Novel jet fluxing application for advanced flip chip and BGA/CGA packages

In many applications today involving flip chip, ball grid array and column grid array technologies; flux application is an important process parameter. Dispensing flux requires reproducibility of dispense weight so that amount of flux residue can be controlled. Flux residue causes defects such as voids and delamination when flip chip is underfilled. This problem is magnified with the size of the die and future applications involving finer pitches. Cleaning the flux residue also becomes more difficult with the increase in die size and reduction of bump pitch. The control of weight is also essential when no clean processes are used. The residue left may also contribute to degradation in reliability by leakage as well as contribute to voids in the underfill. The paper shows an application such as solder column interposer where a column grid array interposer is attached to a package, 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 clean flux becomes imperative. In addition, the area that needs to be covered is quite large e.g. 31 mm./spl times/31 mm. Because of such large area, it is not possible to use conventional methods such as brushing or dipping. These methods are also slow and not amenable to high volume applications. Process development was carried out by developing fluxes with suitable viscosity. An additional need was to provide versatility to accommodate different flip chip and column footprints. This process is now fully programmable so that various packages can be used without having to retool the equipment. We have developed a repeatable process that accurately controls the weight of the flux. We will describe a new high volume application using jet fluxer. This fluxing application has been developed for flip chip, ball grid array and column grid array applications. We will describe the development of the jet fluxer where flux is atomized and sprayed. During the development major hurdles were overcoming the surface tension variations of different packages and development of flux with correct viscosity. The process initially had the capability of only spraying very small number of units due to limitations of the nozzle. The nozzle limitations also limited the ability to control the weight. We will show how these problems have been overcome. The process has been implemented in high volume production successfully for both flip chip and column grid array applications.