Finite element analysis of wire bonding non-stick issue in miniaturized package development

Wire bonding failures, including non-stick on die pad (bond-off) and smash ball of the first bond, are often encountered in manufacturing of miniaturized packages with increased feature density and higher UPH requirement. Many researches have been conducted with an attempt to solve these issues. However, these studies mainly focused on the correlations between bonding pad qualities and robust ball bonding solutions. Few of them investigated these failures from a mechanical point of view, such as deflection of leadframe under vertical bonding force. For today's small outline transistor (SOT) packages, the leadframe has low profiles and complex structures, which makes the wire bonding response very complex and difficult to be predicted. This paper demonstrated that the deflection of leadframe at the first bond position played an important role on the bond-off failure for SOT packages. When the deflection was larger than 5 μm at critical bonding locations, severe bond-off failure occurred. By applying finite element analysis, some ways to reduce this deflection thus eliminating the risk of bond-off failure were discussed and verified by experiments, which include leadframe structure modification at some local areas, change of supporting tape types, and lowering of bonding temperature. This work provides several guidelines for leadframe designers to achieve an optimized leadframe design of miniaturized packages, for the sake of improving wire bonding performances.