Defect analysis of copper ball bonding

Copper ball bonding has gained more and more popularity compared to gold ball bonding due to its better electrical conductivity, better thermal conductivity, higher mechanical strength, lower cost and better reliability and so on. However copper wire has two well-known disadvantages, one is its greater hardness, the other is being easy to be oxidized when forming FAB (free air ball). These disadvantages damage ball bonding process feasibility and stability and bring many problems that are proving difficult to resolve such as none stick, weak bonding, missing ball, different size and shape bonding ball, wire breaking, aluminum sputter, cratering, short tail and finally reduce product yield. Though new materials and improved equipments are also very important to copper ball bonding, promoting bonding process to decrease weld defects is necessary and sustaining. Copper ball bonding process parameters are various and complicated. Many studies have been conducted on the relationship between bonding process and bondability. However many impact of process on bonding are still unclear and misconception, this paper intends to help understand the relevant between. This paper utilizes DOE (design of experiment) to optimize separately FAB parameters such as fire current, fire time and 0.9N₂/0.1H₂ gas flow rate and bonding process parameters such as ultrasonic power, bonding force, bonding temperature and bonding time to obtain the feasible and stabile bonding process. The bonding effect under different process parameters were compared and analyzed by 3D digital microscope. This paper also indicated the failure mode and the failure mechanism of ball shear and wire pull by die tester.