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