| As a kind of lead-free die attachment material,nanosilver paste can achieve the interconnection between power devices and direct bonded copper(DBC)substrate by a low-temperature sintering technique.Nowadays,nanosilver paste is attracting extensive attention because of its high melting point(961?),low sintering temperature(250?300?),low Young's modulus(9 GPa)and high thermal conductivity(240 W/m·K).Currently,an effective atomic diffusion bonding cannot be formed between nanosilver paste and DBC substrate due to the oxidation of the substrate surface under air atmosphere.In this case,sintering densification mechanism of nanosilver paste in formic acid atmosphere and its application of bonding IGBT chips using pressureless sintering was carried out in this dissertation.The main results of this dissertation are as follows:Firstly,the pressureless sintering mechanism at low temperature and densification behavior of nanosilver paste in formic acid atmosphere were studied.The results showed that the formic acid atmosphere has a significant contribution to the densification of nanosilver paste.The porosity of sintered nanosilver in the formic acid atmosphere is 15.5%,and the shear strength of chip bonding is up to 30 MPa.It is found that the densification behavior of sintering nanosilver paste in the formic acid atmosphere is related to the dominant decomposition mechanism of formic acid.When adding formic acid at 180?,formic acid produced H2 by dehydrogenation.The contributions of H2 to nanosilver densification and shear strength are obvious.It is believed that the enhanced sintering process is due to the hydrogenolysis of the binders during sintering,since the hydrogenolysis could break the long cellulose chains into smaller organic components with lower evaporation temperatures.Once the blocking binders are removed,the densification of nanosilver paste can be achieved.However,When formic acid is injected at 280oC,CO is produced by the dehydration of formic acid,and the nanosilver paste does not undergo significant densification.Based on the research results,the mechanism of sintered nanosilver paste in formic acid atmosphere was proposed.The mechanism can be divided into three stages:volatilization stages of low temperature organic compounds,such as diluents and dispersants(25?180oC),hydrogenolysis stages of high melting temperature binder(180oC),volatilization of low-molecular-weight organic and densification stage of silver nanoparticles(180?280oC).Secondly,this dissertation studied the fabrication process of sintered-nanosilver IGBT module based on the sintering process approach mentioned above,and the static and dynamic switching performances of the sintered-nanosilver IGBT module and the same level commercial one are analyzed and compared.The results showed that the on-state voltage drop and switching loss of the two kinds of IGBT modules have no obvious difference,while the thermal resistance of the sintered-nanosilver IGBT module is 12.7%lower than that of the commercial one.Finally,this paper studied the aging behaviors of the sintered-nanosilver IGBT module and the same level commercial one under thermal shock(-55?150?)and power cycling aging(junction temperature:25?150?).Thermal cycling test results show that the thermal resistance of the sintered-nanosilver IGBT module has no significant increase after 1000 cycles,however,the thermal resistance of the commercial one increases from the initial value of 0.45?/W to final value of0.58?/W after 500 cycles and eventually results in the connection failure.The power cycling results showed that the voltage drop of the commercial one significantly increased by 20%of the initial value and finally the module failed after 63 K cycles,However,the lifetime of the sintered-nanosilver IGBT module is about 90 K cycles. |
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