An experimental study of fatigue strength characteristics of beam lead material in μBGA package

Summary form only given. Thermal fatigue cracking in a beam lead in μBGA, a type of CSP (chip scale package), is identified as a major failure mode under a cyclic thermal loading environment. Accurate understanding of the parameters affecting the fatigue behavior of a beam lead is thus crucial to ensure the fatigue endurance of μBGAs. The main purpose of this study is to investigate the fatigue strength and behavior of the beam lead, which consists of an Au thin film-coated Cu core under mechanical loading conditions as an accelerated test condition. Fatigue tests under displacement control mode are carried out with Cu foil (18 μm thick) and Au coated Cu foil (20 μm thick) specimens to assess the effect of Au film on the fatigue life of the beam lead. To impose a mechanical load on a beam lead, we have developed a micro-mechanical testing machine which enables the low-cycle fatigue test of a beam lead. Tests are carried out under tensile loading conditions. From the experimental results, we obtained the curves for strain vs. fatigue life of Cu foils and Au coated Cu foil specimens. Comparisons between fatigue behavior of Cu foils and Au coated Cu foils are also made. The experimental results are compared with the fatigue life of beam leads in a fully assembled μBGA under the temperature cycling test condition