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Electrical contacts. What is an electrical contact?. Why doesn’t it always work?. Lena Sjögren Corrosion and Metals Research Institute. Cable connection between two printed circuit board assemblies.

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electrical contacts
Electrical contacts
  • What is an electrical contact?
  • Why doesn’t it always work?

Lena Sjögren

Corrosion and Metals Research Institute

cable connection between two printed circuit board assemblies
Cable connection between two printed circuit board assemblies

An electrical signal between the two boards will be transmitted through six contacts: solder joint (printed circuit board to contact pin) - connector (contact pin to contact spring) - insulation displacement connection (contact spring to cable) - insulation displacement connection - connector - solder joint

electrical contacts1
Electrical contacts
  • Power transmission
  • Signal transmission
  • Grounding – personal safety
  • Signal ground
  • Electromagnetic shielding
slide8

Current is constricted through the contact spots

The detour required from the current (the electrons) results in an additional voltage drop thus resistance - constriction resistance

slide9

Constriction resistance

one contact spot

plastic deformation

contact area

contact force

hardness

constriction resistance

resistivity

contact spot diameter

slide10

Gold plated connector contact

0,5N contact normal force

Constriction resistance: 1mW

example

10

1

Example

0,5 N contact normal force

hard gold, 160 HV (kp/mm2)

d = 0,02 mm

Rconstr. = 1,3 mW

Measured, new contact: 11,3 mW (bulk 10,0 mW)

after use: 23,0 mW (bulk 10,0 mW)

Resistance increase

Rconstr.

Contact area after use

Initial contact area

1

100

=

=

kontaktr relse terbildning

F

Contact area, cross section

Contact surface seen from above

Kontaktrörelse - återbildning
kontaktr relse terbildning1

F

Contact area, cross section

Contact surface seen from above

Kontaktrörelse - återbildning
resistans som funktion av tid antal
Resistans som funktion av tid/antal

Resistance/contact failures

Time

Ack. number

tenn rensning av ytfilm

F

Contact area, cross section

Contact surface seen from above

Tenn – rensning av ytfilm

Contact area, cross section

rensning av ytfilmer forts
Rensning av ytfilmer, forts.

Noble metal contacts

Base metal contacts

Soft films onhard substrate

Hard films onsoft substrate

Squeeze and shear

Crack

Friction force

Contact force

Cylinder cylinder

Rough surface, Ra>0,5

min 50-100mm wipe

termiska effekter
Termiska effekter

Current constriction through contact spots –- constriction resistance

Contact spot

Kontaktpunkt

- heat emission

failure mechanisms
Failure mechanisms
  • Contact force reduction
    • Initially too low contact force / variations
    • Plastic deformation (manufacturing, closing)
    • Stress corrocion cracking
    • Stress relaxation
failure mechanisms1
Failure mechanisms
  • Contact force reduction
    • Initially too low contact force / variations
    • Plastic deformation (manufacturing, closing)
    • Stress corrocion cracking
    • Stress relaxation
  • Film growth
    • Corrosion
    • Diffusion
    • Contamination
slide26

Pore corrosion

Corrosion creep

failure mechanisms2
Failure mechanisms
  • Contact force reduction
    • Initially too low contact force / variations
    • Plastic deformation (manufacturing, closing)
    • Stress corrocion cracking
    • Stress relaxation
  • Film growth
    • Corrosion
    • Diffusion
    • Contamination
  • Wear
    • Mating wear
    • Fretting
design and materials
Design and materials
  • Where is the spring?design and materials Þspring properties
  • Contact material Þ environmental resistance, requirements on contact force, surface geometry etc.
  • Housing Þ mechanical stability, environmental resistance
agcdo relay with without enclosure open closed

Open contact, not enclosed

Closed contact, not enclosed

Open contact, enclosed

Closed contact, enclosed

AgCdO-relay – with/without enclosure, open/closed

upper limitG2

lower limitG1

Bulletin No 102, ”Corrosion of Electronics”

skruv med jordbricka klimatprovning
Skruv med jordbricka, klimatprovning

Contact resistance

Grounding washer

Tin

  • Initial
  • Change of temp
  • Dry heat +85°C
  • Damp heat
  • Corrosion

Aluminium

PCB

skruv utan jordbricka klimatprovning
Skruv utan jordbricka, klimatprovning

Contact resistance

  • Initial
  • Change of temp
  • Dry heat +85°C
  • Damp heat
  • Corrosion