Interfacial Reaction of Molten Sn on a Strained Cu
Electroplated Layer
Interfacial reactions of molten Sn on a Cu electroplated layer with and without strain were investigated. A Cu layer electroplated on a rectangular Si substrate was subjected to tensile stress by bending the Si substrate on a three-point bending apparatus. Two intermetallic compounds, Cu6Sn5 and Cu3Sn, were formed at the Sn/Cu interface after reflow at 260°C for 10 min to 30 min, regardless of the strain. Top-view examination revealed that the tensile strain had significant effects on the surface morphology of the Cu6Sn5 grains. On the nonstrained Cu layer, ovoid-shaped Cu6Sn5 grains were formed first after soldering, but long, prismatic Cu6Sn5 grains precipitated on the existing ovoid-shaped grains after solidification in air. On the strained Cu layer, only ovoid-shaped Cu6Sn5 grains but no long, prismatic ones were found after solidification. A possible mechanism is proposed to explain the absence of long, prismatic Cu6Sn5 grains on the strained Cu layer.