electrical contacts
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
23-04-19
Electrical contacts
What is an electrical contact?
Why doesn’t it always work?
Lena SjögrenCorrosion and Metals Research Institute
23-04-19 kabels_kurs.swf
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