Problem Description
The assumption of new functionality of electronics will increase the number of faults with perhaps unanticipated fault modes. In addition,the move toward lead-free electronics and MEMS will further result in unknown behavior. It is therefore necessary to gain an understanding of the behavior of deteriorated components. Different environmental and operational conditions from conditions typically seen in ground-based applications may impact the deterioration in different ways. Anecdotal information reports that, for example, the mounting direction of circuit boards has an impact on the frequency of intermittent faults since they are flexing in a particular way when subject to accelerations during take-off, causing temporary disconnects. Some of the conditions, besides high acceleration, that may impact proper operation includes operation in extreme temperatures and temperature cycling, vibrations, humidity, power cycling, radiation, and a combination thereof.
Figure 1: IGBT with extreme damage
Common Extrinsic IGBT Faults
Extrinsic faults are faults that are associated with the external components of the die such as packaging, wires and solder.
Table 1: Extrinsic IGBT faults
| Fault |
Cause |
Effect |
| Wire Lift |
-CTE mismatching
-Bond looping and lagging
-Dendrite growth
-Wire sweep
-Bonding pressure
-Metal electro-migration
|
-Increase in wire resistances
-Wire detaches from package / die causing an open circuit
-Short circuit between bonds due to metal migration or dendrite growth
|
| Die/Package Solder Degradation |
-CTE mismatch between die and package
-Thermal cycling of solder crystalline structure |
-Voids formed in die/package interface
-Increase in internal temperature
-Thermal degradation |
Possible Prognostic Signatures for Wire Lift
Degradation Wire Lift - Degradation wire lift would likely be seen as an increase in resistance across the Gate or Collector/Emitter junction.
An increase in resistance across the gate is directly measurable by holding a constant Vce (Collector/emitter voltage), Vge (Gate emitter voltage) and measuring the Ice (Collector/Emitter current). Internal temperature of the silicon die has a significant effect on the internal resistance of the collector/emitter channel, thus a correction must be applied depending on the internal temperature of the IGBT. This internal temperature must be explicitly calculated, as a measurement made using the package temperature does not account for solder/die bond degradation. Measuring an increase in gate wire resistance proves more difficult, as the gate has complex impedance.
Common Intrinsic IGBT Faults
Intrinsic faults are the faults that are inherent to the operation of the device.
These faults independent of the device packing and usually have a cause rooted in the semiconductor materials or underlying physics of the device.
Table 2: Intrinsic IGBT Faults
| Fault |
Cause |
Effect |
Physics |
| Thermal Runaway |
-Overheating of the IGBT die
|
-Short circuit
-Smoke and fire
|
-Negative resistance/temperature coefficient
|
| Latch Up |
-Heat degraded silicon
-Lowering of parasitic SCR holding voltage
|
-IGBT stuck in on-state
|
-Parasitic NPN BJT forms a SCR with PNP BJT
|
| Time Dependent Dielectric Gate Breakdown/
Acute gate Breakdown |
-Chronical electrical overstress
-Thermal cycles
-Transient voltage spikes
-Manufacturing defects
|
-Decrease in gate speed
-Eventual loss of gate control
-Increase in leakage current
-Change in Vg threshold
|
-Doping concentration, work function, gate oxide capacitance
|
|
Collector/emitter degradation |
-Electrostatic discharge
-Electrical overstress
-Snappy diodes
-Contact migration
-Thermal degradation
|
-Increase in R-on
-Decrease R-off
-Slow I-rise/fall
|
-Dielectric breakdown of depletion region
-Doped silicon gains energy above activation energy
|
| Piping/Contact Migration |
-Local hot spots |
-Increase in ice leakage current
|
-Defects in depletion region (change in doping)
-Metal/Si alloy from heat
|
