Research On The Preparation And Performances Of Cu-pillar Bumps With Sn Cap In The Area Array Packaging

With the rapid development of electronic packaging technology, electronic products become more and more miniaturization and high integration and multifunctionality. The copper pillar bump array interconnection packaging technology is attracting great interest due to its outstanding performances, such as good thermal conductivity, electrical conductivity, and good number of I/Os. At present, due to the difficulties of copper interconnection, the leadfree solder is used to achieve the connection between the copper pillar bump and the pad. But this interconnection strcuture can lead to the serious reliability problem of copper pillar bump joint, due to the diffusion of the Cu-Sn atoms. In particular, as the relentless scaling-down of the electronic product size, the diffusion of Cu-Sn atoms would result in more serious problem. Therefore, how to impede the diffusion of Cu-Sn atoms and the growth of IMC layers at the interface, are the pivotal technology to enhance the interconnection reliability of copper pillar bump. In order to solve the above problems, the way of adding α-Fe₂O₃ nanoparticles was used to prepare novel copper pillar bump with α-Fe₂O₃ nano-composite Sn cap in this paper. Whereafter, the influences of α-Fe₂O₃ nanoparticles concentration on the diffusion of the Cu-Sn atoms, the growth of IMC layers at the interface and the assembly reliability of Cu pillar bump joint during the thermal cycling at-65-150oC and isothermal aging at 150oC were investigated respectively.The results showed that added α-Fe₂O₃ nanoparticles can significantly optimize the morphology and size of intermetallic compound（IMC） at Cu/solder interface during the thermal cycling. With the increase of α-Fe₂O₃ nanoparticles amount, the IMC gradually changed from scallop to flat and the refinement of Cu6Sn5 grain significantly improved. Adding appropriate of α-Fe₂O₃ nanoparticles can remarkably inhibit the overgrowth of the interfacial IMCs, reduce the thickness of the all IMCs and the growth rate of Cu3 Sn layer. Meanwhile, all of copper pillar bump joints in the shear strength first increased and then decreased with cycle times increasing. When the time of thermal cycling was lower, the shear strength of Cu pillar bump joint with Sn-0.024Fe₂O₃ composited solder cap was the largest. When the time of thermal cycling was more than 100, the shear strength of Cu pillar bump joint with Sn-0.032Fe₂O₃ composited solder cap was the largest. The comparison analysis result showed that the change rate of shear strength of the Cu pillar bump joint with Sn-0.024 Fe₂O₃ solder cap was much smaller, indicating its ability better to adapt to changes in temperature. On the basis of the principle of surface adsorption and second phase strengthening, the influences of α-Fe₂O₃ nanoparticles concentration on the growth of IMC layers at the interface and the assembly reliability of Cu pillar bump joint were analyzed respectively.Under the condition of isothermal aging at 150oC, the thickness of interfacial IMCs of copper pillar bump with Sn-xFe₂O₃ composited solder cap were increased with the aging time. At the same aging time, the slight addition amount of α-Fe₂O₃ nanoparticles in the Sn cap would obviously increase the inhibition effect of the growth of interfacial IMC and the shear strength of joint, while excessive addition amount of α-Fe₂O₃ nanoparticles would impair the inhibiton effect and reduce the shear strength. As nanoparticles were added at a concentration of 0.024 g/L, interfacial IMC layer had the minimum growth rate constant and the biggest shear strength of the joint, indicating its better mechanical stability.