Interesting results have been achieved from the experimental works on materials that were assumed candidates for tolerant packaging of components such as power semiconductors. Properties of insulating materials and chemical compatibility between different insulating and packaging materials have been investigated. It has been demonstrated that the termination area of power semiconductor chips is vulnerable for fibers and other particles when the insulation liquid is in direct contact with the chip surface. Even though the insulating liquid was dehumidified and filtered, failures caused by very small fibers were experienced when the liquid was in direct contact with the chip surface, i.e. without the normally used protecting silicon gel. This implies that the chip and substrate interfaces need to be protected from direct contact with liquids by one or more materials with the sufficient mechanical, chemical and electrical properties for maintaining the long-term insulation quality. In the project, candidates for such additional protection has been investigated by experimental research:
- Alternatives with maintaining the silicon gel insulation normally used in industrial components
- Alternatives without silicon gel and liquid in direct contact with silicon chip
- Alternatives without silicon gel, but additional coating with polyimide or Parylene.
The chemical compatibility experiments for investigating compatibility between silicon gel and various relevant insulation liquid candidates concluded that water is very fast absorbed by the gel until the relative humidity equilibrium is reached.
The main focus for the postdoctoral work has been on studying breakdown phenomena of the selected insulation liquid candidates by applying electrical, optical and acoustic detection methods.
The student has focused on partial discharges in critical high electrical field regions of IGBT components depending on insulating materials, voltage waveform and the environmental pressure. The main conclusions from the insulation material breakdown investigations can briefly be summarized as follows:
- PD testing with sinusoidal voltage is not able to reveal defects and ageing mechanisms that may be activated by fast switching power semiconductors.
- Contamination control is essential for critical locations like chip terminations.
- Liquids have varying dielectric properties depending on molecular structure
- Barrier is required to avoid particles, if present, to move into high fields regions