A study on evaluation method for reliability of Nano-metal pastes of high temperature resistant of power devices

Abstract

In recent years, environmental problems and energy problems have been increasingly serious. And reducing emissions of CO2 is required. Research of hybrid and electrical vehicles became one of the top targets. These cars need power devices such as inverter to control electric power. Since operating temperature of an Si power device is about 150°C, a big cooling system is needed to control the module's temperature to below 150°C. These weight and size is big burden for these vehicles, therefore downsizing technologies are the key of the development of new vehicles. On the other hand, SiC power devices can increase the operative temperature up to 300°C. This advantage can downsize the cooling system Furthermore, the electrical consumption of SiC is half of Si. The present packaging technology is reliable under 150°C. However, the reliability at high temperature has not been studied yet. Moreover, automotive power devices particularly need high reliability under temperature cycles and power on-off cycles. Conceptual diagrams of the mismatch absorption by the conventional chip mounting structure and the new high-temperature-resistant chip mounting structure are found, respectively, so a new packaging concept is needed. Two demands are necessary for SiC packaging technology. First is the mounting method should be achieved by low-temperature mounting process and be used in a high temperature environment. The other is high reliability of the packaging structure. The author's group has proposed a new method using Ag-Nano material to mount the SiC chip on pure aluminum board as shown in fig 1. The Ag-Nano is a mixture of Nano-sized Ag particles Organic material can be mounted firmly below about 300°C with low pressure. The stress can be relaxed on the metal board side because of pure aluminum substrate is soft compared with the Ag-Nano. However, it is impossible to stick Ag-Nano onto an aluminum board. Ni plating and Ag plating on the substrate was also used. Ni plating has the facility to prevent substrate from oxidation and warpage. And its reliability has been investigated by other members of the author's group. In this study, the authors proposed a new experimental method to investigate the reliability issues of the Ag-Nano joint. By watching SEM image of cross section of new mounting structure after 1000cycles of thermal cycle test, and temperature profile of the test, the fatigue crack was found from Ni plating layer to Ag-Nano. To improve the reliability First the properties of Ag-Nano are investigated. To improve the bonding strength of the Ag Nano layer and its antioxidation, Ag-plated Ni plating has been adopted in new structure SiC power devices. Plating of normal electronic material is so thin that its mechanical characteristics cannot be noticed so well, but in this structure though compared AL thickness, Ni Plating is very thick, its contribution to the reliability of the new structural is proved under very exacting using environment. The FEA analysis is taken to evaluate the factor of the reliability of the Nano-metal layer. The evaluation for the thickness of the Ni plating and Ag plating and their factors to the reliability of Nano-metal layer has been taken. By analyzing the result of the FEA, to improve the reliability of a new structure, suppression of stress at the edge of the bonding layer is important also it is necessary to thin the plating while maintaining the same functionality of the original plating of this structure, evaluation of fatigue strength of Ag-Nano is necessity.