led placement guidelines for membrane switches
TRANSCRIPT
![Page 1: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/1.jpg)
Membrane Switch LED Design Considerations
![Page 2: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/2.jpg)
LEDs
• Light Emitting Diode– “A light emitting diode (LED) is a semiconductor device
that emits near ultraviolet, visible, or infrared light when electrically biased in the forward direction.”
• Symbol
• Electrical Characteristics– Forward Direction (Typical 20 ma)– Reverse Direction (Open)
• Leakage current (Max 10 microamps typical)
![Page 3: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/3.jpg)
Non-Ohmic Device
• LEDs are Non-Ohmic Devices– Resistance varies with Current
Forward Current vs Forward Voltage
05
1015202530
1 1.2 1.4 1.6 1.8 2 2.2 2.4
Forward Voltage (V)
Forw
ard
Curr
ent (
mA)
Ohmic Device Non-Ohmic Device
![Page 4: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/4.jpg)
LED Selection
• Color– Wavelength, nm (nanometer)
• Brightness (or luminosity)– mcd (milli-candellas)
• Size (length and width)– 1206, 0805, 0603, 0402, 0201
• Cost– Blue and White
• Bi-Color/Tri-Color
![Page 5: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/5.jpg)
LED Size Matters
• Easier to Place
0603 Package(1.5 mm x 0.75 mm)
1206 Package(3.0 mm x 1.5 mm) = 4 X
This is NOT a PCB!!
![Page 6: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/6.jpg)
Thinner Components
• Height– Standard - 1.1 mm (0.044”)– Super Thin – 0.7 mm (0.030”)– Ultra Thin – 0.5 mm (0.020”)
• Thinner Spacers• Eliminate Embossing
– Reduce Emboss Height
Height
Can Reduce Overall Switch Costs
![Page 7: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/7.jpg)
Thin LEDs Specific for Membrane Switches
• Developed for Membrane Switch Industry– Introduced in 2001– Worldwide Usage
![Page 8: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/8.jpg)
MS LEDs
• 1206 LED– 3.2 mm x 1.6 mm x 0.5 mm (0.35mm)– Red, Yellow, Green, Orange, White and
Blue– Brighter then Standard LEDs
• Can be run at lower current– Red, Yellow, Green, Orange, White and
Blue– 1210 Package Available for Bi-Colors
• 3.2 mm x 2.5 mm x 0.5 mm
![Page 9: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/9.jpg)
PAD Design
• Substrate to Ink is Weakest Bond• Bond Direct to Substrate
Be Careful with UV Conductive Ink
![Page 10: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/10.jpg)
Circuit Design
• Series Circuit
Same Current, Same Brightness
![Page 11: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/11.jpg)
Circuit Design
• Parallel Circuit– LED Internal Resistance is not identical– Longer Traces Increase Difference
Different Current, Different Brightness
![Page 12: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/12.jpg)
500 Ohms12V
Current Limiting Resistors
• Limit Maximum Current (eg 24 ma)• Equalize Brightness when within the parallel circuit
– 83% reduction in current variation in example below
Most Common Compliant
![Page 13: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/13.jpg)
LEDAssembly
![Page 14: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/14.jpg)
Adhesive Placement
• Two Adhesive System– Center Drop of Acrylic Adhesive– Two Drops of Conductive Adhesive
Strength and Dielectric
![Page 15: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/15.jpg)
Conductive Adhesive
• One Part Adhesive– Longer Pot Life– Higher Curing Temperatures
• Two Part System– 4 hr Pot Life– Low Temperature Curing
• 48 hrs at room temperature• 60 minutes at 70°C• 15 minutes at 120°C
Dispense Adhesive First
![Page 16: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/16.jpg)
LED Placement
• Manual Placement– Timed Dispensing– Place with Tweezers– 50 to 100 per hour
• Automation– Cost and Quality Improvement
• Correct color and direction• Consistency
– Dispense & Place• 300 to 500 per hour
– Dispense then place• 1000 per hour and up
![Page 17: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/17.jpg)
Successful Placement
![Page 18: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/18.jpg)
Curing
• Batch Ovens– Must Re-Heat Oven to Temperature– Temperature Variation on Racks
• Conveyor Ovens– Infra-Red Vs Convection– Slow-Moving Conveyor– Must Profile Temperature
Oven Cure Increase Bond Strength
![Page 19: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/19.jpg)
Encapsulation
• Encapsulation– Do you Need it?
• Reliability• Subsequent Processing
– UV Material– Is Automation needed?– Problems
• Ink Compatibility
![Page 20: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/20.jpg)
Switch & LEDTesting
![Page 21: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/21.jpg)
Switch Testing
• 100% Testing Required• Membrane Switch Tester
– Tricor, Stingray– Shorts, Opens, and LED Illumination
• Test LEDs at Low Current Levels– Even if they are set to 20 ma– Can show False Negatives– Particularly problematic with Blue and White
LEDs and long traces– May need to retest with Power Supply
Tricor Model 913H
![Page 22: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/22.jpg)
LED Testing
• LEDs are Non-Ohmic– Resistance varies with Current– Very high resistance at low current
• Multi-meters– Separate Setting for Diodes
• Shear Test– ASTM F1995-00(2005)
Test with Constant Current Source
![Page 23: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/23.jpg)
Failure Causes
• Mis-Placed LEDs– Color, polarity– Accuracy
• Excess Adhesive (on top of LED)• Shorts
– Mitigated by Center Drop
• Poor Bonding/Intermittent– Poor Dispensing– Adhesive on top of LED– Adhesive/ink incompatibility
• Micro-Fractures from Handling• ESD Failures (blue & white)
![Page 24: Led placement guidelines for Membrane Switches](https://reader035.vdocument.in/reader035/viewer/2022081421/5881e16d1a28ab36088b4f37/html5/thumbnails/24.jpg)
Electro Static Damage
• Blue & White LEDs– Susceptible at low levels
• Damaged at Any Stage– Placement– Curing– Testing– Subsequent Assembly– Customer Installation
• Difficult to Detect– Reverse Voltage breakdown– Reverse Voltage > 10 microamps