On Resistance Of Nano-Ag-Pd Paste To Electrochemical Migration Behavior At High Temperatures

Nano-silver paste has gradually replaced traditional Sn-Pb solder and lead-free solder as one of the key materials in power semiconductor device packaging with its excellent mechanical properties,conductive and thermal conductivity and green lead-free.However,a large number of studies have shown that silver was susceptible to migrate or better known as electrochemical migration(ECM),and the ECM of metals was one of the main forms of electronic components failure.In the past,the research on the ECM behavior of silver was mainly concentrated in the ambient temperature and humid environment,but with the development of the wide band gap(WBG)semiconductor power device which can be applied to high temperature environment,the ECM behavior of silver in high temperature was also paid attention to.Some researchers have suggested that Ag-alloy powders can be used as bonding materials instead of pure silver to effectively control the electrochemical migration of silver,such as Ag-Pd.However,there were many problems such as complex process and easy agglomeration of Ag-Pd particles.Moreover,there were few reports about the inhibition mechanism of Pd or Ag-Pd alloys.Therefore,it is urgent to explore a new process to improve the production efficiency and clarify the inhibition mechanism of Pd or Ag-Pd alloy on the ECM of silver.In this paper,a hybrid preparation method was proposed to prepare nano-Ag-Pd paste.The high temperature ECM behavior of the paste and the ECM inhibition mechanism of Pd particles on silver were studied,which provided theoretical guidance for the reliability design of the high temperature package of the power semiconductor devices.Firstly,the nano-Ag-Pd paste was prepared by adding Pd particles into the nano-silver paste,and the alloying of Ag-Pd was realized by sintering process.The results showed that the alloying degree of Ag and Pd particles in the nano-Ag-Pd paste increased with the increasing sintering temperature,and the process of alloying was the diffusion process of Ag particles to Pd particles.Then,the ECM failure behavior of nano-Ag-Pd paste and nano-silver paste was compared and analyzed.The results showed that the mean time to failure(MTTF)of sintered Ag-Pd electrode was much higher than that of sintered silver electrode at the same sintering temperature.Therefore,the addition of Pd particles had a good inhibitory effect on the high temperature ECM of silver.For the nano-Ag-Pd paste,the MTTF of sintering Ag-Pd electrode at different sintering temperature was:350~oC>430~oC>800~oC,the corresponding phase composition was respectively,PdO(oxidation process)+Ag-Pd Alloy+Ag+Pd,PdO+Ag,Ag-Pd Alloy.Therefore,the inhibitory effect of PdO and Ag-Pd alloys on the ECM of silver was:PdO(oxidation process)+Ag-Pd alloy>PdO>Ag-Pd alloy.Finally,we investigated the ECM inhibition mechanism of the addition of Pd particles to sintering silver in high temperature environment.During the high temperature ECM of silver,the oxygen competition mechanism in the process of PdO formation determined the MTTF of sintered silver,and not the number of PdO itself and the simple mixing of PdO and Ag-Pd alloy.In addition,the formation of Ag-Pd solid solution alloy decreased the Gibbs free energy of the system to the lowest value,thus enhancing its thermodynamic stability,and therefore,reducing the migration rate of Ag-Pd compound ions and improving the MFFT of ECM.