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Self-assembled monolayers for reduced temperature direct metal thermocompression bonding. Applied

A study on temperature dependence in gold-gold ͑Au-Au͒ thermocompression bonding was performed. Gold studs were bonded to two kinds of surfaces-cofired gold on alumina and electroless nickel covered with immersion Au on silicon. A critical bonding temperature was observed for both substrates. No bonding occurs when the temperature is below this threshold value, whereas bond strength increases with bonding temperature beyond the threshold. This critical temperature can be related to the activation of organic films on the bonding surfaces. Under similar bonding conditions, the critical temperature is lower for a harder substrate than for a softer substrate, primarily because of larger interfacial shear stresses. This is supported by the observation on the interfacial shear stress distribution at the bonding interface based on finite element simulation models of substrates with different hardness.