Vibration Durability of Sn3.0Ag0.5Cu (SAC305) Solder Interconnects: Harmonic and Random Excitation

Abstract

In this study, the durability of lead (Pb)-free tin(Sn3.0)silver(Ag0.5)copper(Cu) (SAC305) printed wiring assemblies (PWAs) is investigated under constant amplitude, narrow-band (harmonic) excitation and under step-stress broad-band (random) excitation, and compared to the durability of Pb-based Sn37Pb PWAs. The results show that Sn37Pb assemblies last longer than SAC305 assemblies at similar excitation levels, for both harmonic and random excitations used in this study. The test specimens are identical for all tests, consisting of a PWA with plastic ball grid array components, quad flat pack components, leadless ceramic chip carriers, and leadless chip resistors. The test matrix includes test boards with different kinds of finishes and different aging conditions. Both the harmonic and random vibration tests are conducted on single-axis electrodynamic shakers. The harmonic vibration excitation is applied to a single specimen at a time, while the random vibration excitation is applied simultaneously to 20 test specimens using a specially designed test fixture. The flexural response of each test specimen mounted in the fixture is first thoroughly characterized before conducting the durability experiment. The flexural strain histories, measured on the PWAs, are used to compare the performance of the assemblies at different excitation conditions and also as inputs in other studies for stress analysis to quantify the damage in the solder joints.1 The durability tests are then conducted and time-to-failure is documented for the entire test matrix. The random durability tests are conducted at three temperatures: 125 °Centigrade (C), 25 °C (room temperature), and -40 °C. The harmonic tests are conducted only at room temperature. Destructive failure analysis (cross-sectioning, polishing, and microscopy) is conducted to confirm the failure modes. The test results presented here are analyzed in related publications1 to extract the high-cycle fatigue properties of the SAC305 and Sn37Pb solder materials.