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2015-04-03 Electrical contacts What is an electrical contact? Why doesn’t it always work? Lena Sjögren Corrosion and Metals Research Institute.

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Presentation on theme: "2015-04-03 Electrical contacts What is an electrical contact? Why doesn’t it always work? Lena Sjögren Corrosion and Metals Research Institute."— Presentation transcript:

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

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

5 Kontaktområde - förstoring

6

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

10 Gold plated connector contact 0,5N contact normal force Constriction resistance: 1m 

11 Example 0,5 N contact normal force hard gold, 160 HV (kp/mm 2 ) d = 0,02 mm R constr. = 1,3 m  Measured, new contact: 11,3 m  (bulk 10,0 m  ) after use: 23,0 m  (bulk 10,0 m  ) = Resistance increase R constr. Contact area after use Initial contact area 10 1 = 100

12 Kontaktrörelse - återbildning Contact area, cross sectionContact surface seen from above F

13 Kontaktrörelse - återbildning Contact area, cross sectionContact surface seen from above F

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

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

16 Soft films on hard substrate Hard films on soft substrate Squeeze and shear Crack Noble metal contactsBase metal contacts Friction forceContact force Cylinder cylinderRough surface, R a >0,5 min  m wipe Rensning av ytfilmer, forts.

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18 Fritting silver, copper

19 Rensningsmekanismer, översikt

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

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 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 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

30 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

31 Anslutningsdon

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

33 AMP-don, tredje fjäder

34 IDC (Insulation displacemenet contacts)

35 Screw

36 Skruv med jordbricka, klimatprovning Tin Grounding washer Aluminium PCB 1Initial 2Change of temp 3Dry heat +85°C 4Damp heat 5Corrosion Contact resistance

37 Skruv utan jordbricka, klimatprovning 1Initial 2Change of temp 3Dry heat +85°C 4Damp heat 5Corrosion Contact resistance

38 Sammanfattning


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