Reliability of Sintered and Soldered High Power Chip Size Packages and Flip Chip LEDs

For automotive exterior lighting application high luminance light sources are required, i.e. high current density. The interconnect between LED and board needs to have a high thermo-mechanical fatigue resistance. Silver and copper sintering was investigated as a replacement for SnAgCu (SAC) solder joints. Sinter processes with and without pressure were applied. For process development and reliability investigation the interconnects were analyzed by X-Ray (area and homogeneity of interconnect) and transient thermal analysis TTA (integrity of joint between LED and board). For development of the sinter processes, as reference, also 1 mm2 thin film dies (TF-LEDs) were used. Silver sintering under pressure revealed as expected high thermal and mechanical performance. Pressureless 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. For the FC-LEDs best thermal resistance mean value of 8.5 K/W is 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). For copper sintering no stable process was achieved. The main reason was due to only partial reduction (under formic acid enriched nitrogen) of the copper paste during the sintering process. The reliability of the soldered LED interconnects using different solders, i.e. SAC305, SnBiAg, Indium and an improved SAC+ solder, was investigated by accelerated stress testing. The LEDs are placed in a temperature shock chamber and the joint quality, i.e. crack formation, is measured after 100/500/750/1000 cycles by TTA. The degradation of the thermal properties of the joints is monitored. The reliability data are presented and discussed in the paper.