using structure function. (1) Page 1/5 Test mode and failure There are two modes in a power cycle test: Tj Power cycle that changes the junction temperature (Tj Temperature) inside of the power semiconductor device specimen (specimen) and Tc Power cycle that changes the temperature of the specimen's case (Tc Temperature). (Test purpose) To evaluate the specimen failure rate in the market through an accelerated life test. The following describes the test modes and failure sites. (Test mode 1: Tj Power cycle) Tj Power cycle is a test that heats or cools the Tj temperature in a short period of time, and mainly evaluates the life of aluminum wire bonding area on the silicon chip in the specimen, and the solder bonding area beneath the silicon chip. (Test mode 2: Tc Power cycle) Tc Power cycle test applies and interrupts the power until the Tc temperature rises or drops to the given upper or lower limit of the temperature, and mainly evaluates the life of the solder bonding area beneath the silicon chip and solder bonding area between the insulating substrate and copper base in the specimen. ( failure) When each power cycle test is conducted, shearing strain due to the difference in the linear expansion coefficient of each structure part will generate cracks in the bonding area of each structure part. When the crack continues to widen, the temperature of the bonding area rises, resulting in destruction of the part. (Failure analysis) In a power semiconductor device, presence or absence of failure, life, and reliability are closely related to the temperature during operation. Thermal resistance is used to analyze and calculate the rise in temperature. Internal temperature of the device Several S - Several min OFF ON OFF ON OFF ON 125 25 Tj (=Tj max - Tj min) in semiconductor chip Semiconductor chip Cu base (heat sink) 半田 Cu base (heat sink) Heating site in the device Cu base (heat Semiconductor chip Wire bonding Crack Failure site in the device
using structure function. (2) Transient thermal resistance and Structure function Page 2/5 (Thermal resistance) There are two types of thermal resistance: "stationary thermal resistance" in a state where the specimen adequately saturates, or for direct current; and "transient thermal resistance" for short-term power pulse application. (Measurement method) Power semiconductor devices vary in shape. The heat discharging time from chip to the external air also varies by each structure part, and the effect is seen in the measurement data of transient thermal resistance. There are two methods for measuring transient thermal resistance: heating method (dynamic method) and cooling method (static method). Both of those measurement methods were prescribed in 1990 by EIAJ (Electronic Industries Association of Japan). (Structure function) Structure function is a graphical representation of data, which is obtained by converting a graph of time/temperature rising characteristics of a power semiconductor device in measurement of transient thermal resistance into a graph of thermal resistance characteristics and heat capacity characteristics. The structure function enables evaluation of thermal characteristics of each structure part and shows the change of failure site over time in a graphical representation. Semiconductor chip Bonding agent Bonding agent Heat sink Example of cross-section structure of power semiconductor device Convert Rth 1 Rth 2 Rth 3 Rth 4 Rth 5 Cth 1 Cth 2 Cth 3 Cth 4 Cth 5 Heat equivalent circuit of heat dissipation path of power semiconductor device Semicond uctor chip temperatu retj [K] Heat absorption and heat rise due to thermal capacity of semiconductor chip Heat absorption from semiconductor chip to insulating substrate Heat absorption from insulating substrate to heat sink Heat absorption to insulating substrate Heat absorption of heat sink Data of measurement result (Relationship between time and temperature rising characteristics in short-term phenomena) t(s) Convert Heat capacity Cth [J/W] Easy to conduct heat =Large thermal resistance =>Small inclination (Bonding agent, grease, etc.) Hard to conduct heat =Small thermal resistance =>Large inclination (Metal, Si chip, etc.) Easy to see with the emphasized inclination Thermal resistance Rth [K/W] Structure function (Relationship between thermal resistance and heat capacity)
using structure function. (3) Heating method and Cooling method (Dynamic method) (Static method) Page 3/5 (Heating method [Dynamic method]) The measurement method is based on the actual use environment, and the measured data of transient thermal resistance is shown on the application note of the power semiconductor device. Measurement of short-term range has limits in the capability for turning ON the sample's Gate and device's power-application capacity. There will also be switching noise; however, Espec's original technology for Gate control circuit and noise control enable high-speed and stable measurement. (Cooling method [Static method]) By smoothing the measurement result, noise can be eliminated and data can be corrected; however, in the case of a power semiconductor device with multiple heat dissipation paths, it will lead to errors in measured values due to the difference between temperature rising characteristics and temperature falling characteristics. In addition, countermeasures against heat of structure system will be required to saturate the temperature in accordance with the power-application capacity. The following pages describes the two methods in more detail. Test Condition Structure Item Applicable device Stress application Test time Data acquisition Cooling mechanism Heating method (Dynamic method) Device with multiple heat flux paths (IGBT, etc.) Application by pulse (Width: several 10mS) Pulse width Number of ON/OFF cycles Measure the data with interval of a few ms after applying pulse. Cooling method (Static method) Device with one main heat flux path (LED, etc.) Turn OFF the stress application after the temperature saturates from continuous application of stress. Heat saturation time + Cooling time Measure the data with interval of 1μS immediately after turning OFF the stress application. Cooling mechanism is needed to attain Temperature saturation is not required. saturation temperature.
using structure function. (4) Heating method (Dynamic method) Page 4/5 This method continuously applies a constant power pulse while gradating the pulse width, and calculates the temperature change and thermal resistance based on the temperature dependence of voltage at the PN joint area inside the power semiconductor device, which can be obtained at each constant power pulse. Test procedure (MOSFET) 1) Acquire temperature coefficient of the power semiconductor device. Apply measured current (lm) to the drain that attains linearity with gate-source voltage (Vgs) and junction temperature (Tj temperature) of the power semiconductor device in advance, use the test chamber to let the Tj temperature change, and observe the relationship between Vgs and Tj temperature under Im. 2) Measure the low temperature before application of constant power pulse. Switch the circuit of the device to measuring system, and measure the Vgs with Im being applied. From the relationship between Vgs and Tj temperature in Step 1), convert the figure to a temperature to obtain the low temperature Tj1. 3) Measure the high temperature during and immediately after the application of power pulse. Switch the circuit of the device to stress system, and apply a power pulse for an arbitrary unit of time. Then, immediately after the power pulse stops, switch the circuit to the measuring system and measure the Vgs with the Im being applied. From the relationship between Vgs and Tj temperature in Step 1), convert the figure to a temperature to obtain the high temperature Tj2. 4) Calculate the transient thermal resistance. Calculate the thermal resistance based on the temperature difference Tj between Tj1 and Tj2 obtained in Step 2) and 3), and applied power. 5) Repeat the above steps 2) through 4). Repeat Steps 2) to 4) with an arbitrary constant power pulse width, measure the thermal resistance at each point and plot the data on a graph. Measure the thermal characteristics of each structure part by obtaining measurement points in small intervals. Is Gate-On voltage Gate-Off voltage Vd Vg Vs Im Power-supply device Stress system Measuring system Circuit diagram of measuring system of the device Current Temperature T Vgs (1) Constant power ON/OFF Power pulse application time (0.01-1000S) Vgs (2) Transient Thermal Resistance ( /W) Calculate the thermal resistance at each pulse, and plot the points on the graph. Pulse Width (S) Schematic graph of thermal resistance measurement data Schematic of measurement with heating (dynamic) method
using structure function. (5) Cooling method (Static method) Page 5/5 This method continuously applies power to the power semiconductor device, and after interrupting the power upon reaching temperature saturation, it measures the transient phenomenon where the voltage-temperature characteristics of the PN joint area in the power semiconductor device gradually changes. Then, it calculates the temperature change and thermal resistance based on the temperature transient response characteristics. In a radiation structure with heat flow in a single direction, the temperature rising characteristics can be equated to the falling characteristic. This method uses these characteristics, and is specified by JEDEC (Joint Electron Device Engineering Council) as JESD51-14. Test procedure (MOSFET) 1) Obtain the temperature coefficient of the power semiconductor device. Apply measured current (lm) to the drain that attains linearity with gate-source voltage (Vgs) and junction temperature (Tj temperature) of the power semiconductor device in advance, use the test chamber to let the Tj temperature change, and obtain the relationship between Vgs and Tj temperature under Im. 2) Apply power and heat the device until the temperature reaches saturation. Switch the circuit of the device to stress system, apply the power, and adequately heat the power semiconductor device until it reaches thermal equilibrium. Obtain the saturated temperature Tj1. 3) After the temperature saturates, interrupt the power supply and perform high-speed measurement of temperature reduction characteristics. After the temperature saturates, suspend the application of power, switch the circuit of the device to measuring system, and perform high-speed measurement of Vgs with the Im being applied. 1) Convert the relationship between Vgs and Tj temperature into temperature, and obtain continuous data of Tj2 while the temperature is falling. (* In MOSFET or IGBT, the measurement starts 1ms after the power interruption due to the transient response characteristics of Gate voltage. Meanwhile, there is no gate voltage for the diode element, and high-speed measurement of transient response characteristics of forward voltage Vf can be performed in 1μS.) 4) Calculate the transient thermal resistance. Calculate the thermal resistance based on the temperature difference Tj between Tj1 and Tj2 obtained in Step 2) and 3), and applied power. Is Gate-On voltage Gate-Off voltage Vd Vg Vs Im Power-supply unit Stress system Measuring system Circuit diagram of measuring system of the device Current Temperature Ih: Heating current Im: Measured current Measurement Transient Thermal Resistance [ /W] 0.07 greaseless_ave. 0.06 grease_ave 0.05 0.04 0.03 0.02 0.01 0 1.E-06 1.E-04 1.E-02 1.E+00 1.E+02 1.E+04 [sec] t0 t1 t2 Schematic graph of thermal resistance measurement data Schematic of measurement with cooling (static) method