Interfacial Reactions and Low Cycle Fatigue of Lead-Free Solders for Electronic Packaging

Doktorsavhandling, 2007

Two key technologies used by the electronics industry are chip technology and packaging technology. Solder plays a crucial role in both of them. During the last decade, there has been a strong worldwide environmental movement towards lead-free electronic products. The Sn-Zn solder system has been presented as a promising lead-free candidate, recommended mainly for its low melting temperature, around 200°C, which is close to the melting temperature of the traditional binary Sn-Pb alloy, 183°C. The alloy composition chosen to be studied in this work was the ternary Sn-8Zn-3Bi. The low cycle fatigue behavior of this alloy was investigated using single lap shear samples. Finite element modeling was performed and the Coffin-Manson equation was given based on results from experiment and simulation both: Nf = 0.0294 (Δγ)-2.833 Another important issue relating to the reliability of solder joints also investigated in this work, is the intermetallic compounds (IMCs). During soldering and the build-up of solder joints, IMCs are formed at the interface between the solder and the pad metallization. The reliability of the whole package is typically affected by the integrity of the solder joints, and the integrity of the solder joints is greatly affected by the IMCs mainly due to their inherent brittle nature and tendency to generate structural defects. The interfacial reactions between the eutectic Sn-0.4Co-0.7Cu alloy and electroless nickel, immersion gold (ENIG) metallization was investigated after reflow soldering. Common Sn-4.0Ag-0.5Cu and eutectic Sn-0.7Cu solders were used as reference. Two types of IMCs were found in the solder matrix of the Sn-0.4Co-0.7Cu alloy, namely coarser CoSn2 and finer Cu6Sn5 particles, while only one ternary (Cu,Ni)6Sn5 interfacial compound was detected between the solder alloy and the ENIG coated substrate. It was noted that the thickness of the interfacial IMC layers in the Sn-Co-Cu solder joint were thinner than both Sn-Ag-Cu and Sn-Cu IMC layers. The interfacial reaction of Sn-3.5Ag and Sn-4.0Ag-0.5Cu solders on ENIG metallization after high temperature storage (HTS) testing was investigated from a metallurgical point of view. It was noticed that only Ni3Sn4 IMCs were found in the Sn-Ag system, while two kinds of interfacial products, (Ni,Cu)3Sn4 and (Cu,Ni)6Sn5 existed in the Sn-Ag-Cu system. The interfacial layer between the Sn-Ag-Cu solder and electroless Ni(P) coating showed better thermal stability than eutectic Sn-Ag solder since no spalling was observed. Furthermore, the coupling effect in both Sn-3.5Ag-3.0Bi and Sn-8.0Zn-3.0Bi solder joints in sandwiched structure was studied as a function of reflow time. The coupling effect between the ENIG metallization and the Cu substrate was confirmed since the type of IMCs on Ni(P) layer changed from being a Ni-Sn phase to a Cu-Sn phase, apparently as a result of the diffusion of Cu atoms from the opposite Cu substrate. Furthermore, the ternary (Cu,Ni)6Sn5 compounds were formed at the interface between Sn-Ag-Bi solder and Ni(P) substrate. One complex alloy Sn-Ni-Cu-Zn was formed at the Sn-Zn-Bi/Ni(P) interface; however the growth of this complex alloy on the ENIG coated substrate was suppressed.