The invention provides a semiconductor assembly, a semiconductor device and a method for forming the semiconductor device. The semiconductor device includes a pad region located on the semiconductor substrate; a bump structure disposed over the pad region and electrically connected with the pad region. The bump structure includes a copper layer and a lead-free solder layer over the copper layer. The lead-free solder layer is a SnAg layer, and the Ag content in the SnAg layer is less than 1.6 weight percent. When the Ag content in the lead-free bump, the hardness of the bump will be reduced. Asofter bump could remove the crack owing to the heat stress.