Failure Analysis Of MOSFET Device In Lithium Battery Protection Circuit

With the rapid development of electronic products and new energy automobile industry,the demand for lithium batteries is increasing,and the production technology of it is also constantly innovating.The MOSFET(Metal-Oxide-Semiconductor Field Effect Transistor)of the lithium battery protection circuit has the advantages of high input impedance,high switching frequency,simple driving,and low conduction loss,but the harsh application scenarios such as over temperature,over voltage and large current make MOSFETs are susceptible to failure,causing circuit malfunction.Therefore,the study of the failure mode and failure mechanism of MOSFET is of great significance to the working life and reliability of lithium batteries.In this paper,the failure analysis of the MOSFET of the lithium battery protection circuit,from the selection of materials to the application of processing and assembly links,and put forward practical cases.The corresponding solution measures provide a certain reference value for the failure analysis and problem solving of MOSFET devices and other electronic components.The specific research contents are as follows:Firstly,the failure mode and failure mechanism of MOSFET are studied in detail.In terms of failure modes,the main failure modes of MOSFETs include open circuit,short circuit,electrical parameter drift,and potential failure;in terms of failure mechanisms,typical failure mechanisms of MOSFET include welding failure,electrostatic discharge failure(ESD:Electrostatic Discharge),over Electrical stress failure(EOS: Electrical Over Stress),chip crack failure,etc.,can be roughly divided into two categories: improper use of MOSEET and defects of MOSFET itself,which provide theoretical support for subsequent MOSFET failure analysis conclusions.Secondly,the five practical MOSFET device failures are studied in detail,and improvement measures are proposed.1.Analyzed the abnormal welding problem of the MOSFET that caused the abnormal battery function.The problem is that the continuous IMC(Intermetallic coupoud)layer was not formed during the welding process.The improvement measures are optimized for the temperature and time during the welding process.2.Theburn-out phenomenon of the MOSFET in the lithium battery protection circuit was studied,which was mainly caused by exceeding the safe operating area(SOA: Safe operating area).Considering the impedance of the actual test equipment,the short-circuit current of the design requirements cannot be released,so the design requirements are proposed.Improvement,that is,replace the lithium battery protection chip model used with MOSFET,so that the MOSFET works in the SOA.3.Analyzed the internal penetrating crack problem of the MOSFET that caused the abnormal battery function.This problem was caused during the assembly of the finished battery.The improvement measures were to change the design of each fixture to effectively avoid the location of the MOSFET element.4.Analyzed a batch of ESD-caused MOSFET failure products.The fundamental reason is that the MOSFET passivation layer has micro-crack defects,resulting in insufficient anti-interference ability.The improvement measures are to replace with other manufacturers' MOSFET devices,and each assembly process strictly avoids static electricity harm.5.The four-in-one chip is analyzed.The chip integrates two MOSFETs and two lithium battery chips.The failure reason is that the root of the bonding wire connecting the MOSFET and the lithium battery chip is broken.It is caused by the chip manufacturing process,and the improvement measures are by the chip manufacturer.Optimize bonding parameters(time,power,pressure).Finally,a detailed and complete failure analysis process is proposed.From the board level failure analysis of lithium battery protection circuit to the chip level failure analysis of MOSFET devices,the process is as follows: 1.Failure information investigation.2.Battery function confirmation.3.PCBA(printed circuit board assembly)electrical performance test.4.Nondestructive analysis(appearance inspection,X-ray inspection,computed tomography)inspection.5.Damage analysis(slicing,unsealing,FIB(focused)It can be used with SEM to observe the failure morphology.6.Chip level physical performance detection technology(Emmi),OBIRCH(optical beam induced resistance change).7.Put forward improvement measures.