Copper bonding on thin top metal bonding pad

Among the key drivers of development today is cost reduction. To achieve this, one of the measures undertaken is to convert the wire used for the wirebond process from gold to copper in several devices. Conversion from Au to Cu wire is particularly challenging for thin top metal devices because the bond pad structure is more prone to damage. This paper discusses the characterization and optimization studies undertaken in successful copper wire conversion of thin top metal devices. The successful conversion from Au wire to Cu wire resulted in 70% cost reduction. A device with circuit under pad (CUP) die technology has been selected as the representative for thin top metal copper wire conversion. SOIC 14L was the package of choice for this qualification. The representative die used has a top metal thickness of ~0.675μm and a pad metal composition of Al with 0.5% Cu. This qualification enables introduction of new products that combine CUP die technology and copper wire combination as well as copper wire bonding capability for thin top metal. At present, production is currently running on copper wire using thick bond pads of >3μm. This parameter was used as baseline for free-air ball and 2^nd bond. A different bond parameter optimization approach was used for this project as the wire being used is harder and the bond pad structure is prone to pad damage. Only the first bond parameter was meticulously optimized, others were just fine tuned. Based on the statistical analysis software, the most significant parameter change is the bond force. Responses to the wire conversion have been closely monitored. Among the responses monitored are free-air ball (FAB) characteristics, non-stick on pad (NSOP) elimination, ball shear test, wire pull test, crater test and pad metal displacement (PMD). Cross-sectioning and delayering of the bond pads have also been performed to verify the result of the pad metal displacement measurement equipment. All the wir- bond responses that have been monitored indicate that the optimization of the bond force proved to be successful since all the responses required passed the criteria set at assembly site. Aside from wire bond response, reliability test responses have also been monitored. No failures were observed after TC 500 and after other reliability tests. By meeting the required wirebond and reliability responses, this study shows that copper wire bonding for thin top metal devices is achievable. The savings expected in successfully converting Au wire to Cu wire for this package has been obtained without any sacrifice to quality.