A device includes a package substrate including a first non-reflowable metal bump extending over a top surface of the package substrate; a die over and bonded to the package substrate; and a package component over the die and bonded to the package substrate. The package component includes a second non-reflowable metal bump extending below a bottom surface of the package component. The package component is selected from the group consisting essentially of a device die, an additional package substrate, and combinations thereof. A solder bump bonds the first non-reflowable metal bump to the second non-reflowable metal bump.