Printed board unit for optical transmission and mounting method

A method of mounting a semiconductor element for light transmission consisting of light emitting or light receiving element, said method comprising the following steps of:

• mounting a light emitting or light receiving element on a sub-mount chip and also mounting solder bumps on respective contact pads of the sub-mount chip;
• preparing an optical wave-guide substrate provided with a post at a position corresponding to a position where the light emitting or light receiving element is joined;
• arranging the sub-mount chip on the optical wave-guide substrate in such a manner that the light emitting or light receiving element faces toward the optical wave-guide substrate;
• joining the light emitting or light receiving element to the optical wave-guide substrate by a reflow process so that the sub-mount chip is joined to optical wave-guide substrate by means of solder balls and the post is joined to the light emitting or light receiving element so as to electrically connect, and regulate the interval between, the light emitting or light receiving element and the optical wave-guide substrate.