Reliability of 100 μm Bi- and In- solder balls

Low-melting solder alloys are considered as one approach to lower assembly process temperature and therefore thermal stress on temperature sensitive component materials. In this study the reliability of 100 mum-solder bumps of low-melting Bi- and In-alloys has been investigated on a thin-film Al₂O₃ substrate (18 times 14 mm²) with more than 5000 I/Os. Two different bumping processes have been assessed: (1) Screen printing of solder paste and subsequent reflow soldering of the solder reservoirs to bumps. (2) Placement of preformed solder balls via screen in printed tacky flux followed by reflow soldering. Silicon chips with Daisy Chain structures (each >1250 I/Os) were assembled via flip-chip technology on the substrate. The hybrids were underfilled. Accelerated life tests like temperature cycling have been performed. A temperature range between -10 and +90degC has been selected according to the melting points of the alloys (between 119degC and 140degC). The reliability and stability of the bismuth alloy and indium alloy joints themselves have been investigated thoroughly by mechanical and electrical tests. The results of the reliability tests of the 100 mum bumps have been compared with reference to their manufacturing process (solder paste or preformed solder balls) and their alloy composition.