Reliability Evaluations and Test Functions Devised for Automotive Power Devices

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1 Technology Report Reliability Evaluations and Test Functions Devised for Automotive Power Devices Batteries & Energy System Equipment Headquarters ESPEC CORP. Michiya Kusaka Norihito Kan Makoto Kitagawa Abstract Concerns over global warming and nuclear power safety are creating the need to supply renewable energy more efficiently to ensure a stable energy supply. Efficient energy use requires the power devices used as conversion devices for inverter control. Since these devices are used in many different environments, reliability improvements for them are crucial. A growing number of electronic components are being used in vehicles. The greater use of these components and the switch to hybrid and electric vehicles are reducing fossil fuel (gas) consumption to reduce CO2 and enable fuel-efficient travel. It is also becoming possible to prevent accidents by mounting electrical sensors and CCDs in vehicles as road safety features. Inverters are needed to efficiently drive electrical energy to motors. Many power devices are used in inverters. For example, supply systems and intake/exhaust systems used to control the output of the engine or motor used to move the vehicle s wheels have electronically controlled timing, and the motors and actuators used to operate these systems are driven by power semiconductors. Power devices used in vehicles need to ensure prolonged failure-free use in many different environments (high temperatures, low temperatures, heat and vibrations). Power devices play a major role in today s era of groundbreaking vehicle innovations in safety, energy-saving and IT use, so finding ways of increasing their reliability is a key challenge. ESPEC is working with many onboard device manufacturers to increase power device reliability, studying how to evaluate problems specific to vehicles, and devising evaluation items. This article presents some of the failure mode evaluation methods we have devised. Onboard Power Devices Devices used for engine or intake/exhaust control in gasoline-engine vehicles or for motor drive in electric vehicles (EVs) require AC control from DC power sources, and their control chips (dies) can be subject to currents of over 300 amps for some drive components. Applying large currents to chips (dies) causes a rapid rise in the chip temperature (the junction temperature, Tj). When efficient motor control is needed, large currents have to be frequently turned ON/OFF to provide fine-tuned control. The result is a heat cycle in which the power device itself is repeatedly heated and cooled, causing structural deformation. Failure to efficiently discharge the heat away from the device will result in wire breaks, cracks and other problems leading to failure.

2 There are two major phenomena responsible for automotive power device failures. One is called lift-off the wire bonding joined to the chip (die) heats up due to the electrical resistance component in the junction created by the large current. The other is cracking caused by heat-induced deformation of insulation or solder material before heat is discharged. It results when the power device s inverter control operation is triggered by stable engine or motor speed, resulting in long current ON/OFF times that cause the entire power module to gradually cycle between high and low temperatures. It is important to identify the failure generation processes quickly and as soon as possible before these phenomena occur in the market. ESPEC devises and uses evaluation methods for identifying these processes. Evaluation Functions Devised for Power Cycle Test System (ESPEC power cycle test system) 1. Identifying power device characteristics 1-1. Temperature characteristic measurement The entire sample is gradually warmed, and the chip (die) temperature in the sample is measured by measuring Vce (Vgs). The K-factor is calculated and the correlation between Tj and Vce (Vgs) is derived Temperature transient response measurement The amount of time needed to reach Tj max after applying the test current is calculated, along with the amount of time needed to reach Tj min after the current is turned OFF. The cycle time of the power cycle is derived. Meas.interval Temp Target Tj Current(A) pre trigger count Target Lower temp. heat up time cool down time The temperature change occurring between the time the chip (die) is heated by the current applied to it, and the time heat is discharged from the case can be read using temperature transient response testing (dynamic method).

3 Benefit Degraded parts can be identified using continuous cycle testing to measure the temperature rise curve for every specified cycle count. Wire bonding junction deterioration (shown here occurring at 0.4 sec) Deterioration of solder junction between insulation boards (shown here occurring at 1.4 sec) Another method is the static method, in which the change in thermal resistance is identified by saturating the sample to a constant temperature, then turning current application OFF to enable heat discharge. When testing automotive power devices, measurement should ideally be done in a vacuum due to the module shape and since during heat discharge, heat is sometimes discharged from the current application terminals. 2. Power cycle test types 2-1. Fixed-time cycle testing Testing is performed with set ON/OFF power cycle conditions. The sample is subjected to power cycles (such as by testing at the maximum standard test current), and Tj (junction temperature) is measured every specified cycle count to enable early detection of lift-off. 3.5 Fixed Time Voltage[V] Vce 0 19:39:22 19:40:48 19:42:14 19:43:41 19:45:07 19:46:34 19:48:00 19:49:26 19:50:53 19:52:19 Time date: :40:20 U-H:78.0 U-L:85.9 V-H:79.7 V-L:86.3 W-H:87.4 W-L:87.3 U-H:77.6 U-L:86.7 V-H:80.1 V-L:86.7 W-H:80.2 W-L:87.7 U-H:78.0 U-L:85.5 V-H:79.3 V-L:87.1 W-H:85.7 W-L: ΔTj (junction temperature) and ΔTc (case temperature) power cycle testing Power cycle testing is performed in strict conformance with the temperature difference specified by the test standard, automatically compensating the ON and OFF times. The diagram below shows a temperature compensation function for an upper limit temperature of 70 C.

4 2-3. Superimposed testing Power cycle testing is performed by varying the ambient temperature, to perform combination testing that subjects the sample to thermal cycle stress and power cycle stress. The diagram below illustrates combination testing designed for an actual vehicle environment. Test Temp Temp A Temp B Duty(%) Power cycle on Powercycle off Temp cycle time 2-4. Other testing Actual specifications mode Power cycle test conditions are divided into test blocks that simulate actual usage conditions, enabling changes in stress conditions. Constant power testing Constant power cycle testing is performed by varying the stress current in accordance with the change in resistance of the chip (die). Continuous current testing Testing is performed with current continuously applied, keeping the power device s Tj (Tc) max temperature at a fixed value. Automotive power cycle testing that applies large currents to the sample can cause unexpected damage to semiconductor chips from overshoot caused by inrush current. Circuit design must ensure that the sample is not

5 subjected to overcurrent, using an external constant current (CC) circuit between the power supply and power device to ensure that the exact specified current continues to be applied even when power device degradation causes saturation voltage changes. Measuring the change in thermal resistance while performing continuous power cycle testing is also very important for identifying failure occurrences. If the sample is removed for thermal resistance measurement every specified time interval, the sample s state could change, possibly affecting the gathered data. So stress application and measurement should ideally be done between sample installation and the end of testing. The evaluation methods ESPEC has devised enable high-precision testing while improving efficiency and saving energy. Contact ESPEC for solutions to all your test method problems and concerns.