| With the miniaturization and high density improving of plastic electronic packaging, the packaging for power devices is in the same trend. Power devices are used to switch and control electric energy. Due to its high-power feature, a power device has special requirements for packaging reliability. In power devices, the interfacial areas between leadframe(Cu) and epoxy molding compound(EMC) are large. Furthermore, the interfaces of the two materials are usually exposed in air and they are sensitive to environment, e.g. the moisture, heat etc.. For instance, moisture can diffuse along the interfaces into the package and be trapped in small voids. The moisture will evaporate during solder reflow to make the interface delaminate by the higher vapor pressure in the package. As the Cu/EMC interfaces delaminated, the function of silicon dies may be affected. On the other hand, the Cu/EMC interfaces are also in terms of Cu surface oxidation. In this thesis, the adhesive strength of the Cu/EMC interfaces was studied in two ways:one is related to moisture absorbing, and the other is the effects of Cu surface oxidation time. The controlling of Cu leadframe oxidation processing and hygrothermal aging time during device storage were suggested.In the thesis, we combined finite element method (FEM) with experiments on interface strength and fracture to analyze the Cu/EMC delamination. The major contents include:To begin with, we carried out 3D finite element analysis(FEA) for some typical power devices. The temperature, stress and stain distribution were calculated under cyclic thermal loading(-45℃～125℃) according to JEDEC standards. The results showed that the stresses at the Cu/EMC interface are in high value, which may lead to the delamination of Cu/EMC interface.To guarantee the samples of Cu/EMC were agreed with the real devices, the package samples were fabricated by standard commercial process without die. Then we studied the shear strength of Cu/EMC interfaces with different Cu leadframe oxidation time under 165℃and hygrothermal aging time under 85℃/85RH. It is showed that the interface strength firstly increases with oxidation time, then falls and finally, it tends to be unstable. For hygrothermal aged samples, the shear strength drops dramatically and then gets stable. Based on the results, we suggest that the optimal oxidation processing is 8 minutes and the most reasonable storage time is less than 24 hours.To study the impacts of Cu surface oxidation and hygrothermal progress on Cu/EMC interface fracture, the small pre-notch cracks at the interface were prepared. Then the shearing experiments were performed after Cu oxidation or hygrothermal aging. Applying the maximum shear force and geometric parameters, the critical J integral, Jc, is calculated assisted by FEA. It is found that Jc varied with oxidation and hygrothermal aging time. The trend is similar to that of shear strengths. When the real devices were subjected to lower temperature, J integral of Cu/EMC interface at different position were calculated. The location of higher J value is agreed to the delamination site observed by SAM. For the cracks at the same position, J integral predicted the effect of temperature and crack length on crack propagation.In summary, the simulation indicated that the interface of Cu/EMC in power devices are in higher stresses and then in higher possibility of delamination. Based on the interfacial experiments and analysis, the Cu oxidation and moisture effect on the interface shear strength and delamination fracture were illustrated. The optimized Cu oxidation and packages storage conditions were suggested. The J-integral analysis predicts the most dangerous position of delamination for Cu/EMC interface. Moreover, the interfacial crack propagation with varied crack length and temperature was delineated using J-integral. |
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