Effect of impact force towards Cu wire bonding reliability

Copper (Cu) wire bonding technology is now widely held in mass production of semiconductor industry. The main benefit is cost advantage against Au wire and secondly enhance product performance and reliability. More new products are design in with Cu wire bonding and new wire bonding machine also equipped with Cu wire bonding capability. The market trend shows acceptance of Cu wire bonding products increasing yearly. Thus, Cu wire become the most accepted interconnect material besides Au & Al wire. However Cu wire property - hardness, is one of the key challenges for wire bonding process. This lead to a more narrow process window compare to Au wire and lead to more reliability risk. To overcome the Cu wire bonding challenge many researches been carried out. From material aspect, softer Cu wire, robust bond pad, etc. are developed to enhance the process ability. From process aspect, multi-steps bonding is the preferred solution. Multi-steps bonding segmented bonding into impact stage and bonding stage. The impact stage is the first contact of the FAB to the bond pad, which is crucial to form the ball size, ball height and bond pad cratering defect. The bonding stage determines the integrity of bonding. This paper presents how the impact force can influence the product reliability. To determine the impact force experience by bond pad during bond a load cell is being used to measure actual impact force. Different impact force by varying the machine setting, the output response is the Al remnant thickness and cratering result. Kim et al. [1] reported reaction rate of Cu/Al IMC formation was obtained using the Arrhenius plot (lnK versus 1/T) and therefore the theoretical IMC thickness can be calculated as a function of time and temperature. Using the theoretical model a minimum Al remnant thickness at zero hour can be target in order for the Cu/Al interface to withstand the thermal reliability stress. Ultimately, optimum setting for impact stage and bonding stage can be identified for a robust bonding.