Flux and its residues impede flip chip deployment in the packaging and integration of microelectronic, optoelectronic, and micro-electromechanical systems. This paper describes a novel fluxless method of bonding Au studs with eutectic solder confined within cavities on a flex substrate. The joint structure was examined by scanning electron microscopy and energy-dispersive X-ray. In solid-state bonding, the joints are weak and unable to endure assembly processes. Bonding can occur with a joint-in-via (JIV) architecture with an optimized bonding condition when the solder is molten. Instantaneous fluxless bonding, known as then-no-mechanical bonding, was assessed and tolerated a standard reliability test. This fluxless bonding technique is applicable to heat-sensitive devices because of its heat-isolating bonding capability. (c) 2005 Elsevier B.V. All rights reserved.