Effect of solid-state intermetallic growth on the fracture toughness of Cu/63Sn-37Pb solder joints

The mode I chevron notch fracture toughness of Cu/63Sn-37Pb solder joints was measured as a function of solid-state copper-tin intermetallic growth at the solder/copper interface. Soldered chevron notched bend samples were aged in a furnace at 170 degrees C to promote the intermetallic growth and the samples were tested at room temperature after 1, 3, 10, 30, and 75 days of growth. The total thickness of the interfacial intermetallic layer and the individual thicknesses of the component Cu6Sn5 and Cu3Sn layers were monitored at each stage. The chevron notch fracture toughness is correlated with the intermetallic layer thickness measurements and the fracture surface morphology. The results show that at a total intermetallic layer thickness below 5 mu m, failure is dominated by microvoid coalescence in the solder, and intermetallic growth has little effect on the fracture toughness. At a total thickness exceeding 7 mu m, however, fracture occurs by cleavage of the interfacial intermetallic particles and the fracture toughness decreases steadily as the intermetallic layer thickness increases. With a total intermetallic layer thickness of 19 mu m, the chevron notch fracture toughness is only 30% of that measured for an as-soldered, nonaged solder joint.