A strong crystallographic orientation relationship between the Cu6Sn5 scallop-type grains and their Cu substrate has been found by synchrotron micro-X-ray diffraction study. Even though the crystal structures of Cu6Sn5 (monoclinic) and Cu (face-centered-cubic) are very different, angular distributions of crystallographic directions between Cu6Sn5 and Cu revealed a strong orientation relationship. Both SnPb solder and pure Sn showed the same result, indicating that this is general behavior between Sn-based solders and Cu. The strong orientation relation suggests that Cu6Sn5 forms prior to Cu3Sn in the wetting reactions. A total of six different orientation relationships were found. In all the cases, the [1 01] direction of Cu6Sn5 preferred to be parallel to the  direction of Cu with a misfit of 0.24percent. Due to pseudohexagonal structure of the Cu6Sn5 , the six relationships can be categorized into two groups. From the orientation distribution, one group was found to be less rigid then the other group. (18 References).