Development of copper nanoparticles for low temperature die-attach applications
Ng, Mei Zhen
Date of Issue2017
School of Materials Science and Engineering
Copper nanoparticles (Cu NPs) that are passivated with thin layers of amine-based organic materials is studied as a low temperature die attach material. Due to the high surface energy of these Cu NPs, they need a protection layer to prevent spontaneous fusion. However, it is important to have these passivation layers removed for effective sintering. Since these layers prevent fusion, the Cu NPs will have high electrical resistance before any thermal sintering. Therefore, a fast and effective technique has been developed based on observation of electrical resistance measurements during sintering of Cu NPs when subjected to different sintering parameters. Based on the change in electrical resistance, a sintering model of the Cu NPs used in this study has been developed. It was observed that the onset temperature of volatilization and onset temperature of fusion are closely related to the final microstructures of the sintered Cu NPs. For good fusion of these Cu NPs, the passivating layers and the organic materials that are present in the Cu NPs paste must be effectively and completely removed. Any presence of passivating materials will hinder the fusion of Cu NPs that leads to low neck growth. Though it was reported that a higher heating rate is beneficial for the Cu NPs to achieve a denser sintered layer, it is not necessary for the case in this study. If the passivating layers are not removed completely when subjected to higher heating rates, the fusion of the Cu NPs is poor and it will lead to lower shear strength. In addition, based on the electrical resistance measurements, it is observed that when the onset temperature of volatilization is high, it will lead to a higher fusion temperature and the degree of necking between Cu NPs will be high. To leverage on the effect of high heating rates, fast and complete removal of all organic materials within the layer must be carried out. However, this usually would not happen when the heating rate is too fast and/or the peak sintering temperature is < 200°C unless sintering is done under a reactive atmosphere. Therefore, to remove the amine-based passivation layers faster, formic acid vapor is introduced into the sintering furnace. When formic acid vapor is used, it reacts with the amines-based passivation layers to separate them from the Cu NPs surfaces. Hence the fusion of Cu NPs will occur. Since the reaction is faster than sintering in N2 ambient, more fusion was observed and the increase in shear strength was significantly higher when formic acid vapor was introduced only at 200°C instead of throughout the sintering process. This again shows that fusion of Cu NPs at 200°C is possible only when the passivation layers are effectively and completely removed. The thermal stability and the level of oxidation of the sintered Cu NPs joint after 1000 hours of isothermal annealing at 150°C in air have demonstrated the potentiality of this material to be used to replace current solder materials. Lastly, application of Cu NPs paste was demonstrated for high power LEDs and compared directly with commercially available AuSn solder. The electrical, mechanical and optical properties of the LEDs modules bonded with Cu NPs are comparable or even better than those LEDmodules that were bonded with AuSn solders. Moreover, the peak sintering temperature of Cu NPs is 40°C lower than AuSn reflow temperature. Thus, this material has the potential to lower the overall thermal budget that eventually will reduce the overall manufacturing cost.