Investigation of nano-copper for electronic packaging application : solder metallization and die attach
Date of Issue2016
School of Materials Science and Engineering
NTU-Lockheed Martin Joint Laboratory
Nano-copper (NC) has been developed recently with good potential for electronic packaging applications. In this work, the interfacial reaction between NC and Sn-3.5Ag and the adhesion between NC and different metallizations were investigated. Thermal Gravimetric Analysis (TGA) results of NC paste were used to help determine the best sintering condition of NC paste. Heating NC paste at 230 ºC for 10 min at N2 atmosphere is the optimal sintering condition. The sintered NC paste shows a porous nanocrystal structure. In the solder interfacial reaction study with Sn-3.5Ag, plated Cu was used as a reference. Both sintered NC/Sn-3.5Ag and plated Cu/Sn-3.5Ag samples were reflowed for different time spans to obtain their morphology and kinetic information. The morphology changes from 10s to 5 hours reflow showed that the interfacial reaction of NC/Sn-3.5Ag is quite different from Cu/Sn-3.5Ag. NC/Sn-3.5Ag produces porous and elongated scallop-type Intermetallic Compounds (IMCs) while Cu/Sn-3.5Ag only forms scallop-type IMCs. The cause of this morphology difference is due to nano-copper’s unique porous structure. Based on the Scanning Electron Microscope (SEM) and (Energy Dispersive X-ray Spectroscopy) EDS analysis of samples of different reflow times, an IMC growth mechanism in NC/Sn-3.5Ag has been proposed. To quantify NC/Sn-3.5Ag and Cu/Sn-3.5Ag interfacial reaction, a kinetics analysis of the interfacial IMC growth was carried out. Results show that NC/Sn-3.5Ag reaction is faster than Cu/Sn-3.5Ag reaction. The growth of Cu6Sn5 in Cu/Sn-3.5Ag follows t1/3 growth kinetics while the growth of Cu6Sn5 in NC/Sn-3.5Ag follows t1/2 growth kinetics. By comparing experiment result with kinetics model, we conclude that the IMC growth in Cu/Sn-3.5Ag is controlled by diffusion through fast diffusion channel such as the IMC grooves while NC/Sn-3.5Ag reaction is mainly controlled by diffusion through the existing IMC layers. In the adhesion study between NC and different metallzations, NC as a potential joining material was evaluated by cross-cut test and die-shear test. Different metallizations were prepared by either sputtering or electrochemical method. These metallizations are chosen because they are either commercialized in the industry or have shown very good performance in lead-free solder applications. Even though NC can directly bond with Cu surface without forming IMCs, for the purpose of its potential as a joining material used in the future, it is essential to understand its adhesion conditions with different metallizations. Different metallizations were prepared and analyzed by SEM, EDS and Atomic Force Microscope (AFM). The surface roughness, element concentration and metallization layer thickness were obtained and analyzed. Cross-cut adhesion test shows that Cu, ENIG, Cr/Ni/Au and Ni-Sn-P have relatively better adhesion with NC than Ni-P, Ni-W-P, Ni-Co-P. Shear test result reveals that Cu has the best adhesion with NC. By fractographic analysis of the shear test result, NC/Cu failure mode is verified as bulk (cohesive) failure, which resulted in higher failure strength. The failure mode of Ni-Sn-P/NC is bulk-partial IMC failure. The failure mode of Cr/Ni/Au and ENIG is bulk-partial pad peeling failure. In summary, the morphology and kinetics study of NC/Sn-3.5Ag interfacial reaction have been carried out. The obtained information could be used as a reference in electronic packaging applications involving NC and Sn-based solders. Besides, the adhesion study between NC and different metallizations examines the potential of NC paste as a die-attach joining material. Experiment result shows that pure Cu has the best adhesion with NC after reflow at 230 ºC for 10 min.