A Study On New Double-sided Cooling Power Electronic Module Packaging

It has been proved that double-sided cooling has the potential to increase powerdissipation and to improve module reliability by lowering the junction operatingtemperature of chip. However, the availability of semiconductors for solder reflowingfrom both sides and the reliability of die-attachment afterwards become the mainchallenges of double-sided cooling technology. As an alternative lead-freedie-adhesive material, nanosilver paste has the potential to be applied in high powerelectronic packaging, due to its low sintering temperature, high melting point, highelectrical and thermal conductivities, and excellent mechanical reliability. As a result,nanosilver paste was employed to joint double-sided IGBT assembly in this study,both experiments and finite element analysis (FEA) were conducted to explore theproperties and reliabilities of nanosilver paste bonded double-sided IGBT assembly.Thermal and mechanical performances studied by finite element method (FEM)of the double-sided sintered assembly indicated that double-sided IGBT assembly hadsuperior thermal performance over the single-sided one, while the thermal stress onchip was higher than that of the single-sided assembly.In the sample fabrication process, an extra cooling stage in the sinteringprocedure of nanosilver paste was added for sintering the doubled-sided IGBTassembly to minimize the residual thermal stress during cooling down to ambienttemperature. In addition, the plasma cleaning process of gold-plated IGBT chip wasstudied to enhance the bonding strength between sintered nanosilver and gold platedIGBT chip. Furthermore, sintering process of nanosilver paste for double-sidedattaching IGBT chip was presented and parameters of hot-pressing stage, i.e.hot-pressing pressure, temperature and time were obtained by orthogonal experiment.Double-sided packaged IGBT samples were fabricated by sample fabricationprocess proposed in this work. Thermal fatigue characteristics of double-sided IGBTassembly were studied by temperature cycling test. Shear tests revealed that bondingstrength of sintered silver joint in double-sided IGBT assembly decreased with thenumber of thermal cycles. Meanwhile, microstructure analysis indicated that voidspercentage of sintered silver joint increased with the number of thermal cycles. At thesame time, cracks growth was observed by Scanning Acoustic Microscopy (SAM) with the thermal cycles increasing. Simulation results by ANSYS software wereconsistent with the experimental results.To reduce the thermal stress of IGBT chip in nanosilver paste bondeddouble-sided IGBT assembly, silver buffer was sandwiched between IGBT chip andDBC substrate. Finite element analysis (FEA) illustrated that nanosilver paste bondeddouble-sided IGBT assembly with silver buffers had lower junction temperature andthe residual thermal stress on IGBT chip was reduced significantly. Temperaturecycling experiments and shearing tests concluded that by adding silver buffer, averageshear strength of double-sided packaged IGBT assembly increased and the drop ratedecreased with the thermal cycles increasing. Microscopic analysis showed thatgrowth rate as well as the number of voids in sintered silver layer was significantlyreduced in double-sided IGBT assembly with silver buffers. Results of ANSYSsimulation were in agreement with the SAM observations which cracks initiated onthe corner of sintered silver layer and expanded with increasing of thermal cycles.Multichip double-sided packaging by nanosilver paste was analyzed by ANSYSsoftware. It was demonstrated that double-sided packaged multichip assemblyproduced less thermal stress and exhibited higher reliability and feasibility by addingsilver tube.