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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 process for a highly reliable ceramic substrate with electric circuit has been developed for power devices of automotive applications. The circuit conductor has high conductivity to accept a current of 10 to 200 Amperes, and has strong joining strength to ceramic substrates. The conductor is made of sintered copper powders with several kinds of diameters to get a porous structure, for reducing Young's modulus and thermal stress between the circuit conductor and ceramic substrate. Sintering shrinkage of the circuit conductor has been restrained, and joining strength to the ceramic substrates kept strong by an anchor effect. The copper circuit conductor realized electric resistivity of 2.3×10-6 ohm-cm, and coefficient of thermal conductivity, 290 W/mK. We have also confirmed superior reliabilities in the joining strength and conductivity by thermal cycling tests.

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.

Alumina substrates have been developed for a flip-chip mount ECU ( Electronic Control Unit ). On this substrate, large sized IC chips as a flip-chip and other electronic components have been mounted. Cu thick film circuit has also been mounted on the other surface. This circuit consists of printed resistors by connecting tungsten metallized layer and Cu thick film through Cu plating. In order to mount the large sized flip-chip IC, the substrate needs strictly precise dimensions. The tolerance of the dimensions are within ± 0.1 % and the warp of the flip-chip area is less than 10 μm per 10 mm.