Nanoscaled solder for low-temperature assembling processes

The change to lead-free soldering due to RoHS directive demands the search for alternative solder materials. At the moment the favorite lead-free solder for reflow processes is SnAgCu that in comparison to eutectic SnPb has higher melting and soldering temperature. The higher soldering temperature leads to more power consumption for the assembly process and can also lead to damage of some sensitive electronic components. By scaling of soldering material size down to nanometer scale the melting temperature can be reduced. For practical usage multilayer structure of alternating nanoscaled tin and temporary interlayer will be interesting. After soldering process at reduced temperature tin layers are melting together and final solder joint has the properties of bulk material that means the higher unsoldering temperature and accordingly higher stability. The properties of nanoscaled layers of pure tin have been studied. The layers were produced by sputtering in different thicknesses between 5 and 100 nm. The surface topography and the influence of process parameters on the sputtering rate were investigated using REM and AFM methods. For the protection from oxidation carbon and silicon nitride layers were RF-sputtered in the same process chamber. The layer thickness was varied between 2 and 10 nm. The function of this coating was tested by x-ray diffraction. The crystallographic structure of tin layer under silicon nitride was measured as sputtered, after melting and after cooling processes. The crystalline tin film after cooling down is a sign for impermeability of oxygen. For investigation of phase change the method of fast differential calorimetry was used. The prepared samples with tin layers scaled down to 15 nm were measured with this method. Measured values have certain deviation from theoretical model values that can be explained with another surface tension due to final preservation by silicon nitride layer.