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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.

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Electrical contacts

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  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

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

  4. Kontaktområde - förstoring

  5. Contact area Force

  6. 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

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

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

  9. 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 = =

  10. F Contact area, cross section Contact surface seen from above Kontaktrörelse - återbildning

  11. F Contact area, cross section Contact surface seen from above Kontaktrörelse - återbildning

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

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

  14. 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

  15. Frittingsilver, copper

  16. Rensningsmekanismer, översikt

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

  18. Silver - temperaturökning

  19. Tenn - temperaturökning

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

  21. Stress relaxation

  22. 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

  23. Pore corrosion Corrosion creep

  24. 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

  25. Fretting oxidation/corrosion

  26. Fretting risk – tangential force control

  27. 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

  28. Anslutningsdon

  29. 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”

  30. AMP-don, tredje fjäder

  31. IDC (Insulation displacemenet contacts)

  32. Screw

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

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

  35. Sammanfattning

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