Electrical contact resistance (ECR) measurements are needed for judging the performance of electrical appliances. Understanding the behaviour of ECR at low temperature gives a unique opportunity for understanding the contact mechanism itself and controlling the contact resistance for its applications in various areas at these temperatures. In many high-end applications, sophisticated electronic devices are being operated below ambient temperature to improve their performance. The availability of cryogens, improvement in Thermo-Electrical (TE) based Peltier coolers, accelerated the development of these devices. In designing such systems, an accurate measurement of electrical contact resistance below room temperature is important. A detailed experimental investigation has been conducted on electrical contact resistance across bare and coated metal contacts at low temperatures. As a part of the experimental investigation, a test facility capable of varying the contact force, surrounding pressure and temperature, is developed. The design, construction, testing and use of this facility are described. Electrical contact resistance at different contact pressures across copper, OFHC copper and brass with and without gold coatings is measured using 4-wire technique with high accuracy. The test specimen preparation, instrumentation and data acquisition are explained in detail. The setup is standardized by comparing the experimental results obtained across copper-copper contacts in vacuum with the theoretical model. The electrical contact resistance is measured as a function of contact force at different temperatures. The effect of loading and unloading, and the existence of hysteresis are experimentally studied. The electrical properties of conductors improve at low temperature but this is not true for contact resistance. At low temperature the contact resistance increases and it depends on applied contact force, hardness and roughness of the contacting surfaces. Gold-coated contacts exhibited an increase in contact resistance at low temperatures.

Board-to-board connectors are used for electrically connecting one printed circuit board to another. In this thesis, a method for assessing the reliability of such connectors will be developed and tested with the goal of determining the reliability of a selection of connectors. Board-to-board connectors are widely used for connecting different electronic modules to each other, forming a modular system. Today, one of the limiting factors when it comes to modularizing electronic systems, is the lack of data on the long-term reliability of board-to-board connectors. The methods detailed in this thesis are meant to enable electronic designers to make informed decisions on connector selection when modularizing electronic systems. Modularization of electronic systems has several advantages, for example an increased first-pass yield (FPY) during assembly, and the ability to modify or replace certain parts of the system without re-spinning the entire thing. The method by which the reliability of connectors will be tested, is to continuously measure the contact resistance of the connectors during a selection of stress tests. The stress tests in question are mechanical vibration tests, thermal cycling tests, and actuation tests, all of which represent conditions that products from Hitachi Energy may be subjected to. There already exist standards on how to measure contact resistance, but those standards are not necessarily scalable to the measurement of thousands of contacts, nor are they made with the expectation that contacts can be measured during stress tests. The methods presented in this report are developed with the existing standards as basis, but have been modified in order to facilitate the measurement of thousands of contacts during stress tests. The results show that there are advantages to continuous monitoring of contact resistances during tests, as behaviors can be observed, which would not have been captured had the tests been performed conventionally. Furthermore the system which was developed forms the groundwork for a more convenient and cost effective method for testing the reliability of a large selection of board-to-board connectors. / Kontaktdon för kort-till-kort-anslutning används för att elektriskt koppla ett kretskort till ett annat. I denna avhandling utvecklas och testas en metod för att bedöma pålitligheten hos sådana anslutningar, med målet att fastställa pålitligheten hos ett urval av existerande kontaktdon. Kontaktdon för anslutning av kretskort till varandra används allmänt för att knyta ihop olika elektroniska moduler till varandra och bilda ett modulärt system. Idag är en av de begränsande faktorerna när det gäller att modularisera elektroniska system, bristen på data för långtidstålighet hos kontaktdon för kort-till-kort-anslutning. De metoder som beskrivs i denna avhandling är avsedda att möjliggöra för elektronikdesigner att fatta korrekta beslut om kontaktdonsval vid modularisering av elektroniska system. Modularisering av elektroniska system har flera fördelar, till exempel en förbättrad “first-pass yield”, FPY, när det gäller montering och möjligheten att modifiera eller ersätta vissa delar av systemet utan att behöva göra om hela systemet. Metoden som här används för att testa pålitligheten hos kontaktdon är att kontinuerligt mäta kontaktresistansen hos kontaktdonen under ett antal stresstester. De aktuella stresstesterna är mekaniska vibrationsprov, termocykeltester och aktiveringstester, vilka alla representerar vanliga situationer som Hitachi Energy’s produkter kan utsättas för. Det finns redan standarder för hur man mäter kontaktresistans, men dessa standarder är ofta inte skalbara till mätningar av tusentals kontakter samtidigt, och de är inte heller utformade för att mäta kontakter under stresstester. De metoder som presenteras i denna rapport är utvecklade med befintliga standarderna som grund, men har modifierats för att underlätta mätning av tusentals kontakter under olika stresstester. Resultaten visar att det finns fördelar med kontinuerlig övervakning av kontaktresistanser under tester, eftersom vissa beteenden kan observeras som inte skulle ha upptäckts om testerna hade utförts på ett konventionellt sätt. Dessutom utgör det system som utvecklades grunden för en mer bekväm och kostnadseffektiv metod för att testa pålitligheten hos ett stort urval av kontaktdon för kort-till-kort-anslutningar.