Alternatively, simultaneously with the etching, an uneven portion may be formed on the surface (the upper surface) of the metal substrate 10 on which the precoat 60 is formed. In other words, the etching and the formation of the uneven portion may be simultaneously performed by irradiating laser light once.

The pattern 50 or the uneven portion is formed by irradiating laser light in the shape of the edge of the metal layer 30 (for example, in a rectangular shape) and then irradiating laser light in the same shape at a smaller size. In other words, the pattern 50 or the uneven portion is formed by irradiating laser light in the shape of a closed curve at a gradually reducing size.

Then, the metal layer 30 is formed in the pattern 50. In this way, the metal layer 30 is formed on the upper surface of the metal substrate 10 on which the precoat 60 is formed. Accordingly, the uneven portion is formed on the surface on which the metal layer 30 is formed. Thus, the metal layer 30 is effectively bonded to the metal substrate 10.

The metal layer 30 thus formed is made of a conductive material and serves as a bump. The metal layer 30 is formed on the surface of the metal substrate 10 so as to protrude upward at a predetermined height.

As shown in FIGS. 5 and 6, an uneven portion may be formed on the upper surface of the metal layer 30 in a shape corresponding to the shape of the uneven portion formed on the metal substrate 10. Due to the existence of the uneven portion formed on the metal substrate 10, the uneven portion on the upper surface of the metal layer 30 is formed without having to perform a separate process.