Lead-free solder flip chip-on-laminate assembly and reliability

This paper examines the assembly process for flip chip die with SnAgCu solder bumps and the results of liquid-to-liquid thermal shock testing. The SnAgCu alloy required a thicker dip layer of flux to achieve good wetting compared to the SnPb eutectic alloy. A liquid spray flux yielded more consistent solder wetting with the SnAgCu alloy. With both fluxes, a nitrogen reflow atmosphere was necessary with the SnAgCu alloy. A peak reflow temperature of 246°C was used for the assembly of the SnAgCu thermal shock test vehicles. A lower peak temperature of 235°C did not yield sufficient solder wetting. Liquid-to-liquid thermal shock testing was performed from -40°C to +125°C. The SnPb alloy performed slightly better than the SnAgCu and the dip flux was better that the spray flux. The degree of delamination with the SnAgCu alloy was significantly higher than with the SnPb alloy. Cracks in the underfill between adjacent solder balls were observed. The SnPb alloy extruded into these cracks more readily than the SnAgCu and created electrical shorts