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High Reliability KeypadDesign and Assembly

09.26.13

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Topics Covered

Target Markets Terms and Definitions Basic Core Switch Technology Factors That Can Help or Harm Long-term Performance Extending Electrical and Mechanical Operating Life

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Target Markets

Target markets are end applications requiring critical-mission performance, long service life, or capital equipment applications where the cost of the keypad assembly is a tiny portion of the equipment cost and expected service life.

Markets that are not considered target markets are applications where the cost of the keypad is a large portion of the application cost or the application service life is very short such as “throw-away” electronics or applications with rapid obsolescence.

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Target Markets

Target Markets– Military Electronics

– Satellite Communications Ground Support Equipment

– Mobile Communications

– Medical Electronics

– Electronic Test Equipment

– Larger Process Control Equipment

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Terms and Definitions

Single Pole Switch– A simple on-off switch with two terminals or contacts which are either

connected together (closed switch) or disconnected from each other (open switch). Each single switch is also called a “pole”, so the symbol shown is a single pole switch.

Normally Open Switch– Switch terminals or contacts that are not connected when the switch is not

pressed or actuated. Tactile metal dome switches are all normally open switches.

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Terms and Definitions

Contact Bounce– Unwanted circuit noise caused by the switch mechanism. When the

contacts strike together, their momentum and elasticity act together to cause the switch contacts to bounce apart one or more times before making steady contact. The result is a rapidly pulsed electric current instead of a clean transition from zero to full current. The effect is usually unimportant in power circuits, but causes problems in some logic circuits that respond fast enough to misinterpret the on‑off pulses as a data stream.

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Terms and Definitions

Switch Array– A group of switches connected together in a scheme to minimize the

number of switch terminal connections for interfacing with electronics. Common switch arrays are “common pole” and “Row-Column matrix” or X-Y matrix”.

– Common-connected switch arrays have oneswitch terminal from each switch connectedto a common circuit and the other switchterminals are connected individually.

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Terms and Definitions

Row-Column– Or X-Y matrix arrays have switches connected where switches share

interface circuits but no two switches share the same two interface circuits. X-Y matrix arrays decrease the number of required interface circuits for a larger number of switches.

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Terms and Definitions

Logic-Level Circuit Switches– Resistive only switching with open circuit voltage less than 30 volts and

closed circuit current less than 20 milliamps. Tactile metal dome switches are not suitable for high voltage, high current or capacitive or inductive loads. Logic level switches can control power circuits but require suitable buffering.

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Terms and Definitions

High Reliability Low-Profile Keypads– A keypad assembly using tactile metal dome switches providing

momentary, normally open switches in an environmentally sealed switch array with suitable interface connectors and finished with graphic overlay or silicone molded rubber boots. Keypads can include shielding, displays, discrete LEDs, backlighting, other electronics and mechanical hardware such as studs.

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Basic Core Switch Technology

Core Switch Technology revolves around tactile metal dome switches with supporting structure and esthetic touches designed for long service life while looking like the keypad assembly belongs in the application.

Switch domes were developed in the early 1970s for consumer electronics requiring a low-profile, long life keypad including hand-held electronic calculators and television onset and remote controls.

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Basic Core Switch Technology

Virtually all switch domes in use today are stamped from high tensile strength stainless steel for predictable mechanical life.

Properly designed keypad applications using tactile metal switch domes have very low closed switch resistance, very high open switch resistance, and very low closing and opening switch contact bounce.

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Basic Core Switch Technology

Dome switches provide tactile snap because the stainless steel has high tensile strength and the stamped dome shape defines the applied force versus tactile snap (force-deflection curve).

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Performance Factors

Factors That Can Harm Long Term Performance– Non-conductive oxides forming on the tactile metal dome switches

– Non-conductive oxides forming on the printed circuit board pads directly under the tactile metal dome switches

– Outside contaminants entering into the switch dome cavity

– Exceeding switch dome voltageor current limits

– Dome switches controlling circuitsother than resistive loads(no capacitive or inductive loads)

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Performance Factors

Non-conductive Oxides Forming on the Tactile Metal Dome Switches– Virtually all tactile metal dome switches available today are stamped from

stainless steel alloys.

– All stainless steel alloys contain a small amount of chromium in the alloy. Chromium reacts with oxygen in the air and forms a thin layer of chromium oxide on the surface of stainless steel.

– The chromium oxide is transparent so you cannot see it. This oxide layer seals the iron particles in the stainless steel from the oxygen in the air so that there's no formation of iron oxide (iron rust). This action is what makes stainless steel "stainless.” However, chromium oxide is also nonconductive – big problem for any switch. Chromium oxide can be temporally worn off by dome switch use but quickly forms again.

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Performance Factors

Non-conductive Oxides Forming on Printed Circuit Board Pads Under Tactile Metal Dome Switches– Printed circuit board plating or finishes over copper other than gold will

form oxides or will not be flat as required for proper dome switch operation.

Outside Contaminants Entering the SwitchDome Cavity– Any airborne contaminants such as dirt, dust, lint or dissolved

contaminates in liquid form must not enter the switch dome cavity as virtually all contaminates are non-conductive.

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Performance Factors

Exceeding Dome Switch Voltage or Current Limits – Exceeding dome switch open circuit voltage of 30 volts or closed circuit

current of 20 milliamps quickly deteriorates the switch contact surfaces and causes pitting.

– Contact pitting increases contact resistance, intermittent operation and over-heating.

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Performance Factors

Dome Switches Controlling Circuits Other Than Resistive Loads (No Capacitive or Inductive Loads)– Non-resistive switch dome loads causes switch contact surface pitting

faster than exceeding voltage and current limits.

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Performance Factors

Solutions to Factors That Can Harm Long Term Performance– Prevention of Non-conductive Oxides Forming on Tactile Metal

Dome Switches • The best solution is gold plated metal dome switches for prevention of

chromium oxide. • The problem with standard gold plating process is the hydrogen-based plating

chemistry causes hydrogen embrittlement in the stainless steel that dramatically reduces the mechanical life of the dome switches.

• Our proprietary gold plating process does not use hydrogen-based plating chemistry, prevents chromium oxide on surfaces of stainless steel metal dome switches and does not affect mechanical life.

– Prevention of Non-conductive Oxides Forming on Printed Circuit Board Dome Switch Pads

• Gold plated printed circuit boards are necessary for the longest performance.

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Performance Factors

Solutions to Factors That Can Harm Long Term Performance– Stopping Outside Contaminants Entering the Dome Switch Cavity

• Printed circuit boards used with metal dome switches must be designed to prevent printed circuit board vias under or near dome switches from acting as outside air vents. Many commercial industry custom keypad manufacturers purposely place printed circuit board vias under the dome switches to make construction easier and maximizing the dome switch tactile feel.

• Design and construction techniques allow Suncoast Digital Technology to internally vent the switch cavities without venting to the outside atmosphere.

• Dome switch cavities need to be environmentally sealed for best long term performance.

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Performance Factors

Solutions to Factors That Can Harm Long Term Performance– Stay Within Dome Switch Voltage and Current Limits and Using Only

Resistive Loads• Logic-level tactile metal dome switches should only interface with CMOS or

high impedance circuits at logic-levels to prevent contact surface pitting. • Dome switches should never directly control reactive loads such as discharging

capacitors or switching inductive loads..

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Performance Factors

Other Design and Manufacturing Characteristics of High Reliability Keypads– Most high reliability custom keypad assemblies require shielding from

internally or externally generated RF energy, or electrostatic discharge.

– Shields can be incorporated into keyboard designs and must be positioned between the user’s finger that pushes against the switches and all circuitry below including the dome switches.

– Electrostatic discharges follow the path of least resistance from a charge contained on a user's finger to earth or chassis ground. It is imperative that this discharge path be intercepted prior to entering any of the keypad and electronic circuitry including the tactile metal dome switch.

– Shield selection and termination methods must consider RF frequency and level of interference attenuation required. At the same time, the shield material must be flexible to allow the dome switch to operate properly while preserving tactile feel.

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Performance Factors

Other Design and Manufacturing Characteristics of High Reliability Keypads– Best tactile feel for unambiguous switch operation for the end-user

requires that the tactile feel of the raw dome switch is not diminished by the keypad’s construction methods and materials.

– Materials between the dome switch circuitry and the user's finger and sealing the dome switch cavities can reduce the tactile feel.

– Overall keypad design and construction must be coordinated with the customer’s engineering staff forsuccessful user interface products.

– All of these factors must be dealtwith as a combination instead ofindividual parts to preserve thebest tactile feel.

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Our Products

Battery Packs Flex & Rigid-Flex PCB’s User Interfaces

Fans & Motors Cable Assemblies Printed Circuit Boards

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Design Centers &Technical Support

Battery Pack & Power Management – Denver, CO User Interfaces – Largo, FL Fans & Motors – Wales, UK PCB’s – New Bedford, MA & Shenzhen, China Flex & Rigid Flex – Toronto, Canada Cable Assemblies – New Bedford, MA

Our Engineering and Design teams are ready to help our customers create world class and cost effective product solutions.

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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, wewill reply via email.

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