Early fatigue failures in copper wire bonds inside packages with low CTE green mold compounds

This paper reports a second reliability concern related to the introduction of the low-CTE green mold compounds in advanced IC packaging. The increased mismatch between the CTE´s of copper wire and mold compound causes high mechanical stresses in the copper wire bonds during temperature cycling tests. The repeated plastic deformation in each temperature cycle results in fatigue cracking of the copper wire bond. This paper gives an explanation for this new failure using thermo-mechanical finite element modelling. It shows that only under the combination of copper wire and low CTE overmold, the stresses become high enough in the wire in order to get plastic deformation, which finally leads to low cycle fatigue cracking.