Characterization and simulation of LTCC/adhesive and alloy 42/adhesive interface strength for automotive applications

Thermoset-based adhesives are used as thermal and electrical interfaces. In automotive applications, they are required to have excellent adhesion since delamination may precipitate other electrical, thermal or mechanical failure mechanisms. A vast amount of literature is available on the investigation of molding compounds and various material interfaces. However, only very few studies focus on delamination of adhesive interfaces. The reason is that apparently it was not possible to initiate an interface crack in a delamination sample. In various attempts, random cracking in the adhesive was obtained instead. Yet interface cracks are found in real products and really form a reliability issue. But so far the absence of adequate interface strength data makes it hardly possible to design for reliability of products with adhesive interfaces. The present paper solves the above problem. We succeeded to get an interface delamination between the adhesive and two different materials (e.g. Low temperature cofrred ceramic (L TCC) and alloy 42). The specimens are made by identical fabrications processes as during the fabrication of the electronic control unit under study. The interface to be investigated is preconditioned for delamination initiation, by adding a single step to the fabrication process, thus enabling the investigation of different interfaces that have the same processing conditions as the real product. The presented specimen preparation method and the testing methodology can be used for determination of critical adhesion properties of different interfaces (including brittle materials like L TCC) in electronic control units. Specimens are investigated by delamination experiments near Mode-I loading conditions at room temperature. The obtained interface data is interpreted via image processing and finite element modeling of the J-integral method. In particular, cohesive zone modeling is used to validate the critical energy release rates for different interfaces.