Novel jet fluxing application for advanced flip chip and BGA/CGApackages

摘要

In many applications today involving flip chip, ball grid arrayand column grid array technologies; flux application is an importantprocess parameter. Dispensing flux requires reproducibility of dispenseweight so that amount of flux residue can be controlled. Flux residuecauses defects such as voids and delamination when flip chip isunderfilled. This problem is magnified with the size of the die andfuture applications involving finer pitches. Cleaning the flux residuealso becomes more difficult with the increase in die size and reductionof bump pitch. The control of weight is also essential when no cleanprocesses are used. The residue left may also contribute to degradationin reliability by leakage as well as contribute to voids in theunderfill. The paper shows an application such as solder columninterposer where a column grid array interposer is attached to apackage, the gap between the package and the interposer is very small.In such cases, it is not possible to clean and therefore use of no cleanflux becomes imperative. In addition, the area that needs to be coveredis quite large e.g. 31 mm.×31 mm. Because of such large area, itis not possible to use conventional methods such as brushing or dipping.These methods are also slow and not amenable to high volumeapplications. Process development was carried out by developing fluxeswith suitable viscosity. An additional need was to provide versatilityto accommodate different flip chip and column footprints. This processis now fully programmable so that various packages can be used withouthaving to retool the equipment. We have developed a repeatable processthat accurately controls the weight of the flux. We will describe a newhigh volume application using jet fluxer. This fluxing application hasbeen developed for flip chip, ball grid array and column grid arrayapplications. We will describe the development of the jet fluxer whereflux is atomized and sprayed. During the development major hurdles wereovercoming the surface tension variations of different packages anddevelopment of flux with correct viscosity. The process initially hadthe capability of only spraying very small number of units due tolimitations of the nozzle. The nozzle limitations also limited theability to control the weight. We will show how these problems have beenovercome. The process has been implemented in high volume productionsuccessfully for both flip chip and column grid array applications