The main failure modes of thyristors are electrical characteristics damage, thermal stress damage, mechanical and structural damage, etc. The liquid-solid diffusion phenomenon between Al and Si at 700 °C was revealed by Al-Si liquid-solid diffusion experiment. The experimental results show that above the melting point of Al, a significant liquid-solid diffusion reaction occurs between the Al coating and the silicon chip, forming a liquid Al-Si mixture. This reaction destroys the PN structure of the silicon chip, resulting in abnormal device function. In addition, it was also observed that during the cooling process, the Si element in the Al-Si mixture precipitated in a dendritic structure without forming a new intermetallic compound, which was consistent with the prediction of the Al-Si phase diagram.