1. IGBT Module Structure
An IGBT module mainly consists of several IGBT chips connected in parallel, with the chips electrically connected to each other via aluminum wires. In a standard IGBT package, a freewheeling diode is also connected in parallel with each IGBT. Then, a large amount of silicone gel is poured on top of the chip, and finally it is encapsulated in a plastic shell. The stacked structure of the IGBT unit is shown in Figure 1-1.
From top to bottom, it consists of three parts: the chip, the DBC (Directed Bonding Copper), and a metal heat sink (usually copper). The DBC is made of three layers: the top and bottom layers are metal layers, and the middle layer is an insulating ceramic layer. Compared to ceramic substrates, DBCs offer superior performance: they are lighter, have better thermal conductivity, and are more reliable.
2. Failure Mechanism of IGBT Packaging
The reliability of power devices refers to their ability to perform their intended functions under specified conditions, usually expressed as lifespan. Since semiconductor devices are primarily used for current switching, they generate significant power losses; therefore, thermal management of power electronic systems has become a top priority in design. During the operation of power electronic devices, thermal issues must be addressed first, including steady-state temperature, temperature cycling, temperature gradients, and the matching of packaging materials with operating temperatures.
Because IGBTs employ multilayer packaging technology, this not only increases packaging density but also shortens the interconnect length between chips, thereby improving device operating speed. However, this very structure has raised questions about the reliability of IGBTs. It's easy to imagine that IGBT module package-level failures mainly occur at bonding wire connections, chip solder joints, substrate solder joints, and on the substrate itself.
During typical power or temperature cycles, the chip, solder layer, substrate, baseplate, and package all experience varying degrees of temperature and temperature gradients. The coefficient of thermal expansion (CTE) is a crucial material performance indicator, representing the ratio of the increase in linear dimension to its length at 0 degrees Celsius for every 1-degree Celsius increase in temperature within a given temperature range. Due to the different CTEs of various materials, the thermal strain between them differs with temperature changes, leading to thermal stress fatigue losses at the joints between interconnecting layers. Therefore, the thermal behavior of a device is closely related to the structure of the module package. Studies show that for every 10°C increase in operating temperature, the temperature-induced failure rate doubles.
Detachment of aluminum-bonded wires
The diameter of the aluminum bonding wires within IGBTs is typically 300-500µm, and their chemical composition varies depending on the manufacturer. However, in almost all cases, adding one-thousandth of an alloy, such as a silicon-magnesium or silicon-nickel alloy, to pure aluminum significantly increases its hardness, thus controlling corrosion resistance. The current capacity of the bonding wires decreases due to their disproportionate length and slight dependence on substrate temperature. The maximum DC current is limited by melting caused by the ohmic thermal effect of the wires themselves. Since the aluminum bonding wires are directly connected to the chip or pressure buffer, they are subject to significant temperature variations. Because IGBT modules are constructed from materials with different coefficients of thermal expansion, significant thermal fatigue is inevitable during operation. This fatigue becomes increasingly pronounced over time due to the ohmic effect of the wires, eventually leading to cracks at the root of the bonding wires.
In thermal cycling tests, the remodeling of aluminum conductors causes periodic compression and tension on the bonding surface due to the mismatch in thermal expansion coefficients, an effect far exceeding the material's inherent expansion and contraction limits. Under these conditions, pressure is released through various mechanisms, such as diffusion creep, particle sliding, and misalignment. The remodeling of aluminum leads to a reduction in the effective contact area, resulting in an increase in sheet resistance. This explains why Vce increases linearly with periodic testing.
Solder fatigue and solder voids
Cracks in the solder layer between the chip and the substrate due to differences in thermal expansion coefficients increase the contact resistance of the wires. This increased resistance amplifies the Ohmic effect, creating a positive temperature feedback loop that exacerbates the cracking and ultimately leads to device failure. Voids within the solder layer affect thermal cycling, reducing the device's heat dissipation and further accelerating temperature rise, thus speeding up module damage. Furthermore, the hysteresis phenomenon between stress and strain, where the material's shape changes continuously during temperature cycling, increases thermal fatigue in the solder. Additionally, voids introduced into the solder due to manufacturing processes can disrupt thermal cycling during operation, causing localized overheating, another significant cause of module failure.
Wafer and ceramic cracks
In a seven-layer IGBT structure, the mismatch in thermal expansion coefficients can cause significant mechanical stress in each layer. Under temperature variations, the deformation of each layer differs, and even within the same layer, different parts can deform to varying degrees due to temperature differences. This inevitably leads to excessive localized stress, which can cause the material to crack.
