led placement guidelines for membrane switches
TRANSCRIPT
Membrane Switch LED Design Considerations
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)
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
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
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!!
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
Thin LEDs Specific for Membrane Switches
• Developed for Membrane Switch Industry– Introduced in 2001– Worldwide Usage
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
PAD Design
• Substrate to Ink is Weakest Bond• Bond Direct to Substrate
Be Careful with UV Conductive Ink
Circuit Design
• Series Circuit
Same Current, Same Brightness
Circuit Design
• Parallel Circuit– LED Internal Resistance is not identical– Longer Traces Increase Difference
Different Current, Different Brightness
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
LEDAssembly
Adhesive Placement
• Two Adhesive System– Center Drop of Acrylic Adhesive– Two Drops of Conductive Adhesive
Strength and Dielectric
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
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
Successful Placement
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
Encapsulation
• Encapsulation– Do you Need it?
• Reliability• Subsequent Processing
– UV Material– Is Automation needed?– Problems
• Ink Compatibility
Switch & LEDTesting
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
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
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)
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