Eternal Packages: Liquid Metal Flip Chip Devices

Gallium liquid metal joints are described as an alternative to higher melting point tin-or lead-based solder joints in flip chip packages. A complete assembly process is described, including the bumping process with the electrodeposition of gallium on corrosion resistant bonding pad, a low-temperature and low bonding force chip joining process with a HCl flux, and an underfill process with a low stiffness material. An analysis of the projected reliability of Ga joints is presented, using known properties of Ga in its liquid and solid states, finite element models of flip chip modules, and preliminary experimental results on test modules. While experimental data suggest that damage to Ga joints produced in their solid state can disappear when the joints melt, adjustments to the elastic modulus of the underfill material are discussed to prevent plastic deformation and potential fatigue failure of the solid Ga joints.