Samples 1, 2, and 3 were prepared such that each of them included a ceramic board and a plurality of the electrode pads 4 formed on the ceramic board. Each of the electrode pads 4 was formed according to the first embodiment as shown in FIG. 1, and was sized such that: a length of each side of the electrode pad 4 in planar view was 920 μm; a thickness of the electrode pad 4 was 16 μm; the interval D between the through holes 4A was 20 μm; and the width of the central part 41 was 350 μm.

Each of the through holes 4A was formed to have sides of: 25 μm in the sample 1; 21 μm in the sample 2; and 17 μm in the sample 3. The samples 1, 2, and 3 were heated at 330° C. and checked for the bulge in the electrode pad 4. As a result, none of the samples included the bulge in the electrode pad 4.

Samples 4, 5, and 6 were prepared such that each of them included a ceramic board and a plurality of the electrode pads 4 formed on the ceramic board, wherein each of the electrode pads 4 was same in size with that of the example 1, except for the thickness of the electrode pads 4 which was set to 5 μm.

Each of the through holes 4A was formed to have sides of: 25 μm in the sample 4; 21 μm in the sample 5; and 17 μm in the sample 6. The samples 4, 5, and 6 were heated at 330° C. and checked for the bulge in the electrode pad 4. As a result, none of the samples included the bulge in the electrode pad 4.