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

• Main Authors: C.E. Ho, S.P. Yang, P.T. Lee, C.Y. Lee, C.C. Chen, T.T. Kuo
• Format: Article
• Language: English
• Published: Elsevier 2021-03-01
• Series: Journal of Materials Research and Technology
• Subjects: Cu; Surface finish; IGEPIG; IMC; Molten solder channel; HSBS
• Online Access: http://www.sciencedirect.com/science/article/pii/S2238785421001551

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.