A theoretical study to improve wire sag of ultra-long wire bond loops for 3D/MCM packaging.

An innovative theoretical model by altering cross section of gold wire is proposed to reinforce the wire sag stiffness of a wire bond that suffered low yield for the applications of 3-dimensional and multi-chip module (MCM) packaging. The flexural rigidity of a wire bond is found to be a function of the material properties in the moment of inertia of a bond wire. Suffice to say, if the moment of inertia of the bond wire can be raised, the associated sag deflection will be diminished. By manipulating the moment of inertia of the bond wire, the sag deflection of the wire bond can be regulated appropriately to avoid wire sag and even the wire sweep enigma. The ellipse-like cross section of a wire bond is applied as a numerical example to improve the wire sweep stiffness or wire sag stiffness in this study. The results show the sag deflection of a wire bond can be reduced significantly if the ellipse-like cross section is oriented in the preferred direction of flexural rigidity of a wire bond. Furthermore, a systematic study of matching design is performed to control wire sag and wire sweep for the wire bonding applications of overhang cross-stacked and step-stacked packaging. It shows that to use a single diameter of bonding wire in these complicate bonding environments of overhang stacked configurations is a favored possibility. The advantages shown from the theoretical study need to be implemented in further work in a production environment on current wire bonding equipment.