Electromigration Behavior Of Ni/Sn-3.0Ag-0.5Cu/ENEPIG(OSP) Solder Joints In Flip Chip Packaging

With the continuous miniaturization and high integration in manufacturing electronic devices, the solder bumps are downsizing and the current densities passing through solder joints are increasing. The diameter of flip chip solder joints will decrease to 50μm in the near future. As a result, the average current density will be up to 104 A/cm2 if a current of 0.2 A is applied. Under such a high current density, electromigration (EM) damage in ultra fine solder joints becomes a critical reliability issue. In addition, due to health and environmental concerns, lead-free solders are used in the electronic industries. As a result, the effect of EM on reliability of lead-free flip chip solder joints has attracted extensive attentions of the academe and industry. In this work, the EM behavior of Ni/Sn-3.0Ag-0.5Cu/ENEPIG and Ni/Sn-3.0Ag-0.5Cu/OSP flip chip solder joints were studied under a current density of 5.0×103A/cm2 at 150℃. Furthermore, three-dimensional (3-D) finite element simulations on current density and temperature were performed to better understand the current crowding and Joule heating in flip chip packaging.The simulation results showed that the serious current crowding occurred in the regions where the current is flowing in and out of the solder joints. Due to the Joule heating, the temperature of the chip increased remarkably under current stressing. However, the temperature in a single solder joint was almost uniform, the difference of temperature in the single solder joint was below 1℃and there was no large temperature gradient across the solder joints.The experimental results showed that:(1) For the Ni/Sn-3.0Ag-0.5Cu/ENEPIG solder joints, the (Cu,Ni)6Sn5 IMCs formed at the interfaces of as-soldered solder joints and massive spalling of (Cu,Ni)6Sn5 IMCs occurred at chip side. During aging at 150℃, the interfacial IMCs transformed from (Cu,Ni)6Sn5 into (Ni,Cu)3Sn4. However, the spalled IMCs remained (Cu,Ni)6Sn5 type. When electroless Ni-P was the cathode, EM significantly enhanced the consumption of Ni-P layer and formation of Ni3P layer with columnar voids. The dissolved Ni atoms were driven along the direction of electrons flow and formed a large amount of (Ni,Cu)3Sn4 in solder matrix. Once the Ni-P layer was completely consumed and transformed into Ni3P, the reaction between Ni3P and Sn would lead to a fast growth of Ni2SnP layer. When Ni UBM was the cathode, no significant consumption of Ni-P layer occurred. However, the current crowding induced a rapid and localized dissolution of Ni UBM and Cu pad on chip side (cathode). The dissolved Cu atoms were driven to the anode and led to the transformation of IMC type from (Ni,Cu)3Sn4 into (Cu,Ni)6Sn5 at Ni-P/Sn-3.0Ag-0.5Cu interface (anode). The (Au,Pd,Ni)Sn4 phases, coarsening with increasing time, were distributed not only in the solder matrix but also at both interfaces during aging at 150℃. However, during EM at 150℃, the Au and Pd atoms, dissolved from (Au,Pd,Ni)Sn4 phases in the solder matrix, were driven along the flowing direction of electrons to the anode interface, and resulted in a thick (Au,Pd,Ni)Sn4 layer at the anode. (2) For the Ni/Sn-3.0Ag-0.5Cu/OSP solder joints, the (Cu,Ni)6Sn5 IMCs formed at the interfaces of as-soldered solder joints. During aging at 150℃, the interfacial IMCs remained (Cu,Ni)6Sn5 type. However, with increasing aging time, the Cu content in interfacial IMCs increased and the Ni content in interfacial IMCs decreased. During EM under a current density of 5 X 103 A/cm2 at 150℃, when the Cu pad on the PCB was the cathode, current crowding induced a rapid and localized dissolution of Cu pad and formation of voids at the electron entry point. The dissolved Cu atoms were driven to the anode side by EM, and a large amount of Cu6Sn5 was formed in the solder along the direction of electron flow. With increasing EM time, the Cu pad on the PCB was almost consumed completely and the voids extended the Cu6Sn5/Cu interface which induced the failure of the solder joint. When the Ni UBM on the chip was the cathode, no serious consumption of Ni UBM and Cu pad on the chip occurred and no large number of Cu6Sn5 IMCs formed in the solder matrix. The type of interfacial IMCs did not change and remained (Cu,Ni)6Sn5. There was no evidence of EM-induced failure in the solder joints with the electrons flowing from the chip to the PCB.