Fluxless Chip Join Process Using Formic Acid Atmosphere in a Continuous Mass Reflow Furnace

There is an ever growing need for a fluxless chipjoin process to alleviate the difficulties associated with fluxcleaning in small gaps and to target fluxless packagingapplications such as required for advanced photonics. Moreover, a fluxless process can enable new process flows such as in-line underfill of fragile structures or acombination tack and mass reflow operations for 3D packages. This paper discusses the development of a continuous mass reflow chip join furnace using formic acid atmosphere for fluxless, flip chip organic packaging applications. A proposed reaction mechanism between formic acid and tin oxide will be discussed. This understanding provides the basis for the reflow furnace design and the associatedexperimentation. A description of the prototype mass reflowfurnace is provided including the formic acid delivery andabatement systems. Experimental data includes the choice of temporary adhesive material and dispense pattern to hold the assembly in place prior to reflow for chip and capacitors as well as wettability data for different formic acid concentration profiles, temperature profiles and soak times. Several test vehicles were used to investigate the impact of bump metallurgy, die size and gap between the die and organic substrate on the effectiveness of formic acid to reduce tin oxide. The impact of oxygen level was found to be critical for wettability and temporary adhesive behavior - data to support very low oxygen level is presented. A cleanliness assessment was performed on samples after chip removal and includes visual inspection, SEM and EDS data as well as XPS surface study with focus on tin redeposition. Reliability data for DTC testing of modules is presented as well as cross-sections of theinterconnections formed under formic acid atmosphere.