A Study on Electromigration in Lead-free Solder Joints-using Electroless Nickel and Electroless Cobalt as Diffusion Barrier

• Main Authors: Chun-Chieh,Wen, 溫俊傑
• Other Authors: Chao-hong,Wang
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/70849361453131724165

碩士 === 國立中正大學 === 化學工程研究所 === 101 === Electromigration has become a serious reliability issue in flip chip packaging. Therefore, electromigration on interfacial reactions between electroless Ni-P or electroless Co-P and Sn-based solders was studied in this study. The reaction couples are prepared for Sn-based solders/Ni-P or Co-P/Cu/Ni-P or Co-P/Sn-based solders, which have two advantages better than real flip chip solder joints. The first is that the current distribution in the structure is uniform; the second is that the nonsymmetrical Joule heating effect can be avoided. Using Ni-P alloy as barrier layer in electric current test (current density is set at 5000A/cm2 under 180oC), the result shows that layered structure IMCs is always formed at the cathode interface (electron current flows from solder layer to Ni-P). In the contrast, massive amount of IMC spall into the solder at the anode interface (electron current flows from Ni-P to solder), and two stage phase transformation is observed for Ni-P layer. It is first transformed into P-rich nickel layer due to the diffusion of nickel atoms, and then the P-rich nickel layer reacts with Sn-based solder to form Ni2SnP. TEM image shows that the layers in P-rich nickel are composed of different Ni-P IMCs, and the difference of electric current direction cause the formation of different phase. When increasing the amount of P in deposition layer, P-rich nickel layer is formed rapidly and causes the failing of diffusion barrier. On the other hand, using Co-P (4.5wt%P) alloy as barrier layer in Sn solder and conduct electric current test. The result shows that CoSn4 metastable phase is formed at the interface of both electrodes; CoSn4 is then transforms into CoSn3 when the barrier layer is completely comsumed. The rapid growth of CoSn4 is also observed under high current density. Sn-Ag-Cu alloy solder is used to react with Co-P alloy, the formation of (Cu,Co)6Sn5 phase is observed at the interface, the result shows that this phase can reduce the consuming speed of Co-P alloy in electric current test.