In the previous article the question of Disadvantages of Power Semiconductor Modules with Standard Design was raised. Today we continue to investigate this matter.

Influence of the Environment

For a series of standard industrial modules, there is no significant work to reduce the risk of negative environmental impact, since these modules meet the corresponding standard industrial requirements, and the end user understands that.

Additional protection measures are relevant for products that are used in special equipment, i.e., modules with increased reliability for railway transportation, power grid, aviation, space, military equipment, etc.

Insufficient Thermodynamic Stability

The insufficient thermodynamic stability of the modules of the standard design is very typical for them and cannot be significantly improved.

A solution to the issue to ensure thermodynamic stability is possible only within the framework of other structural and technological concepts, for example, for hermetically sealed pressure contact modules.

Ineffective Pressure Contact

The problem is relevant for high-current modules with a large baseplate area. To ensure the minimum thermal resistance of the case/heat sink contact, this baseplate must be pressed against the heat sink with a force evenly distributed over the area.

To ensure the clamping force in the middle of the baseplate, profiled baseplates are used, which in the unpressed state have some bulge in the middle.

Some companies use the following solution to achieve guaranteed uniform heat removal from the baseplate: the baseplate of the module has a profiled pressure contact surface in the form of a regular grid of cylinders, so called pin fin cooling technique.

Minimizing the Internal Inductance of the Wiring

The problem is quite relevant, especially for high-current modules, since the internal inductance largely determines the area of safe operation of the module, and there is no space for the damping circuits inside the housing.

The general approach to minimizing internal inductance includes the following measures:

  • minimization of the busbar length of the internal wiring;
  • increasing the width of the busbars;
  • using paired busbars in the form of low-inductive strips;
  • symmetrization of the current path inside the module, so that the parasitic circuits are directed counter to the magnetic flux.

Optimization of the Thermal Resistance of the Junction to Case

The thermal resistance of the junction to case in the module is determined by the thermal resistances of all layers in the package from the semiconductor to the baseplate, as well as the ratio of the areas of silicon chips in the module and the area of its baseplate. According to the importance of the possible contribution to the total thermal resistance, the following ranking can be proposed:

  • Quality of solder ceramics/case, chip/ceramics

The quality of solder is one of the most important problems that determine the quality and reliability of the module of the standard design as a whole. It is necessary to ensure a solid connection without non-soldered voids and bubbles. The matter is also very important because the presence in the junction of voids with linear dimensions of a fraction of a millimeter, occupying an insignificant fraction of the area, can lead to a noticeable local overheating of the semiconductor structure, and therefore greatly reduce the reliability and overload resistance of the entire module.

  • Material and thickness of the ceramic substrate

In almost all versions of the modules, the isolation of the power semiconductor from the heat sink is realized using a ceramic substrate. The most common ceramics are Al₂O₃. The thickness of it in recent years for use in modules at voltages up to 1700 V has been reduced from 0.63 to 0.38 mm to reduce the thermal resistance of the junction/case. With this, power semiconductor module manufacturers also partially managed to compensate for the increase in thermal resistance caused by a decrease in the area of new generations of chips.

A significant effect is provided by the replacement of the ceramic material. AlN ceramics are very promising, which has a fairly close coefficient of the linear thermal expansion to silicon and significantly exceed Al₂O₃ in thermal conductivity. Its use is hindered by a higher price, however, the application will likely expand, as it reduces the thermal resistance of the module by 20-30%.

Alternative materials like doped with zirconium Al₂O₃ and Si₃N₄ at a thickness of just 0.32 mm and copper layers up to 0.6 mm lead to better properties compared with standard Al₂O₃ ceramics. For relatively low-voltage applications up to 1200 V, hybrid insulating substrates consisting of molybdenum or tungsten with a thin layer of AlN are considered to be promising.

Insulating substrates based on a polycrystalline diamond are very promising as well. This material has a coefficient of linear thermal expansion close to silicon and the thermal conductivity is about 1.5 times higher than copper. This justifies the use of relatively thick substrates, since the intense lateral heat flow, in a certain thickness range, an increase in the thickness of the diamond substrate leads to a decrease in the overall thermal resistance.

At the same time, it becomes justified to use chips of relatively small sizes, with minimized losses.

The use of diamond substrates in high-voltage modules is advantageous. The increased thickness of the board is also justified by an increase in the insulation voltage. The only limiting factor is the very high price of polycrystalline diamond substrates.

  • Material and thickness of the baseplate

The baseplate, made of copper with a thickness of about 3 mm, makes a small contribution to the overall thermal resistance but increases the heat capacity and heat spreading under the chip and slightly reduces the transient thermal resistance in a time period from 0.1 to 1 second.

Alternative composite materials have about half the thermal conductivity, as well as a lower heat capacity, so the contribution of such baseplates to the overall thermal resistance becomes noticeable, especially in transient conditions.

  • Ratio of the area of the chips and the baseplate

Reducing the total area of the chips reduces the cost of the module, but the technical characteristics including thermal resistance, as a rule, deteriorate.

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