3mil Copper wire bonding capability study in 4um Al top metal by thermosonic bonding

Abstract

The heavy copper wire with 1.2 to 2mil diameter in thermosonic bonding are already widely used in high power devices at different packages throughout semiconductor industry as part of low cost, better mechanical properties and better electrical performance with lower resistivity. Thus, this capability study intended to evaluate more heavy copper of 3mil diameter by thermosonic bonding in DFN (Dual Flat No-leads) package using Mosfet device. This paper discussed the challenges in development of 3mil copper wire in terms of bondability, workability and reliability with 4μm Al pad thickness. The study consists of machine platform selection with heavy copper wire capability, capillary design and copper wire material type. Technical Risk Assessment was considered for each wire bonding step or cycle process from FAB (Free Air Ball) Formation till 2nd bond (Stitch on lead) thru Wire Tail Cutting sequence. A CTQ (Critical To Quality) requirements for copper bonding were identified as responses for this study such as ball shear test, wire pull test, ball dimension, stitch dimension, cratering test, PMD (Pad Metal Displacement) or Al remain through ball cross section, ball flatness and Ball Shear Strength. Machine stoppages were monitored during evaluation run. The FAB characterization showed the main EFO (Electronic Flame-Off) parameter of EFO Current and EFO Time is significantly higher than existing heavy wire setting in order to achieving the free air ball diameter target. Bonding integrity test were achieved with high ball shear and wirepull test without pad metal damage. In part of robustness test, no cratering or crack on pad was seen in several unit samples and good PMD (pad metal displacement) remain were measured and no underlying metal was collapsed or damage was observed. In this study, 3 evaluation lots and 1 control lot were used and completely passed HTSL, Temperature Cycle, High Humidity High Temperature Reverse Bias; up to extended read points based on standard heavy copper wire reliability requirements. This study results showed that 3mil heavy copper wire with 4 μm and above AL pad thickness could be used for thermosonic bonding in power Mosfet devices for future D/QFN packages of ON semiconductor.