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Electrical contacts

What is an electrical contact?

Why doesn’t it always work?

Lena SjögrenCorrosion and Metals Research Institute

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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

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Power transmission

Signal transmission

Grounding – personal safety

Signal ground

Electromagnetic shielding

Electrical contacts

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Kontaktområde - förstoring

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Contact area

Force

H

FA

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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

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contact areacontact forcehardnessconstriction resistanceresistivitycontact spot diameter

Constriction resistanceone contact spotplastic deformation

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Gold plated connector contact

0,5N contact normal force

Constriction resistance: 1m

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Example

0,5 N contact normal force

hard gold, 160 HV (kp/mm2)

d = 0,02 mm

Rconstr. = 1,3 m

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

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

=Resistance increase

Rconstr.

Contact area after use

Initial contact area

10

1=

1

100

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Kontaktrörelse - återbildning

Contact area, cross section Contact surface seen from above

F

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Kontaktrörelse - återbildning

Contact area, cross section Contact surface seen from above

F

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Resistance/contact failures

Time

Resistans som funktion av tid/antal

Ack. number

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Tenn – rensning av ytfilm

Contact area, cross section

Contact area, cross section Contact surface seen from above

F

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Soft films onhard substrate

Hard films onsoft substrate

Squeeze and shear Crack

Noble metal contacts Base metal contacts

Friction force Contact force

Cylinder cylinder Rough surface, Ra>0,5

min 50-100m wipe

Rensning av ytfilmer, forts.

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Frittingsilver, copper

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Rensningsmekanismer, översikt

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Termiska effekter

KontaktpunktCurrent constriction through contact spots –- constriction resistance- heat emission

Contact spot

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Silver - temperaturökning

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Tenn - temperaturökning

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Failure mechanisms Contact force reduction

Initially too low contact force / variations Plastic deformation (manufacturing, closing) Stress corrocion cracking Stress relaxation

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Stress relaxation

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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

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Pore corrosion

Corrosion creep

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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

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Fretting oxidation/corrosion

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Fretting risk – tangential force control

0

2

4

6

8

10

12

0 100 200 300 400 500 600Time (s)

Co

nta

ct

vo

lta

ge

dro

p (

mV

)

1.97 N 1.95 N 1.9 N 1.7 N 1.5 N

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Design and materials Where is the spring?

design and materials spring properties

Contact material environmental resistance, requirements on contact force, surface geometry etc.

Housing mechanical stability, environmental resistance

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Anslutningsdon

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Open contact, not enclosed Closed contact, not enclosedOpen contact, enclosedClosed contact, enclosed

Bulletin No 102, ”Corrosion of Electronics”

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

lower limitG1

upper limitG2

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AMP-don, tredje fjäder

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IDC (Insulation displacemenet contacts)

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Screw

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Skruv med jordbricka, klimatprovning

Tin

Grounding washer

AluminiumPCB

1 Initial

2 Change of temp

3 Dry heat +85°C

4 Damp heat

5 Corrosion

Contact resistance

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Skruv utan jordbricka, klimatprovning

1 Initial

2 Change of temp

3 Dry heat +85°C

4 Damp heat

5 Corrosion

Contact resistance

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Sammanfattning