Effects of fourth alloying additive on microstructures and tensile properties of Sn-Ag-Cu alloy and joints with Cu

The effects of the fourth elements, i.e., Fe, Ni, Co, Mn and Ti, on microstructural features, undercooling characteristics, and monotonic tensile properties of Sn-3 wt.%Ag-0.5 wt.%Cu lead-free solder were investigated. All quaternary alloys basically form third intermetallic compounds in addition to fine Ag3Sn and Cu6Sn5 and exhibit improved solder structure. The precipitates of Sn-3Ag-0.5Cu (-0.1 wt.%X; X = Ni, Ti and Mn) alloy are very fine comparing with the other alloys. The effective elements for suppressing undercooling in solidification are Ti, Mn, Co and Ni. All quaternary bulk alloys exhibit similar or slightly larger tensile strengths; especially Mn and Ni can improve elongation without degrading strength. The interfacial phases of Sn-3Ag-0.5Cu (-0.1 wt.%,X; X = Fe, Mn and Ti)/Cu joints are typical Cu6Sn5 scallops. Sn-3Ag 0.5Cu (-0.1 wt.%X; X = Ni and Co)/Cu joints form very fine Sn-Cu-Ni and Sn-Cu-Co scallops at interface. The Cu/Sn-3Ag-0.5Cu-0.1Ni/Cu joint exhibits improved tensile strength prior to thermal aging at 125 and 150 degreesC. The fracture surface of Cu/Sn-3Ag-0.5Cu/Cu joint exhibits mixture of ductile and brittle fractures, while Cu/Sn-3Ag-0.5Cu (-0.1X; X = Ni and Co)/Cu joints exhibit only brittle fracture at interface. The Sn-3Ag-0.5Cu-0.1Ni alloy is more reliable solder alloy with improved properties for all tests in the present work. (C) 2002 Elsevier Science Ltd. All rights reserved.