Plastic Power Devices Hierarchical Failure Mechanism And Process Improvement

Power semiconductor devices are the basis and core part of modern power electronics technology.With the rapid development of power semiconductor,the packaging technology of power devices has been developing comparatively stagnant which constraints the further improvement of device performance.Plastic encapsulation is the main form of power devices packaging and package delamination was one of the most common failure modes which induced by the non-airtaightness of plastic packaging.Delamination would decrease the packaging reliability and cause failure issues such as wire-bond defect,die surface damage,pop-corn effect and metal corrosion.Generally,the delamination occurs with the combined action of moisture and thermal stress.The wide application of power devices set the increasing demands for higher packaging reliability especially for the devices which working in harsh environments.The study on packaging delamination is extremely limited and the explanation from the perspective of adhesive theories has not been reported now.This paper is to study the mechanism of the delamination and propose the packaging process improvement solution which possesses high practical value.Failure analysis was performed to investigate the delamination on DAP and die surface.In order to find out the fracture location and mechanism,surface morphology was inspected by SEM and composition by EDX and Raman spectrum.Then this paper presents the concept of boundary layer of the adhesive structure.Corrosion test was employed to study the electrochemical characteristic of Sn-Cu-SnPb system which is coincident with the packaging structure.The corrosion mechanism and process of Cu early-stage crevice corrosion and Cu-SnPb galvanic corrosion were determined.It was found that electrochemical corrosion in high temperature and high humidity would enhance the expansion of delamination.For process improvement,the feasible method may be the oxidation of Cu leadframe.Influence of the oxdation on surface roughness,contact angle,surface morphology was investigated,and the thickness and interface morphology of oxidation layer were tested by FIB and SEM.The experiment results demonstrated the feasibility and effectiveness of oxidation for surface modification from the perspective of adhesive theories.According to the failure analysis and oxidation characteristic conclusions, process improvement solution was proposed.The shear strength of a certain L/F with different oxidation conditions and KTMC5900GM(EMC) was studied,on the basis of the mechanical testing,an optimized process condition was obtained.The performance in the high temperature & humidity aging test and V-notch testing after process optimization were also been evaluated.The experiments results indicated that,the delamination mainly occurred in the Cu oxide layers and the adhesive boundary layer,the expansion mechanism were hydrothermal stress and electrochemical corrosion.The oxidation treatment of the leadframe could enhance the adhesion strength between the Cu L/F and EMC,and improve the anti-delamination performance of the plastic encapsulated power device. But the thickness of the oxide layer must be strictly controlled,or it will cause adverse effect.