1. Manufacturing That Eliminates Risk & Improves Reliability
How to Properly Select LEDs for
Your Keypad
November 19th 2021

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Agenda
 Membrane & Keypad Overview
 LED Options for Keypads
 Design Options to Reduce Risk
 Case Study
 How To Get Started With Your Custom Build

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Membrane & Keypad Overview

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Membrane Switch & Keypad Options
 Membrane Switches
– A momentary switch device in which at least one
contact is on, or made of, a flexible substrate-
ASTM
– LEDs, dome switches, and adhesive
 Silicone Rubber Keypads
– Compression molded silicone elastomer
– 40-60 Shore A hardness
– Pair with dome switch or conductive carbon pill
 High Reliability & Low Cost
 LEDs for Backlighting
– LED windows
– Uniform backlighting
Silicone overlay with circuit
LED
Locations

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Membrane Switch & Keypad Options

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Keypad Circuit Options
 PET- Polyester Circuit
– Silkscreened conductive ink
– Flexible laminated layers, limited layer count
– Typically, 0.007” – 0.025”
 Rigid PCB
– Etched copper traces with Laminated FR4
– Ideal for surface mounted components
– Typically, 0.063” – 0.250”
 FPC
– Etched copper with polyimide base circuit
– Fine pitch ZIF tails
– Typically, 0.007” – 0.025”
Circuit type will drive LED selection
Flex FPC PET Circuit
Rigid PCB

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LED Options for Keypads

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LED Design Options
 Wikipedia LED Definition
– A light-emitting diode (LED) is a semiconductor
light source that emits light when current flows
 Applications
– Front panels & HMIs
– Automobile headlights
– Interior lighting
– PCBAs
 Electrical Operation
– Most operate of 5VDC or less
– Act as a diode - current flows one way
– Must include a current limiting resistor
LED polarity matters for custom installations
Printed Circuit Board with LEDs
LED Strip

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LED Types to Consider
 Leaded LEDs
– Through-hole LEDs
– Wide range of lenses and viewing angles
– Must be used with rigid PCB base circuits
 SMD LEDs
– Surface Mounted Devices
– No wires, solder direct to circuit
– Wide variety of sizes and colors
 Side-firing LEDs
– Right angle surface mounted LED
– Usually paired with Light Guide Film
– Limited availability of sizes and colors
Not all LEDs have drop in replacements
Side-firing LED
Rigid PCB with through-hole LEDs
SMD LED
Leaded LED
Single surface mounted LED
LGF backlit text

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Light Guide Film
 Use with Side Firing LEDs
– Place LED outside the viewing window
– LED is not visible to the user
 Uniform Backlighting
– Uniform brightness
– Reduced hotspots
 Selective Backlighting
– Individually lit keys
– Lightblock in strategic locations
– Needs sufficient space
Not all selective backlighting designs are manufacturable

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LED Size and Color
 Lots, and lots, and lots to chose from
– Manufacturer
– Package size
– Electrical performance
– Brightness and chromaticity
 Design Flexibility
– Specify high level requirements for non-
critical applications
– Add verbiage to drawing notes permitting
alternate LEDs
Not all LEDs have drop in replacements

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LED Size and Color
 Keep It Simple
– What are you trying to accomplish?
– Must Have’s vs. Nice To Have’s
 Plan On Making Changes
– Build samples quickly, then assess
– Obsolescence risk
Not all LEDs have drop in replacements

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Other Keypad Backlighting Options
 Fiber Optic Backlighting
– Uses light source with glass or plastic
fibers
 Light Pipes
– Uses injection molded plastics as an
elongated lenses
LEDs are the primary light source for most backlighting arrangements

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Design Options to Reduce Risk

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High Risk LED Choices
 Small Package Sizes
– Some membrane switch circuits are hand
populated
– Choose 0603 size and larger
 Yellow vs Amber
– Brightness must be tested
– Can either color be used?
Dual source all LEDs where possible
Yellow LED vs Amber LED

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High Risk LED Choices
 ESD Threshold
– Is there shielding?
– Will these be used in ESD prone areas?
 Bicolor & Tricolor LEDs
– Bicolor & Tricolor LEDs are more scarce
– Obsolescence risk
– May require a respin to change
Dual source all LEDs where possible
Bicolored Red/Blue LED

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Adjusting Keypad Brightness
 Perceived Brightness
– Must be tested at the end of assembly
– Is it too late to make changes?
 What Impacts LED Brightness?
– Materials above the LED will change its intensity
– Translucent silicone and graphic overlays
Reduce risk by dual sourcing LED Driver ICs

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Adjusting Keypad Brightness
 Swap LEDs vs Modify Power
– Potential for circuit respin
 PWM Local Brightness Control
– Pulse Width Modulation
– Added to parent PCBA
Reduce risk by dual sourcing LED Driver ICs
250 mcd luminance LED

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Night Vision Imaging Systems (NVIS)
 Mil-STD-3009
– Military specification that details the requirements for
aircraft cockpit indicators and lighting systems.
– NVIS Green A and Green B
 NVIS Compliant LEDs
– Use Off-The-Shelf LEDs
– Pair white LEDs with caps
– Consider NVIS films
 Chromaticity
– Requires specialty instrumentation
– Transparent materials may filter
NVG compatible LEDs are scarce and known for long leadtimes

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LED Placement
 Mounting locations and techniques are critical
 Spacing Guidelines
– Avoid placing LEDs within 4mm perimeter keep-out
zone
– 15mm keypad spacing for selective backlighting
 Rear Mounted LEDs
– Typically reserved for rigid PCBs
– Optimize overall height
– Necessary for backlit center hole domes
 Location Drives Intensity
– LEDs can be placed adjacent or behind switches
Clever mounting arrangements can yield truly custom results
Selective Backlighting Spacing

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Case Study

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Bezel Assembly- Case Study
 Custom backlit bezel assembly
 Narrow PCBA 0.75” wide
 Local brightness and voltage control
 6-week development timeline

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Bezel Assembly- Case Study
 PCBA Footprint and Layer Count
Drives the Design
– Limited circuit routing options with 4-layer
PCB
 Electrical Requirements
– Aircraft power 28VDC
– DC-DC converter necessary
– 25 x LEDs in series
 Local Brightness Control
– PWM LED driver added to circuit
LEDs mounted to PCBA

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How to Get Started

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Initial LED Backlighting Considerations
 Backlit button vs discrete LEDs
 Number of buttons to illuminate
 Colors of the LEDs
 Backlight uniformity requirements
 LED package size
 Overall height concerns
 LED mounting arrangements
 Are alternate LEDs allowed?
 Dimmable brightness
 Are LEDs in series with switches?
 Will light guides be necessary?
 Approval stakeholders
Schematic and a dimensioned drawing are required to launch development

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Development Timeline
Most projects yield functional samples in 5 weeks
Initial Concept
Drawings & Schematic
Samples
Evaluation
Production
Typical Development Lead-Times
Standard Keypad 4 - 6 weeks
Complex Keypad 6 - 8 weeks
SMART HMI 8 - 20 weeks

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Summary

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Summary
 LEDs are widely available, reliable and inexpensive
 Countless sizes, colors, and package sizes
 Susceptible to long lead-times, high minimum order quantities
 Circuit redesigns can be expensive
 Clear delineation of Must Have vs. Nice To Have requirements

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Our Products
Battery Packs Flex & Rigid-Flex PCBs Cable Assemblies Printed Circuit Boards
RF Products User Interfaces Flexible Heaters EC Fans & Motors

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Q&A
 Questions?
– Enter any questions you may have
in the Control Panel
– If we don’t have time to get to it, we
will reply via email

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Thank You
Check out our website at www.epectec.com.
For more information email sales@epectec.com.
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