An array of solder balls is attached to solder pads of one of a first substrate and a second substrate. After aligning the array of solder balls relative to solder pads of the other of the first substrate and the second substrate, a thermal-mass-increasing fixture is placed on a surface of the second substrate to form an assembly of the first substrate, the second substrate, and the array of the solder balls therebetween, and the thermal-mass-increasing fixture. The thermal-mass-increasing fixture is in physical contact with at least a surface of a periphery of the second substrate. The thermal-mass-increasing fixture reduces the cool-down rate of peripheral solder balls after a reflow step, thereby increasing time for deformation of peripheral solder balls during the cool-down and reducing the mechanical stress on the solder balls after the cool-down.