The Importance of Cooling Rate in Developing the Totally Controlled Reflow Process for Lead Free and Eutectic Tin Lead Processing

摘要

The impact cooling rates exert on the reflow process is investigated in this study. The trends in shear strength and the microstructural evolution of the solder joints are described. Lead free (Sn/3.5Ag/0.7Cu) and tin lead (63Sn/37Pb) assemblies on copper organic solder preservative (Cu-OSP), electroless nickel-immersion gold (ENIG), and immersion tin (Imm Sn) surface finishes were considered in this experiment. A Differential Scanning Calorimeter (DSC) was used to simulate the heating and the cooling cycles experienced by solder in the reflow process. The strength values and the analysis of microstructure were documented using the Instron Machine and Scanning Electron Microscope (SEM): 1. Slower cooling rates result in the increased formation of the Ag3Sn and Cu6Sn5 intermetallics. These will form both in the bulk solder as well as at the pad solder interface. Faster cooling rates inhibit the growth of these intermetallics. 2. Aging at 125℃ results in decreased shear strength values. Pad adhesion, solder migration, and increased intermetallic thickness at the solder-pad interface impact the shear strength. 3. The intermetallic thickness at the solder-pad interface is proportional with the Time Above Liquidus. 4. Faster cooling rates result in stronger joints as observed in those samples with a Cu-OSP board finish. 5. Two failure modes were observed during shear testing. Pad lifting was predominantly observed in the immersion tin finished boards while bulk solder cracking was observed in the Cu-OSP and ENIG finished boards. 6. The impact of profile type. Linear heating versus Ramp-Soak-Spike, on joint quality was observed. However, joint quality is greatly dependent upon all reflow process parameters, especially flux-profile compatibility.