2.4 Conclusions

This chapter performed MD simulations of six micro-scale contact material pairs (Cu-Cu, Ag-Ag, Au-Au, Cu-Ag, Cu-Au, and Ag-Au) for wire bonding processes, and investigated the microweld formation and breakage mechanism by the analysis of the force between the wire and substrate ( F _w-s ), atomic displacement, atomic von Mises stress, and four critical displacement points. The following conclusions were made.

(1) During the loading process, soft material pair (Ag-Ag) and stiff material pair (Cu-Cu) would generate of the largest and the smallest von Mises stress, separately.

(2) The smaller value of Z _LB-HAFI , the earlier atomic contact between atoms of the wire and substrate occurs, which implies the material pair could be easier to be bonded. Among homogeneous material pairs (Cu-Cu, Au-Au, and Ag-Ag), the order of Z _LB-HAFI value is Ag-Ag＜Au-Au＜Cu-Cu. Among heterogeneous material pairs, the order of Z _LB-HAFI value is Cu-Ag＜Ag-Au＜Cu-Au.

(3) One material (like Au) from the homogeneous material pair with small Z _LB-HAFI value could be used to form heterogeneous material pairs having improved bonding ability. It suggests to add an Au coating to the surface of Cu wire to reduce both bonding difficulty and high cost of using pure Au. varUFy3atYueAGFj05/vEQVi+rXpucbFA9wCLKKztstq2wuZrSE4vSiKqOSwdlEs