is necessary to consider which material it will be adhered to, and choose a strong double-sided adhesive that can adhere closely to the adhering surface. The membrane switch is an integrated unit, and all material selection must consider the overall sealing, waterproofing, insulation, and anti-aging properties. For outdoor membrane switches, it is also necessary to consider wind and rain, temperature changes between winter and summer, and strong ultraviolet rays from the sun.

Engineers should comprehensively consider the overall design of membrane switches, taking into account both scientific principles and technological feasibility, to ensure that the produced products are stable, reliable, economical, and practical.

2. Printing

Most of the printing for membrane switches is done through screen printing

The preliminary stage of silk screening involves plate exposure. Before plate exposure, it is necessary to use a film to compare with the sample to confirm whether the film is correct and free from damage. Check whether the screen is clean and free from dirt and damaged small holes. Generally, a 100T screen is used for circuits, and a 120T screen is used for panel exposure (especially fine ones may use a 140T or 160T screen). Then, compare with the sample to determine whether the film should be exposed normally or reversely (special attention should be paid to circuits, with the lower circuit, insulating oil, and jumper wires exposed normally, the upper circuit exposed reversely, and the shielding layer exposed normally, printed on the front of the upper circuit).

The mixing of ink is a crucial task in the silk-screen printing process, as it directly impacts the appearance quality of membrane switches. The color mixing master must possess a solid understanding of color knowledge and ink mixing techniques to ensure that the mixed ink exhibits vibrant colors, good brightness, and accurate hue.

          The variation pattern of the three primary colors

Theoretically, any color can be created by mixing different proportions of the three primary colors (magenta, yellow, and cyan). When equal amounts of the three primary color inks are mixed, they can be turned into black (approximately). By mixing equal amounts of the three primary color inks and adding white ink in different proportions, various shades of gray ink can be prepared. By mixing the three primary color inks in various proportions, a variety of intermediate or compound colors with different hues can be created, with the hue leaning towards the primary color with a larger proportion. When equal amounts of two primary color inks are mixed, they can form a standard intermediate color; when two primary color inks are mixed in different proportions, they can create intermediate colors with various hues, with the hue tending towards the hue of the primary color with a larger proportion. Furthermore, adding white ink to any color ink will make the hue appear brighter. Conversely, adding black ink will make the hue appear darker.

Screen printing includes sample printing, semi-automatic machine printing (for small batches), and automatic machine printing. Before loading the screen, it is necessary to check against the sample to ensure the screen is correct, the screen film is not damaged, and it is not reversed (with special attention to the circuit). During the printing process, it is important to repeatedly check whether the color registration of the printed products is consistent and accurate, and whether there are issues such as ink smearing, dry screen, or dust on the screen. Any problems must be promptly corrected by adjusting the machine. Each step should be checked repeatedly to ensure the product is flawless. Develop a good habit of self-inspection, and at the same time, pay attention to keeping the workshop and workstations clean and tidy, creating a harmonious and efficient working environment.

3. Assembly

Assembly is subdivided into drilling, bubbling, placing spring clips, bonding, forming, and terminal crimping, among other steps.

Drilling is a crucial aspect of ensuring precise bubbling. Inaccurate hole positioning can lead to bubbling keys not being centered, and the resulting shape may also be skewed, resulting in defective products. When setting up hole sampling, it is important to check whether the drilled holes are centered (the hole position deviation should not exceed 0.05mm). If not, sampling should be redone.

The quality of bubbling is closely related to the quality and precision of the mold. If the curvature of the mold keys is too steep, it is easy for the panel to become bubbled and not to rebound. If the curvature of the mold keys is too large, the panel will feel uncomfortable after bubbling. It is required that the height of each key (0.3~0.5mm) be equal, and the force (160~390g) be balanced.

Thin film switches generally use nickel-plated dome switches (also known as dome chips). When there are high requirements for resistance stability and button sensitivity, gold-plated dome switches (or gold-plated terminals in special cases) can be used. Before installing the dome switches (or dome chips), arrange them properly. Use tweezers to pick up the dome switches and place them at the button positions on the circuit, ensuring they are centered. After placing each dome switch, press it with the tweezers to ensure there is no double dome phenomenon. If there is a double dome phenomenon, the feel of the thin film switch will be significantly heavier, or even lose its tactile feedback.

Lamination is the concentrated embodiment of membrane switch assembly, which involves pasting each structural layer of the membrane switch into a unified whole. First, the interlayer is pasted onto the lower circuit. Then, the upper circuit is pasted onto the lower circuit in alignment (for those with spring clips, the spring clips are embedded onto the key positions of the interlayer, followed by pasting the spring clip fixing layer). The front side of the upper circuit is pasted with top glue, while the back side of the lower circuit is pasted with back glue. If the LED wiring is a separate structural layer, the light-transmitting part of the LED needs to be punched first, followed by pasting the LED wiring layer, and pasting the back glue on the back side. Finally, the panel is pasted (if the panel has transparent windows and light holes, the glue layers at the windows and light holes need to be punched out first). During each step of pasting, care should be taken to ensure there are no bubbles or impurities, and the glue should not wrinkle (for specifics, refer to the article "Assembly Techniques for Membrane Switches").

Stamping and forming have strict requirements for dimensions. For products with small quantities, die-cutting molds can be used. During the production process, it is recommended to randomly check the dimensions with a caliper to ensure accuracy and check for mold deformation. It is best to conduct shell trial attachment. If the mold is deformed, production should be stopped immediately and a new mold should be made. For products with larger quantities, steel molds should be used, as they have less deformation and more precise dimensions. Depending on the specific situation and customer requirements, the allowable tolerances for drawings without specified tolerances are generally as follows:：

           General dimensions(mm)               permissible tolerance(mm)

              18 below                    ±0.1

              18-50                       ±0.15

              51-125                      ±0.2

              126-250                     ±0.3

              251 above                    ±0.4

The burr and mold bending size of the membrane switch after stamping and forming should not exceed 0.2mm, and the position should be towards the side without conductors.

The gold finger PIN spacing is 2.54mm, and it is generally required to punch terminals and install black shells (with very few requirements for 1.27mm PIN spacing to punch terminals). Terminals are divided into male and female ends, with most membrane switches using female ends. For PIN spacings of 1.27mm, 1.0mm, and 0.5mm, terminals are generally not punched. Instead, reinforcement is applied to the back of the gold finger, with the reinforcement position generally flush with the gold finger carbon paste applied to the back of the gold finger. In special cases, it is applied to the designated position.

4. Detection

In terms of appearance, the panel of the membrane switch must be free of obvious defects, such as missing strokes or uneven thickness of characters; stains, light spots, ink removal, color spots, and scratches in the background color blocks; unclear transparent windows (special attention for F150 and F200PET printed with UV window liquid), glue overflow, residual glue, etc.; no misalignment (printing plate misalignment ≤ 0.15mm; combination of upper and lower line key positions misalignment ≤ 0.3mm; combination of lines and keypads, panel and button misalignment ≤ 0.3mm; blank area misalignment around the panel and line combination ≤ 0.2mm; bubble and substrate misalignment at the panel button position ≤ 0.5mm).

Functional testing requires a 100% inspection of membrane switches, ensuring no open circuits, short circuits, poor insulation, or other defects; LED-equipped switches must not exhibit LED leakage, as LEDs with resistors will quickly deteriorate. The resistance value of each button circuit of the membrane switch should not exceed 80Ω. For circuits with lengths greater than 400mm and special requirements for circuit resistance, additional specifications are provided on the drawing. In a normal temperature and humidity environment, the insulation resistance between conductor lines should be greater than 5000KΩ (500VDC for 1 minute). After high-temperature and constant humidity testing, as well as salt spray testing, the insulation resistance should not be less than 2000 KΩ.

The lifespan test requires that a single button, when pressed with a force of 2N, should have a contact conduction count exceeding 1 million times (or up to 2 million times for special requirements). However, merely inspecting the surface of the button for deformation or damage, tearing open the structural layer to check for internal damage, and examining the key position for wear, the lifespan test is only a test within a relatively short time frame, without considering the span of actual usage time and changes in usage environment.

For high and low temperature testing, the membrane switch is adhered to the housing and placed in an oven after one week, with the temperature adjusted to 60℃ (±2℃). After 72 hours of high temperature exposure, the membrane switch is removed from the oven, cooled, and tested for electrical performance compliance and dimensional changes, as well as any signs of cracking or peeling of the adhesive. Similarly, the membrane switch is placed in a freezer and subjected to a constant temperature of -20℃ (±2℃) for 120 hours of low temperature exposure. After being removed from the freezer and returned to room temperature, any water droplets on the surface are wiped off, and the membrane switch is tested for electrical performance compliance, dimensional changes, and any signs of cracking or peeling of the adhesive. For products requiring resistance to acids, alkalis, and salts, the membrane switch is immersed in a solution containing 5% of these substances, removed, and then subjected to high and low temperature testing. Finally, various electrical properties and appearance are tested for compliance.

5.  Packaging into warehouse

The key point for packaging and warehousing is to avoid heavy pressure. It is strictly prohibited to stack multiple products flatly; they must be arranged vertically. When packing, consideration can be given to placing smaller boxes inside larger ones to prevent the buttons from failing due to prolonged heavy pressure. The outgoing wires of membrane switches must be well protected to avoid any possibility of bending under any circumstances. Additionally, the warehouse must be well-equipped with moisture and dust prevention measures, and the surrounding air must be free of acidic, alkaline, or other corrosive gases.

6. Key parameters

Operating voltage: ≤50VDC

Operating current: ≤100mA

Contact resistance: 0.5~10Ω

Insulation resistance: ≥100MΩ (100VDC)

Base material withstand voltage: 2kVDC

Rebound time: ≤6ms

Loop resistance: ≤80Ω (within a loop length of 40cm for a single button)

Insulating ink voltage resistance: 100VDC

Reliability and service life: >1 million cycles

Key press force: 160~450g force

Operating temperature: -30℃ to +50℃

Storage temperature: -20℃~+45℃, humidity 95%±5%