IMC microstructure modification and mechanical reinforcement of Sn-Ag-Cu/Cu microelectronic joints through an advanced surface finish technique

Intermetallic compound(s) (IMC) that nucleates at the interface between solder and Cu trace during a soldering reaction, is one of the most crucial factors for microelectronic packaging reliability. This study was conducted to modify the IMC microstructure and to reinforce the mechanical strength of Sn-Ag-Cu/Cu microelectronic joints through various surface finish coatings, including organic solderability preservative (OSP), immersion Ag (ImAg), immersion Sn (ImSn), Au/Pd (electroless palladium/immersion gold, EPIG), and Au/Pd/Au (IGEPIG) layer(s). We confirmed that the type of surface finish dominated the IMC growth morphology and mechanical characteristics of the Sn-Ag-Cu/Cu solder joints, even though these surface finishes were eliminated in a few seconds of the soldering process. A dense Cu6Sn5 layer with a scallop-like appearance was obtained for the traditional OSP case, while a prismatic, loose Cu6Sn5 microstructure was produced for the alternative cases (metal films). This loose Cu6Sn5 microstructure offered numerous molten solder channels for the in-diffusion of Sn to Cu, retarding undesired Cu3Sn growth at the Cu6Sn5/Cu interface; consequently, a brittle-to-ductile transition in the joint fracture mode with a high shear strength and fracture energy was obtained in the high-speed ball shear (HSBS) test. A significant mechanical reinforcement of the Sn-Ag-Cu/Cu microelectronic joints can be achieved with the replacement of the traditional OSP coating by the newly developed IGEPIG trilayer surface finish. (c) 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).