Electrical contacts n What is an electrical contact

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

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

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

Kontaktområde - förstoring

Contact area Force

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

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

Gold plated connector contact 0, 5 N contact normal force Constriction resistance: 1 m. W

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

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

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

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

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

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 Rough surface, Ra>0, 5 cylinder min 50 -100 mm wipe

Fritting silver, copper

Rensningsmekanismer, översikt

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

Silver - temperaturökning

Tenn - temperaturökning

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

Stress relaxation

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

Pore corrosion Corrosion creep

Failure mechanisms n Contact force reduction q q n Film growth q q q n Initially too low contact force / variations Plastic deformation (manufacturing, closing) Stress corrocion cracking Stress relaxation Corrosion Diffusion Contamination Wear q q Mating wear Fretting

Fretting oxidation/corrosion

Fretting risk – tangential force control

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

Anslutningsdon

Ag. Cd. O-relay – with/without enclosure, open/closed enclosed Open Closedcontact, not enclosed upper limit G 2 Bulletin No 102, ”Corrosion of Electronics” lower limit G 1

AMP-don, tredje fjäder

IDC (Insulation displacemenet contacts)

Screw

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

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

Sammanfattning