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Purpose The purpose of this paper is to study the phenomenology of Al2O3-DBC substrate thermal-cracking under different high temperature cyclic loadings. The extremely low cycle fatigue (ELCF) life prediction model for ductile materials was used to describe the thermal fatigue life of Al2O3-DBC substrates. Design/methodology/approach Four groups of thermal cycling tests using Al2O3-DBC substrates with 0.65 mm thick copper were conducted using different peak temperatures. The failure samples were observed by optical microscope. The thermal plastic strain distribution in the Al2O3-DBC substrates was analyzed using a finite element method with the Chaboche model for describing plastic deformation of copper. The ELCF life prediction model was used to predict the life of Al2O3-DBC substrates under high temperature cyclic loadings. Findings Interface cracking was observed to initiate at the short edge of the bonded copper and deviated into the ceramic layer when the crack grew beyond the critical length of 0.1-0.8 mm. The interface crack deviated into the ceramic layer at different thickness and grew parallel to the interface layer between the ceramic layer and copper layer. The crack propagation stopped after certain cycles. The copper layer with 10-20 μm thick alumina inside was not split away totally from the ceramic layer. The ELCF life prediction model could predict the life of Al2O3-DBC substrates well under high temperature cyclic loading. The material constants in the extremely low fatigue life prediction model were obtained using thermal fatigue tests results. Research limitations/implications The influence of copper layer thickness and ceramic layer thickness on thermal cracking characteristics of DBC substrate should be studied in the future. Failure models should also be further investigated. Originality/value The failure model of Al2O3-DBC substrates under high temperature cyclic loading was studied. A method for predicting the life of the substrate samples under high temperature cyclic loading was proposed.

PurposeDirect‐bond‐copper (DBC) substrates crack after about 15 thermal cycles from −55 to 250°C. The purpose of this paper is to study the phenomenology of thermal‐cracking to determine the suitability of DBC for high‐temperature packaging.Design/methodology/approachThe thermal plastic strain distribution at the edge of the DBC substrate was analyzed by using a finite element method with the Chaboche model for copper. The parameters of the Chaboche model were verified by comparing with the three‐point bending test results of DBC substrate. The thermal analyses involving different edge tail lengths indicated that susceptibility to cracking was influenced by the edge geometry of the DBC substrate.FindingsInterface cracking was observed to initiate at the short edge of the bonded copper and propagated into the ceramic layer. The interface crack was caused by the accumulation of thermal plastic strain near the short edge. The edge tail can decrease the thermal strain along the short edge of the DBC substrate. Thermal cycling lifetime was improved greatly for the DBC substrate with 0.5 mm edge tail length compared with that without edge tail.Research limitations/implicationsThe thermal cracking of DBC substrates should be studied at the microstructure level in the future.Originality/valueThermal cycling induced failure of DBC was analyzed. A method of alleviating the thermal plastic strain distribution on the weakest site and improving the thermal fatigue lifetime of DBC substrates under thermal cycling was proposed.

Sintered nanoscale silver paste was used as thermal interface material in an electronic package. Based on the microstructure of the sintered silver paste, die shear stress-strain curves and thermal cycling test results, a model combined with the model of Navarro-de los Rios and small cracks interaction physical model is put forward to predict the thermal fatigue lifetime of the sintered nanoscale silver paste. Meanwhile, Die-shear strength experiments instead of thermal loading were conducted using Al2O3 and AlN substrates to check the model.