Power Cycling Test Stations for Power Semiconductors

During active power cycling, the device under test (DUT) is periodically heated by its own forward current and cooled during the off-phase by a cooling system. The resulting cyclic temperature swings (ΔTj) create thermomechanical stresses at interfaces between materials with different thermal expansion coefficients. Typical failure mechanisms are:

• Bond Wire Lift-off: Different thermal expansion of aluminum bond wire and chip surface creates shear stress leading to bond detachment. Manifests as increase in forward voltage VCE(sat).
• Chip Metallization Degradation: Recrystallization and crack formation in aluminum metallization on the chip, accelerated by high temperature swings.
• Solder Fatigue: Crack propagation in solder joints between chip and DCB substrate (die attach) and between substrate and baseplate. Manifests as increase in thermal resistance Rth(j-c).

The ECPE guideline AQG 324 defines two different power cycling modes that target different failure mechanisms:

• PCsec (ton < 5 s): Short heating pulses create steep temperature gradients that primarily stress chip-near connections -- bond wires, chip metallization and the upper solder layer (die attach). This mode simulates applications with fast load changes like motor inverters.
• PCmin (ton > 15 s to minutes): Longer heating times allow heat to penetrate deep into the assembly structure. Thermal stress extends to substrate, baseplate solder and package attachment. This mode is relevant for applications with slower load cycles.

The end-of-life criteria per AQG 324: +5% increase in VCE(sat) or +20% increase in Rth compared to initial value.

The standard method is the VCE(T) method (also TSEP method -- Temperature Sensitive Electrical Parameter). The semiconductor itself serves as temperature sensor:

1. Calibration curve: Before testing, the temperature-dependent forward voltage VCE is recorded at a defined measurement current (typically 1-500 mA) for various temperatures.
2. Measurement during test: After each load cycle, the load current is switched off and a small measurement current is applied. The measured forward voltage is converted to virtual junction temperature Tvj via the calibration curve.
3. Zth determination: From the cooling curve Tvj(t), the transient thermal impedance Zth(t) can be calculated -- a sensitive indicator for incipient solder layer degradation before the steady-state Rth changes measurably.

For SiC MOSFETs, the calibration curve behaves differently depending on gate bias conditions (VGS = 0 V vs. VGS = -10 V) and may shift with aging.

Wide-bandgap semiconductors differ from conventional Si-IGBTs in several aspects:

• Higher operating temperatures: SiC modules can be qualified at junction temperatures up to 200°C and above. The test system must reliably measure and control these temperatures.
• Different assembly technologies: Modern SiC modules often use sintered chip attachments (silver sinter instead of solder) and copper bond wire instead of aluminum. These connection technologies have different fatigue characteristics and require adapted test parameters.
• Smaller chip areas: At equal current carrying capability, SiC chips are significantly smaller than Si-IGBTs, leading to higher local heat flux densities.
• Calibration curve stability: For SiC MOSFETs, the VCE(T) calibration curve may shift due to gate oxide aging (threshold voltage drift), affecting temperature measurement.

Our systems provide complete measurement infrastructure for standards-compliant power cycling tests:

• Load current generation: 150 A (TLW 820 LoPo) to 1,000 A (TLW 763), configurable to module requirements
• Measurement current source: up to 1,000 mA for the VCE(T) method, including automatic calibration curve determination
• Test stations: up to 36 measurement points in 3 independent strings (TLW 800), up to 20 test stations in one string (TLW 820)
• Temperature measurement: Type K thermocouples at each test station plus string temperatures
• Cooling: Individually controllable water cooling circuits per string with PT100 sensors
• Optional: Recording of cooling curves and transient Zth measurement
• Data management: PLC-controlled continuous operation with automatic recording, email notification on test interruptions, network interface for remote access
• Safety: Protective hood with safety interlock, automatic limit monitoring