The main disadvantages or problems of the power semiconductor modules with standard design can be divided into:
- cycling resistance due to the large difference in coefficients of the linear thermal expansion of baseplate/ceramics, ceramics/chip, ceramics/terminals, as well as insufficient resistance of the aluminum wire in places of contact welding;
- environmental exposure, especially during long-term operation, since the protection of chips is carried out by compound only;
- insufficient thermodynamic stability and fire safety; in the event of an accident (short circuit of a chip or a group of chips), aluminum wires burn out, an electric arc occurs, and with sufficient energy, the burnout of the compound leads to a gas-dynamic shock and the arc expanding outside the housing, often accompanied by the destruction of the plastic housing of the module; this can also lead to ignition of the power converter system;
- ineffective pressure contact of the baseplate to the heat sink for high current modules with a large area baseplate;
- minimizing the internal inductance of the wiring in the module;
- optimization of the thermal resistance of the junction to case.
Let’s take a closer look at the first problem – cycling resistance.
The most unreliable elements of the structure are marked with numbers during cycling. The figure at the same time means the priority of the problem for the original design of the module with a copper baseplate, an aluminum oxide ceramic, a wire bonding system of 200-300 microns thin of aluminum wires, and a soldered contact of the terminal.
To solve the most acute problem of insufficient cycling resistance of the baseplate/ceramics junction, composite baseplate materials have been developed that replace copper and have high thermal conductivity and a coefficient of the linear thermal expansion closer to ceramics. The most widely used composites are aluminum and silicon carbide AlSiC, however, copper-molybdenum composites CuMo are also used. Cheaper graphite-based composites are also considered to be very promising.
Recently, an original solution was proposed that allows using aluminum as a baseplate material. At the same time, the technology of direct splicing of an aluminum baseplate and non-metalized ceramics from below is applied. The upper metallization on ceramics is also made of aluminum, on which chips are soldered. Switching to a new module design triples the resistance of modules to thermal cycles, and replacing copper with aluminum reduces the total weight of the module by one-third.
The most radical way to solve the problem is to abandon the baseplate at all and ensure the pressure contact of the ceramic substrate directly to the heat sink.
To increase the cycling resistance of the chip/ceramics junction, the following measures are taken:
- replacement of aluminum oxide Al₂O₃ ceramics with a more consistent with silicon in terms of the coefficient of the linear thermal expansion aluminum nitride AlN;
- optimization of solder properties, use of more robust and high-temperature solders;
- optimization of solder layer thickness;
- replacement of the soldering operation with low-temperature diffusion welding.
Insufficient cycling resistance of aluminum wire bonds in the module, after carrying out the above-described optimizing structural and technological improvements in terms of baseplate/ceramics and ceramics/chip junctions, is a factor limiting the power cycling of the entire module.
Despite the small area of the welded contact, it is under a very severe cycling effect. In this case, a relatively long heating cycle of the entire module and significantly shorter, but repeated with the clock frequency of the converter, cycles of temperature changes of the structure are summed up.
The situation is aggravated by the negative impact on the junction of the high-density current, which leads to the movement of aluminum atoms under the combined influence of the electron flow and electric field, and, as a result, the formation of cavities and microcracks in the contact zone.
These effects can lead to two types of defects:
- peeling of the wire from the contact surface, i.e., destruction of the junction itself;
- cracking and breaking of the wire near the junction, i.e., destruction of the wire.
Studies conducted by the leading manufacturers of power semiconductor devices show that the first type of defect occurs after 20k-30k thermal cycles with a temperature range of 100 ° C, the second after 50k-70k cycles.
Ways to deal with it:
- minimizing overheating of the silicon by reducing the transient thermal resistance of the junction to case;
- optimization of welding;
- bonding of each wire with several welded contacts to the contact pad.
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