Effect of intermetallic compounds on the shear fatigue of Cu/63Sn-37Pb solder joints

The effect of Cu-Sn intermetallic compounds (IMC) on the fatigue failure of solder joints has been studied by means of shear cycling. The samples consist of leadless ceramic chip carriers (LCCC) soldered onto FR-4 printed circuit boards (PCB), and are prepared by conventional reflow soldering using a 63Sn-37Pb solder paste and then aged at 150°C for 1, 4, 9, 16, 25, 36, and 49 days. The specimens are subjected to low cycle fatigue shear tests controlled by the displacement. The results indicate that the fatigue lifetime of the solder joints depends on the thickness of the LMC layer between the Cu-pad and bulk solder, and the quantitative relationship between the lifetime and thickness can be described as a monotonically decreasing curve. The greatest decrease is over the thickness range up to 2.8 μm, when the IMC/bulk solder interface becomes flat, corresponding to a lifetime decrease to 62% of the as assembled value. For further increase in IMC thickness the lifetime decreases more slowly. Evidently, the effect of the Cu-Sn intermetallic compounds on the joint fatigue lifetime is not only concerned with the IMC thickness hut also the interface morphology. A thick and flat LMC layer has a deleterious effect. The results of X-ray diffraction and metallographic analysis show that cracks initiate underneath the component metallization, and propagate along the IMC/solder interface, then toward the fillet. The Cu ₃Sn (ε-phase) is formed between the Cu-pad and p-phase, and grows more quickly than the η-phase during storage and long term operation or aging tests. However, the Cu₃Sn makes only a small direct contribution toward fatigue failure