<div class="htmlview paragraph">As the car-mount electronic control devices are increasing in number and variety, power train control devices for controlling actuators such as motors and solenoids are intensively developed at great speed. The electronic circuits used in these devices contain microcomputers, LSIs and ICs that have arithmetic processing functions, and power transistors that drive the actuators, and therefore they are always accompanied by the problem of temperature rise of electronic circuit elements due to heat generation of power transistors caused by operation of the devices, which made it difficult to reduce the size of the device while guaranteeing the reliability.</div> <div class="htmlview paragraph">We have newly developed an aluminum base flexible printed wiring board, and invented the structure of releasing heat and lowering the impedance in the signal lines of the substrate, and succeeded in reduction of the volume of the power train electronic circuit device to 1/5 of the existing products by dividing the mounting of electronic components on the electronic circuit device into the heat generation region and non-heating region. As a result, the electronic circuit devices of power steering control and suspension control are made much smaller as reported in this paper.</div>