Cu bonding development & challenges on NiPd bond pad

Cu bonding continues to gain its popularity due to cheaper cost & better product performances. However, conventional bond pad structure such as Al, AlCu or AlSiCu pose higher risk of pad crater or lifted metal, as Cu bonded ball is 30% harder than Au bonded ball. Hence, this limits the progress of Cu bonding conversion for <1µm pad thickness & sensitive BPOA (Bond Pad Over Active) devices. With the plan of speed up Cu bonding conversion, harder bond pad structure that made of NiPd stack was introduced to overcome such risks that encountered on conventional bond pad structure. This paper will discuss the development work to establish 1.0 mils Cu bonding process for NiPd bond pad on QFN package. Feasibility study stage includes NiPd stack plating thickness evaluation; CMP (Chemical Mechanical Polishing) bond pad topography impact on bondability; plasma cleaning evaluation on NiPd pad. Response surface methodology (RSM) was used as design of experiment (DOE) tools to analyze critical bonding characteristics & determine process window through prediction profiler & contour plot. Look Ahead Reliability (LAR) units were built to assess the reliability performances of Cu bonding on NiPd bond pad with standard stress tests such as MSL1, Autoclave, TMCL & HTOL. Zero reliability failure was reported on Cu bonding to NiPd pad upon completion of LAR. However ground bond broken weld was observed at TMCL. Existing ground bond loop distance from die edge was too short, causing steep looping profile & it was broken under thermo-mechanical stress. New design rule for loop distance from die edge will be studied to understand the shortest distance allowable for Cu wire. With all the thorough process assessments & design rule limitation understanding, qualification build could be arranged to qualify NiPd bond pad with Cu bonding. More products would be converted to NiPd bond pad for Cu bonding conversion once the new technology has been qu- lified.