Physical and Mechanical Characteristics of Pb-free Solder - Copper Joints Project funded by OFES (Switzerland) Within the framework of :Cost Action 531: J. Janczak-Rusch*, T. Rütti*, A. and John EPFL

New generation particle reinforced Pb-free solders: initial tests Experimental setup –tensile test specimen dimensions: 20mm x 1mm cross section –solder alloy: Sn-3.5Ag-0.5 Cu –soldering cycle: 6 min (  30 s) to melt, 1 min (  10 s) in liquid phase, fast cooling (solder jig placed into water); peak temperature: 230°C –tensile test conditions: Instron 5848 MicroTester in displacement control mode at 1µm/s; strain measured with a 50mm clip gauge Test matrix –effect of solder gap width: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8 and 1.0mm –effect of room temperature ageing (T H = 0.6): 1day, days, 1 week, 2 weeks, 1 month, 2 months –effect of ageing at elevated temperatures: 1 week and 2 weeks at T H =0.75 and T H = 0.9

Thermal cycling

h = 50 mm b = 20 mm e = 1 mm t = variable Mechanical testing

Preliminary results Results –a notable number of gas pores form in the solder joint under the given soldering conditions –yield stress, tensile stress, Young’s modulus and strain at fracture are not significantly influence by ageing at room temperature –ageing at high temperatures reduces yield and tensile strengths and increases strain (ductility) –decreased solder gap width increases yield and tensile strengths and decreases strain (ductility) Ageing temperature solder gap width Yield strength Tensile strength Young’s modulus Strain at fracture [mm][MPa] [GPa][%] T H = ± ± ± ± T H = ± ± ± ± T H = ± ± ± ± T H = ± ± ± ± 0.013

Typical microstructures cross-section (etched) Sn-4.0Ag-0.5Cu solder joint typical fracture surface

Purpose  Numerical simulation of solder joints mechanical behavior Model description:  Geometry (h = 50 mm, extensometer’s length)  Boundary conditions: same as experiment, T=25°C  FE Mesh (Plane stress, 2 axial symmetries)  Constitutive law Copper: Elastoplastic (E=112 GPa, =0.33,  y =69 MPa) Solder: elasto-viscoplastic (E=56 GPa, =0.35,  y =32.5 MPa) creep: Garofalo’s type power law Finite Element Analysis

Calibration of constitutive model: Finite Element Analysis

Simulation of different thickness specimen: Finite Element Analysis

New generation particle reinforced Pb-free solders Next steps –soldering under vacuum/ reduced pressure to eliminate gas pores –testing of different base solder alloys: Sn-Ag and Sn-Cu compared to Sn-Ag-Cu –incorporation of strengthening particles into solder alloy