Low Cycle Fatigue Reliability and Interfacial Reaction of Lead-free Solder Joints for Electronic Packaging
Licentiate thesis, 2005
Two key technologies in electronics industry are chip technology and packaging technology. Solder plays a crucial role in the assembly and interconnection in the electronic packaging industry. As a joint material, solder provides electronic, thermal and mechanical continuity. The eutectic Sn-Pb alloy has been widely used in surface mount technology (SMT) for the interconnection and mechanical support between electronic components and printed circuit boards (PCB).
During the last decades, there have been strong worldwide environmental movements towards "green" non-toxic electronic products. Initiatives and legislation forced the electronics industry to replace lead containing products with more environmental friendly, lead free products.
Sn-Zn solder system is therefore a promising candidate, recommended for its low melting temperature, around 200°C, which is close to the melting temperature of the traditional binary tin lead solder. But the low cycle fatigue data of Sn-Zn solder system has not been reported yet and the lack of reliability data is one of the barriers to the application of the Sn-Zn system solder alloy. The aim of the present work is to investigate the low cycle fatigue behaviour of the lead-free Sn-8Zn-3Bi solder joint using lap shear test samples. Finite element simulation is presented and the Coffin-Mason equation for Sn-8Zn-3Bi solder joint is given based on the experimental and simulation results:
Nf = 0.0294 ()-2.833
Where the Nf is the number of cycles to failure and is the plastic shear strain.
Another important issue, related to reliability of solder joints that has been investigated in this work, is the intermetallic compounds (IMCs). Generally, the interconnection by the solder joint is completed by the formation of IMCs at the interface and the reliability of the whole package is affected by IMCs due to their inherent brittle nature and tendency to generate structural defects. The interfacial reaction of Sn-3.5Ag and Sn-4.0Ag-0.5Cu solders on Au/electroless Ni metallization after high temperature storage (HTS) testing at 150°C is investigated for the reliability of electronic production from the metallurgical viewpoint in this work. It is noticed that Ni3Sn4 intermetallic compounds (IMCs) are found in the Sn-Ag system, while two kind of interfacial productions that are (Ni,Cu)3Sn4 IMC layer and (Cu,Ni)6Sn5 spalling compound gains exists in the Sn-Ag-Cu system. Kirkendall voids are found in both lead free solder joints and clear gap exists at the interface for the case of Sn-Ag solder joint after 1000 hour aging.
Finite Element Simulation
Low Cycle Fatigue