Interfacial reaction and failure mode analysis of the solder joints for flip-chip LED on ENIG and Cu-OSP surface finishes

• Flip-chip LEDs were bonded to ENIG and Cu-OSP surface finishes by soldering method.
• The Au layer on the diodes affected the microstructures of the solder joints.
• Large IMCs were observed in the solder layers after isothermal aging.
• The anodes and cathodes appeared different failure modes during the shear test.

The interfacial reactions and failure modes of the solder joints for flip-chip light emitting diode (LED) on electroless nickel/immersion gold (ENIG) and Cu with organic solderability preservatives (Cu-OSP) surface finishes were investigated in this study. The experimental results demonstrate that the interfacial reactions in the Au/Sn–Ag–Cu(SAC)/ENIG and Au/SAC/Cu systems are different but the failure mechanisms of the two types of solder joints are similar during the shear test. For the Au/SAC/ENIG system, the Au layer on the surface finish of the diodes dissolved into the molten solder and transformed into a continuous (Au, Ni)Sn4 IMC layer at the diode/solder interface during reflow and the interfacial IMC at the solder/ENIG interface is dendritic Ni3Sn4 IMC grains which are surrounded by (Au, Ni)Sn4. For the Au/SAC/Cu system, however, no IMC layers can be observed at the diode/solder interface. The interfacial IMC at the solder/Cu interface is (Cu, Au)6Sn5 and a Cu3Sn IMC layer at the (Cu, Au)6Sn5/Cu interface. Tiny (Au, Cu)Sn4 IMC grains distribute in the solder layer and surround the (Cu, Au)6Sn5 grains. For the two types of systems, the primary failure mode for the cathode is due to the broken of the Si-based insulation layer which led to a high residue stress and poor connection between the Si-based layer and the solder layer. Meanwhile, the failure of the solder joint for the anode is mainly because of the failure of the solder layer under the conductive via. The crack generally forms at this area and then propagated along the diode or the diode/solder interface.