Optimization And Implementation Of Reliability For Automotive Level Bipolar Transistors In SOT23 Package

Bipolar transistors are widely used in automotive electronics and are commonly used as amplifiers and switches in electrical circuits.Semiconductor devices used in automotive electronics need to pass the automotive level reliability test,and the test severity is far beyond the general requirement of consumer level reliability test.The high reliability requirement of automotive semiconductors has kept most semiconductor manufacturers out of the automotive supply chain.In this paper,representative bipolar transistors assembled in SOT23 are selected for automotive level reliability test design and reliability test qualification.Failure root causes are found through failure analysis for reliability failure samples,and improvement actions are applied according to the root causes.The goal is to improve the reliability level of bipolar transistors assembled in SOT23 and meet the automotive level reliability qualification requirement,thus can provide reference methods for the reliability improvement of other package types and other devices.The main research methods in this paper are:(1)Analysis of automotive level reliability test theory and test design.(2)Performed reliability test qualification and analyze the data for reliability test results.(3)Find the root cause of failure through detailed failure analysis for the failure samples.(4)Discuss the failure mechanism for both package level and wafer level,and give the improvement actions.(5)Re-qualification for samples after optimized according to automotive level reliability test.The key conclusions of this paper are:(1)The bipolar transistors assembled in SOT23 fail in HAST test,and the failure parameters and failure phenomenon are h FE attenuation shift and leakage exceeding the upper limit.(2)The test data analysis results of reliability test show h FE attenuation shift under different current test conditions.The shift ratio under small current is the biggest one compare to 10% of other current,and the maxium shift value under small current is above 40%.(3)The root cause for leakage failure is delaminations between leadframe and molding compound.Moisture and other contaminations can enter into the package and formed leakage channel which caused leakage failure.The root cause for h FE failure is passivation layer crack.The mobile ion can enter into the passivation layer and silicon oxide layer which can change the charge distribution of the die surface,then reduce the life of minority carrier in the base and emitter,and finally caused h FE attenuation shift.The failure mechanism is mainly in the die surface which lead to the h FE attenuation shift obviously appear under small current condition.(4)There are four actions for reducing the delaminations,including choosing molding compound with lower thermal expansion and water absorption,adjusting the molding temperature and injecting pressure,increasing V groove on the leadframe,and reducing the electrolytic current in tin plating.(5)The passivation layer crack can be reduced by adding PSG to the structure of passivation layer and adjusting the stress type of passivation layer to compressive stress.Bipolar transistors assembled in SOT23 can pass the automotive level reliability qualification test after implement the improvement actions mentioned above.