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.

Eternal Packages: Liquid Metal Flip Chip Devices

Some aspects of the plastic deformation of α-Ga deduced from slip line observations or X-ray topography cannot be explained using a simple criterion based on the specific energy of the dislocations. In particular, glide systems in which the dislocations, which are assumed to be perfect, have a specific energy higher than those of other systems which are not activated by the same resolved shear stress are often observed. Our initial transmission electron microscopy observations provide additional information which allows us to propose an explanation for this behaviour.

N. Durbec

Papers

Glide systems in gallium: justification by dislocation specific energies