Electrical contacts What is an electrical contact? Why doesn’t it always work? Lena Sjögren Corrosion and Metals Research Institute 2017-04-10

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 2017-04-10

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Electrical contacts Power transmission Signal transmission Grounding – personal safety Signal ground Electromagnetic shielding 2017-04-10

Kontaktområde - förstoring 2017-04-10

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Contact area Force 2017-04-10

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 2017-04-10

Constriction resistance one contact spot plastic deformation contact area contact force hardness constriction resistance resistivity contact spot diameter 2017-04-10

Gold plated connector contact 0,5N contact normal force Constriction resistance: 1mW 2017-04-10

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 = = 2017-04-10

Kontaktrörelse - återbildning F Contact area, cross section Contact surface seen from above 2017-04-10

Kontaktrörelse - återbildning F Contact area, cross section Contact surface seen from above 2017-04-10

Resistans som funktion av tid/antal Resistance/contact failures Time Ack. number 2017-04-10

Tenn – rensning av ytfilm Contact area, cross section Contact surface seen from above Contact area, cross section 2017-04-10

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 50-100mm wipe 2017-04-10

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Fritting silver, copper 2017-04-10

Rensningsmekanismer, översikt 2017-04-10

Current constriction through contact spots – - constriction resistance Termiska effekter Current constriction through contact spots – - constriction resistance Contact spot Kontaktpunkt - heat emission 2017-04-10

Silver - temperaturökning 2017-04-10

Tenn - temperaturökning 2017-04-10

Failure mechanisms Contact force reduction Initially too low contact force / variations Plastic deformation (manufacturing, closing) Stress corrocion cracking Stress relaxation 2017-04-10

Stress relaxation 2017-04-10

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 2017-04-10

Pore corrosion Corrosion creep 2017-04-10

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 2017-04-10

Fretting oxidation/corrosion 2017-04-10

Fretting risk – tangential force control Vid mekaniska rörelser 2A möjlighet kontrollera effekter även av termiska rörelser 2017-04-10

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 2017-04-10

Anslutningsdon 2017-04-10

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” 2017-04-10

AMP-don, tredje fjäder 2017-04-10

IDC (Insulation displacemenet contacts) 2017-04-10

Screw 2017-04-10

Skruv med jordbricka, klimatprovning Contact resistance Grounding washer Tin Initial Change of temp Dry heat +85°C Damp heat Corrosion Aluminium PCB 2017-04-10

Skruv utan jordbricka, klimatprovning Contact resistance Initial Change of temp Dry heat +85°C Damp heat Corrosion 2017-04-10

Sammanfattning 2017-04-10