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Reliability of electronic components assembly mounting technologies used on multilayer alumina substrates has been investigated for Electronic Control Units (ECU's), specifically for use in harsh locations, such as under the hood or directly attached to the transmissions of automobiles. The technology mainly consists of lead-free solder and bare chip assembly. Reliability evaluation tests have been performed in a practical, severe environment, similar to under the hood of automobiles. The experiments mainly consist of thermal cycling and corrosion tests. Computer simulation techniques have also been conducted in order to obtain accurate prediction of product lifetime. The estimated lifetimes of the lead-free solders agree fairly well with the experimental results. Through these studies, ECU's with higher reliability have been applied to automatic transmissions of commercial passenger vehicles.

A new alumina ceramic substrate has been developed for an Electronic Control Unit (ECU). This substrate has a thick conductor circuit made of molybdenum for a power line, accepting high electric current up to 50 Amperes (standard design). The design concept can integrate control and power circuits in one substrate, directly mountable on engines or power train system without any forced cooling system. Cross-sectional shape of the conductor is designed to minimize thermal stress and reduce cracking in the substrate, caused by a thermal expansion mismatch between alumina and molybdenum. Optimization of the design is performed by FEM analyses and X-ray diffraction measurement. We confirmed that the substrate can work after 10,000 cycles of temperature cycle test, and 600,000 cycles of electric cycle test.