Cu core ball, solder joint, solder paste and formed solder

A Cu core ball comprising:

• a Cu ball, and
• one or more metal layer for covering a surface of the Cu ball, each layer including one or more element selected from a group of Ni, Co, Fe and Pd,
• wherein the Cu ball contains at least one element selected from a group of Fe, Ag and Ni in a total amount of 5.0 ppm by mass or more to 50.0 ppm by mass or lower, S in an amount of 0 ppm by mass or more to 1.0 ppm by mass or lower, P in an amount of 0 ppm by mass or more to less than 1.0 ppm by mass, and remainder of Cu and inevitable impurities,
• wherein the Cu ball contains purity which is 99.995% by mass or higher to 99.9995% by mass or lower, and sphericity which is 0.95 or higher, and
• wherein the Cu ball contains a diameter of 1 μm or more to 1000 μm or lower.