Process development and reliability of sintered high power chip size packages and flip chip LEDs

Silver and copper sinter processes were investigated as a replacement for SnAgCu (SAC) solder joints for high power Thin Film (TF) LEDs (die bond) and Flip Chip (FC) LEDs. Processes with and without pressure were applied. The joint quality was investigated by non-destructive tests, i.e. transient thermal analysis and X-Ray, as well as destructive tests, i.e. shear tests and cross sections. Silver sintering under pressure revealed as expected high thermal and mechanical performance. Also pressure free Ag sintered joints showed not as good but promising results. Printing and drying conditions have a major impact on the quality of the sinter joint, in especially for the FC LEDs. The process had to be adjusted to avoid shorts between the electrical contacts. For the FC LEDs best thermal resistance mean value of 8.5 K/W are observed for silver sintering under pressure and a mean shear strength of 64 MPa in comparison to the reference SAC solder (8.5 K/W and 68 MPa). Under pressure silver sintered TF LEDs achieved a thermal resistance of 6.1 K/W and 45.1 MPa shear strength in comparison to 6.5 K/W for SAC solder. Copper sintering was performed under formic acid enriched nitrogen. For copper sintering no stable process was achieved. The main reason was due to non-sufficient reduction of the copper paste during the sintering process.