Morphology of interfacial reaction between lead-free solders and electroless Ni–P under bump metallization

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Abstract
The morphology of interfacial reaction products between four lead-free solder alloys on electroless Ni–P was characterized. The four Pb-free solders were 99.3Sn0.7Cu, 95.5Sn3.8Ag0.7Cu, 96.5Sn3.5Ag, and 96Sn2Ag2Bi (in wt%) alloys. After reflow, the interfacialintermetallics in the first two solders that contain Cu (99.3Sn0.7Cu and 95.5Sn3.8Ag0.7Cu) had good adhesion with electroless Ni–P. However, the 96.5Sn3.5Ag and 96Sn2Ag2Bi alloys formed interfacialintermetallics with a needle shaped morphology that spalled off the surface of electroless Ni–P. This difference is attributed to the role of Cu in the solders (which modified the chemical potential of the interfacialintermetallics), the volume change that occurs during intermetallic formation, and the interfacial properties of the compound. In solid state aging experiments, the consumption of electroless Ni–P by intermetallic growth was not significant (approximately 1 μm) and all the intermetallics had good adhesion to the electroless Ni–P. The electroless Ni–P exhibited some damage at the outer edge of the bond pad due to stress imposed during solid state aging. Large Ag 3 Sn and Cu 6 Sn 5 intermetallics were observed in the bulk of the solders (except for SnAgBi solder), and these intermetallics are discussed in terms of soldering reaction at the interface and phase equilibrium.