The key component of a photovoltaic inverter is its power electronics: The inverter circuit itself. This is where the DC voltage from the solar modules is converted into a grid-compatible AC voltage. That electrical energy conversion takes place in the power electronics and its components - the power transistors. This task is usually performed by so-called IGBT modules (Insulated Gate Bipolar Transistor).
Among the possible faults in an IGBT module, a basic distinction is made between chip faults and module faults. An error does not always have to take the form of a clear defect, but rather corresponds to a degradation of the component. A failure can be caused by progressive degradation.
The cross-sectional view in figure 1 shows the individual elements within the structure. The individual silicon chips are applied to the top of the DBC structure (DBC: Direct Bonded Copper, a substrate structure made of copper, ceramic and copper) by means of a chip plot and connected to each other with high-purity aluminium in the form of bond wires. The copper underside of the DBC is connected to the baseplate using the base solder. The base plate is galvanically isolated from the circuit by the oxide layer of the DBC. The baseplate is then screwed to a heatsink in the application using a thermal interface material (e.g. thermal paste) suitable for the application. The substrate structure is encapsulated with a silicone gel to protect it from the environment.
Figure 1: Cross-sectionial diagram of the IGBT module
Table 1 summarises each of the failure mechanisms and where they are located in the silicon chip or module. In the following, each of these items will be discussed individually with regard to the risks involved.
Table 1: Fault and failure mechanisms of an IGBT module [1]
Location | Failure mechanism | Localisation | Failure type |
1.1 Overvoltage | Vertical breakdown / Gate oxide | Short Ciruit / Open Circuit | |
1.2 Overheating | Vertical breakdown / Gate oxide | Short Ciruit / Open Circuit / Bond wire fatigue / Solder degradation | |
1. Chip | 1.3 Dielectric breakdown | Gate oxide | Increasing of leakage current / Reducing the breakdown voltage / Short Circuit gate and emitter |
1.4 Static discharge | Gate oxide | Short Circuit gate and emitter | |
1.5 Electrochemical migration | Metallisation of the chip surface / connections | Increasing of leakage current / Reducing the breakdown voltage | |
1.6 Metallisation reconstruction | Metallisation of the chip surface | Increase in voltage drop (and thus increase in power losses) | |
2.1 Overheating | Housung / Bond wire / DBC / Solder joints / Silicone gel | Housing breakage / bond wire fatigue / cracks in the DBC / solder degradation | |
2.2 Mechanical shock | Housung / Bond wire / DBC / Solder joints / Silicone gel | Housing breakage / bond wire fatigue / cracks in the DBC / solder degradation | |
2.3 Bond wire fatigue | Bond wire | Bond wire lift-off or bond wire breakage, change in various characteristic properties of the component | |
2. Module | 2.4 Solder degradation | Solder joints | Increasing the heat transfer resistance between the chip and the top of the DBC |
2.5 Vibration | Bond wire / Solder joints | See 2.2 and 2.3 | |
2.6 Flashover | Gate oxide / Terminals | Short circuit gate emitter / Change in the current path | |
2.7 Corrosion | Terminals / Chip / DBC top side | Changing the contact resistance at the connections | |
2.8 Ageing of the insulation | Silikone gel / DBC | Trapping of gases or water in silicone gel / Corrosion of the DBC | |
2.9 Electromigration | Bond wire / Metallisation of the chip surface | Bond wire rupture / Change in various characteristic properties of the component |
1. Chip
When we talk about the IGBT chip, we are referring to the silicon structure of the power transistor itself. The IGBT is a fusion of a MOSFET and a bipolar transistor.
Figure 2 shows the silicon structure with the individual doping layers, the contacts and the flow of electrons and electron holes of an IGBT. When a defined gate-emitter voltage is applied, an N-channel opens between the N+-doped region and the P-well, which acts as a direct connection to the N-drift region. As a result, the drift region is flooded with electrons, causing the depletion region J2 between the drift zone and the P+ injection layer to break down and allow electron holes to flow directly through. This results in the bipolar characteristic of the IGBT: there is a flow of electrons on the one hand and a flow of electron holes on the other.
Abbildung 2: Silicon structure of an IGBT
1.1. Overvoltage
There are several events that can lead to an overvoltage, which are listed below.
1.1.1. Overvoltage between collector and emitter
Input voltage surges (e.g. due to lightning)
Control signal anomalies
Voltage spikes
Unexpected load changes
During operation, a power transistor is driven by a pulse-width modulated signal, allowing the device to switch more power from the DC-link. Depending on the load, this results in a high current variation di/dt, which in turn can cause a voltage swing due to the stray inductances in the system, which together with the voltage already applied can reach beyond the device's dimensions. If the maximum allowable reverse voltage is exceeded, an avalanche breakdown may occur. A direct way to limit the current change is to use gate resistors in the control system and to set a maximum load. [2]
1.1.2. Overvoltage between gate and emitter
External voltage spikes (e.g. from stray inductance)
Unwanted oscillations in the gate circuit
If an overvoltage occurs in the control unit, i.e. between the gate and the emitter, the oxide layer of the insulated gate electrode can be damaged, which can eventually lead to the destruction of this layer. [3], [4]
1.2. Overheating
Overheating can be caused by several factors, which are discussed in the following subsections.
1.2.1. Overheating due to increased power loss
Increased heat generation can be caused by a change in switching and conduction losses. This can cause the junction temperature to rise above the devices maximum ratings and damage the chip. An increased junction temperature also leads to a change in the device characteristics as these are directly temperature dependent. [5], [6]
1.2.2. Overheating due to significant overcurrent
If an overcurrent occurs, for example due to internal or external faults (e.g. unacceptable driver events or an abnormal load), a thermal breakdown can occur. This can lead to thermal hotspots within the IGBT structure, which can cause localised damage.
This can lead to the second breakthrough [7], which can generally be divided into two phenomena:
Electronic phenomenon:
Increasing the current increases the channels volume charge density, which reduces the breakdown voltage. This leads to a further increase in current density. This process continues until the region of high current density becomes so small that a current filament is formed, leading to a rapid local temperature rise and eventually a voltage collapse. [8]
Thermal phenomenon:
As the temperature in the IGBT increases, the gate-emitter voltage required to turn on the IGBT decreases. The temperature in the N-channel is inversely related to the voltage difference between the gate-emitter voltage and the turn-off voltage: a larger difference leads to higher charge generation, which increases the current density in the channel. [4]
1.2.3. Overheating due to malfunction of heat distribution
If the IGBT power module is inadequately screwed to the heatsink, or if a poor thermal interface material is used, the thermal conductivity between the baseplate and the heatsink will be reduced, resulting in a general increase in temperature in the IGBT module itself. This can also cause the maximum junction temperature to be exceeded. [5], [6]
Effects such as the pumping out of the thermal interface material (e.g. due to an overly thick layer of thermal paste or poor attachment to the heatsink) also have an effect on thermal conductivity. [9]
1.3. Dielectric breakdown
The application of high voltage can damage the insulation material, in the case of the IGBT: the gate oxide, over time due to degradation. This may be caused by incorrect dimensioning of the driver circuit. This can reduce the breakdown voltage and increase the leakage current. Ultimately, this can lead to a defect in the gate, resulting in module failure. [3], [4]
1.4. Static discharge
A static discharge leads to a voltage pulse that can cause various faults. This often results in a flashover on the gate oxide, which destroys the functionality of the IGBT module.
1.5. Eletrochemical migration
Metal ions can migrate into insulating material under the influence of an electric field. This creates conductive filaments that lead to localised internal short circuits. These filaments can carry very high currents, resulting in local thermal hotspots. [10], [11]
This circumstances can lead to an increase in leakage current and a gradual loss of blocking capability.
1.6. Metallisation reconstruction
The forward current causes the temperature to rise, while the off-phase causes the temperature to fall. This results in thermomechanical cycles that cause temperature fluctuations. Due to the different thermal expansion coefficients of the materials used (i.e. silicon and the aluminium metallisation layer), these cycles cause mechanical stress which, under extreme conditions, can cause the metallisation to melt and solidify again. This phenomenon is called “reconstruction”. [12]
Such surface reconstruction affects current distribution and increases component voltage drop and hence power dissipation. It also increases the junction temperature, which threatens the reliable operation of the power module. [13]
2. Module
Figure 3 shows an example of a faulty IGBT module. In this case, a short circuit fault has occurred due to overvoltage, resulting in a voltage drop in one IGBT chip on the high side and one on the low side. The two faults are each indicated by a yellow circle.
Figure 3: Defective IGBT module with overvoltage damage
2.1. Overheating
Overheating can accelerate various failure mechanisms in electronic components, leading to gradual degradation or, in extreme cases, thermal runaway and immediate destruction if temperatures exceed permissible limits. The rate of degradation is proportional to the amount of overheating.
Thermal runaway in IGBT modules occurs when the heat generated exceeds the heat dissipation capacity of the cooling system, resulting in a self-reinforcing temperature rise. Causes include failure of the cooling or control system, overvoltage and overcurrent. This can lead to rapid destruction of components such as chips, bond wires and solder joints.
2.2. Mechanical shock
Sudden, strong physical shocks occur during transport, handling or operation and can also be caused by rapid temperature changes. Mechanical stress in the module can cause cracks and fractures in components such as solder joints, chips, packages or DBCs.
2.3. Bond wire fatigue
It is clear from the previous points that an IGBT module is exposed to temperature variations. These variations cause thermomechanical fatigue at junction and transition points due to the different coefficients of thermal expansion. This also applies, for example, to the bonding wires: Bond wires can lift due to shear effects, leading to OC behaviour at this point. [4], [14]
However, lifting one bond wire causes an asymmetrical load distribution to the other bond wires. This increases the temperature and the thermomechanical effect is increased, leading to even faster bond wire fatigue. This results in a positive feedback mechanism of accelerated bond wire fatigue. [15]
2.4. Solder degradation
There are two solder layers in an IGBT module, which are subject to mechanical stress due to the different thermal expansion coefficients of the materials:
Chip solder (connection between the Silicon chip and the top of the DBC)
Base solder (connection between the bottom of the DBC to the base plate)
The solder material is polycrystalline in its solidified state and has grain boundaries. The individual solder grains expand and contract as a result of thermomechanical cycles, which can lead to small voids (microvoids) at the grain boundaries. If this process is repeated, micro-cracks and eventually macro-cracks can form. This can result in localised thermal hotspots, which can lead to an increase in junction temperature. Ultimately, there is a risk of delamination of the solder layer, which severely compromises thermal conductivity and contact. [16], [17]
2.5. Vibration
Vibration can cause mechanical fatigue that damages bond wires, solder joints and semiconductor chips. This leads to cracking, delamination and degradation of electrical function and structural stability of the module. Vibration can also exacerbate existing defects and affect thermal management through misalignment. Resonant vibrations further exacerbate the damage.
2.6. Flashover
Inadequate protection of the inverter hardware from the environment, especially in humid environments, can lead to the accumulation of salt traces and other conductive deposits on the IGBT modules and gate driver circuit boards. These deposits can cause electrical arcing and affect the operation of the driver, resulting in unexpected malfunctions for which the device is not designed.
2.7. Corrosion
In environments with high humidity and salt spray, corrosion on the outside of the module is a significant failure factor. The metal interfaces exposed to these conditions are the weakest link and their corrosion can directly affect the electrical properties of the module. Corrosion by-products can enter the interior of the module through moisture and react with the components, leading to long-term deterioration. The corrosion process is highly dependent on the materials and impurities used. [18]
2.8. Ageing of the insulation
Aging of insulation materials in IGBT modules can occur over time due to various influences and contribute to device failure. These materials are critical to ensure electrical isolation between components and prevent unwanted electrical connections. Aging is caused by a combination of thermal, electrical and environmental stresses. For example, partial discharges at the ceramic/metal interface can cause small discharges at the DBC and larger discharges at the metal edges. This leads to decomposition of the silicone gel into gaseous products, which reduces the insulating capacity. Moisture has a particularly negative effect on the insulating properties of materials such as silicone gel, especially at high temperatures and under prolonged stress. [1]
Prolonged storage of IGBT modules, especially under extreme temperature or humidity conditions, can also significantly reduce their lifetime. Even when the modules are not in use, these conditions can cause thermal stress, material degradation, corrosion and moisture damage that can affect the performance and reliability of the modules.
2.9. Electromigration
Electromigration is the phenomenon of material transport caused by an electric field. As a result, metal atoms (e.g. aluminium anions) are moved by the electron wind induced by the current flow. This material transport leads to progressive thinning and the resulting rupture of bond wires, for example. The phenomenon of electromigration can contribute to bond wire fatigue. [10], [11]
3. Conclusion
Of course, there are also interactions between the individual failure modes and their effects on the power transistor. For example, solder degradation leads to poorer heat transfer, which affects thermal integrity and increases the likelihood of bond wire fatigue.
The biggest accelerator of power semiconductor degradation is temperature. And this is where users and operators can play a role:
Selecting a suitable location for the inverter (or inverter station) with regard to temperature, humidity and other environmental influences
Adequate cooling of the inverter
Respecting maintenance intervals (cleaning the heat sink after green keeping, checking the operation of the fans, replacing the filter mats for central inverters, checking the pressure and volume of water-cooled systems, ...)
PV system design (what is the overload? Does the inverter regularly reach or exceed its load limits?)
With an general overhaul from Eternus Technology GmbH, you can be sure that a thermal interface material suitable for the application is used and that components suspected of accelerated ageing are replaced. We use components that have extended life limits and are therefore more reliable.
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