Abstract: In a package of a semiconductor device mounted a semiconductor element and micro balls as electric contact points, the micro balls are composed of composite micro balls each of which has a core ball and an electrically conductive film around the core ball. The core balls have a sufficient rolling property with the diameter ranged from 30 to 100 &mgr;m. The diameter accuracy thereof is excellent. The electrically conductive film is formed by a solder plated layer which has a thickness of at least 10 &mgr;m uniformly formed on the outer surfaces. The composite micro balls are mounted onto a substrate. The dimensional accuracy of the Z axis of the package can be precisely controlled. Accordingly, the composite micro balls with the solder plated film has a thickness of at least 20 &mgr;m on the outer circumference of each core ball. The production method thereof, and the semiconductor package mounted a semiconductor element using the composite micro balls can be provided.