1. Electrical contacts What is an electrical contact?
Why doesn’t it always work?
Lena Sjögren
Corrosion and Metals Research Institute

2. 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

4. Electrical contacts Power transmission Signal transmission
Grounding – personal safety
Signal ground
Electromagnetic shielding

5. Kontaktområde - förstoring

7. Contact area
Force

8. 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

9. Constriction resistance
one contact spot
plastic deformation
contact area
contact force
hardness
constriction resistance
resistivity
contact spot diameter

10. Gold plated connector contact
0,5N contact normal force
Constriction resistance: 1mW

11. 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. 10 1
Contact area after use
Initial contact area 1
100 = =

12. Kontaktrörelse - återbildningF
Contact area, cross section
Contact surface seen from above

13. Kontaktrörelse - återbildningF
Contact area, cross section
Contact surface seen from above

14. Resistans som funktion av tid/antal
Resistance/contact failures
Time
Ack. number

15. Tenn – rensning av ytfilm
Contact area, cross section
Contact surface seen from above
Contact area, cross section

16. Rensning av ytfilmer, forts.
Noble metal contacts
Base metal contacts
Soft films on hard substrate
Hard films on soft substrate
Squeeze and shear
Crack
Friction force
Contact force
Cylinder cylinder
Rough surface, Ra>0,5
min mm wipe

18. Fritting silver, copper

19. Rensningsmekanismer, översikt

20. Current constriction through contact spots – - constriction resistance
Termiska effekter
Current constriction through contact spots – - constriction resistance
Contact spot
Kontaktpunkt
- heat emission

21. Silver - temperaturökning

22. Tenn - temperaturökning

23. Failure mechanisms Contact force reduction
Initially too low contact force / variations
Plastic deformation (manufacturing, closing)
Stress corrocion cracking
Stress relaxation

24. Stress relaxation

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

26. Pore corrosion
Corrosion creep

27. Failure mechanisms Wear Mating wear Fretting 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

28. Fretting oxidation/corrosion

29. Fretting risk – tangential force control
Vid mekaniska rörelser 2A
möjlighet kontrollera effekter även av termiska rörelser

30. Design and materials
Where is the spring? design and materials Þ spring properties
Contact material Þ environmental resistance, requirements on contact force, surface geometry etc.
Housing Þ mechanical stability, environmental resistance

31. Anslutningsdon

32. AgCdO-relay – with/without enclosure, open/closed
Closed contact, enclosed
Open contact, enclosed
Closed contact, not enclosed
Open contact, not enclosed
upper limit G2
lower limit G1
Bulletin No 102, ”Corrosion of Electronics”

33. AMP-don, tredje fjäder

34. IDC (Insulation displacemenet contacts)

35. Screw

36. Skruv med jordbricka, klimatprovning
Contact resistance
Grounding washer
Tin
Initial
Change of temp
Dry heat +85°C
Damp heat
Corrosion
Aluminium
PCB

37. Skruv utan jordbricka, klimatprovning
Contact resistance
Initial
Change of temp
Dry heat +85°C
Damp heat
Corrosion

38. Sammanfattning