Assembly issues in replacing a high cost hermetically sealed hybridwith a low cost non-hermetic assembly for prolonged use at180° C

摘要

By directly mounting automotive electronics on the engine, it ispossible to reduce the wiring harness length, improve sensor responsetimes and contain electrical noise within the engine compartment.However, engine compartments are aggressive environments, and reliablecircuit construction and packaging costs can be high. In this work,control circuits were constructed using conventional high reliabilitymethods, consisting of bare die that were epoxy mounted, wire bonded andinterconnected using Au thick-film ink, and then hermetically sealed ina gold-plated Kovar package. To reduce circuit cost, the circuit wasredesigned using an Ag thick-film interconnect system and nonhermeticplastic packaging. This circuit was then evaluated with respect toassembly and qualification of the materials used in the substrate andinterconnects, component assembly, wire bonding and encapsulation. Thecircuit was subjected to ageing at 180° C for 1000 hours, temperaturecycling from -65° C to 150° C for 100 cycles, 85° C/85%humidity tests and thermal shock testing. The Ag thick-film circuitsshowed a 50% decrease in conductor to substrate adhesion strength afterageing. After a 1000 hour humidity test, vertical Ag migration betweentracks separated by 50 μm of dielectric caused 2% of circuits tofail. Component and production methods were found to cause circuitfailures after ageing at 180° C for 1000 hours. This work shows thatit is possible to manufacture Ag thick-film, low cost circuits fordirect engine mount applications. However, there are risks with thistype of circuit and recommendations are made on the construction andmaterials required