Assembly issues in replacing a high cost hermetically sealed hybrid with a low cost non-hermetic assembly for prolonged use at 180°C

By directly mounting automotive electronics on the engine, it is possible to reduce the wiring harness length, improve sensor response times and contain electrical noise within the engine compartment. However, engine compartments are aggressive environments, and reliable circuit construction and packaging costs can be high. In this work, control circuits were constructed using conventional high reliability methods, consisting of bare die that were epoxy mounted, wire bonded and interconnected using Au thick-film ink, and then hermetically sealed in a gold-plated Kovar package. To reduce circuit cost, the circuit was redesigned using an Ag thick-film interconnect system and nonhermetic plastic packaging. This circuit was then evaluated with respect to assembly and qualification of the materials used in the substrate and interconnects, component assembly, wire bonding and encapsulation. The circuit was subjected to ageing at 180°C for 1000 hours, temperature cycling from -65°C to 150°C for 100 cycles, 85°C/85% humidity tests and thermal shock testing. The Ag thick-film circuits showed a 50% decrease in conductor to substrate adhesion strength after ageing. After a 1000 hour humidity test, vertical Ag migration between tracks separated by 50 μm of dielectric caused 2% of circuits to fail. Component and production methods were found to cause circuit failures after ageing at 180°C for 1000 hours. This work shows that it is possible to manufacture Ag thick-film, low cost circuits for direct engine mount applications. However, there are risks with this type of circuit and recommendations are made on the construction and materials required