Abstract:

The time-dependent viscoplastic behavior of a hypoeutectic Sn3.0Ag0.5Cu solder is presented and compared to that of near-eutectic Sn3.9Ag0.6Cu [1] and of baseline eutectic Sn37Pb solder [1]. Time-dependent viscoplastic behavior is investigated under two loading conditions; constant force creep behavior and constant displacement stress relaxation behavior. This study is a continuation of previously reported stress relaxation studies, and investigates the validity of using creep model constants derived from constant load conditions to predict stress relaxation behavior (where the stress is constantly varying under constant displacement).

Time-dependent viscoplastic creep behavior is measured at constant stress levels between 3 MPa to 20 MPa at room and high temperatures, using a custom-built thermo-mechanicalmicroscale (TMM) test system. This test system uses a modified Iosipescu shear specimen with a 180 microns wide solder joint. Secondary and primary creep model constants are obtained from the creep measurements. Secondary creep strain rate measurements indicate that hypoeutectic Sn3.0Ag0.5Cu creeps more than near-eutectic Sn3.8Ag0.7Cu solder. Primary creep behavior of both Pb-free solders is compared with that of eutectic Sn37Pb solder.

The measured primary and secondary creep constants are utilized in viscoplastic finite element models to compare with stress relaxation behavior measured earlier on the TMM specimens at various temperatures and strain levels [2]. The current set of preliminary creep constants are found to underpredict the stress relaxation seen in solder. Further work is suggested, to improve the agreement.

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