Novel flip-chip interconnection technology for millimeter wave applications

This paper introduces a new gold-to-gold interconnection process for high precision flip-chip assemblies: the wedge stud bump bonding (WSBB). Miniature, precisely reproduced gold studs are formed in a first step on chip-side or substrate-side bond pads. Device attachment and electrical connections are established by means of a second step: an ultrasonic or thermo-compression bonding process which produces single- or multi-site micro welded contacts. The WSBB process is being successfully applied in millimeter wave hybrid multi chip module (MCM) fabrication. Motivation, requirements, impact on circuit performance and test results are given. As a practical example the surface mounting of Schottky diodes to form up- and down-converting mixers in a hybrid low temperature co-fired ceramic (LTCC) -based front end is described. For the first step, the wedge stud bump (WSB) formation, very favorable results have been achieved with a double cross groove wedge tool, that creates two ridge-shaped micro bumps on top of a flat plateau. Its height above the substrate or chip surface with practically useful process parameters is approx. 10 mum. This plateau serves as spacer establishing a well-defined air-gap between device and substrate. The latter is crucial for good device and interconnects performance in the millimeter wave range. Using 25 mum Au wire the overall WSB footprint is 120 mum by 40 mum, with micro bump areas of 15 times 40 mum² on the top. Micro bumps project typically 20 mum above plateau prior to device attachment and are flattened down to negligible height during the later process step. Further miniaturization of WSBs is possible, limited by wedge bond process tolerances, feasible tool shape and wire diameter. Shear tests have shown typical break loads of 25 cN per WSB with two micro bumps.