High temperature automotive application: A study on fine pitch Au and Cu WB integrity vs. Ni thickness of Ni/Pd/Au bond pad on C90 low k wafer technology

For high temperature automotive application, IC products are required to pass stringent high temperature storage stress test (e.g. 5000hrs at 150 deg C), hence requires reliable wire bonds. Such requirement is especially challenging with fine pitch Au & Cu wire bond (e.g. bond pad pitch <; 70um and bonded ball diameter <; 58um), more-so on low k wafer technology with bond-over-active requirement. In the area of fine pitch wire bond reliability of an IC product, Au wire on Al pad is known to have ball bond failure after prolong thermal aging at elevated temperature. Studies have shown that the root cause is due to diffusion of Au during intermetallic phase transformation, resulted in excessive Al precipitation. Void formation proven to be associated with oxidation (corrosion) process of excessive Al precipitated during intermetallic phase transformation. Bond fails due to void coalescence and crack propagation in IMC level. Hence, to prevent bond failures especially during high temperature thermal aging, over-pad-metallization (OPM) with NiPdAu plating is an option by inhibiting the diffusion of Au in the intermetallic (non Au-Al type), hence prevents/minimizes void formation. It also helps to reduce stress on bond pads during wire bonding. Similar long term reliability concern exists in people´s mind with Cu wire on Al pad, and more-so in Cu due to higher sensitivity towards corrosion. However, as there is tremendous cost saving opportunity through the conversion from Au to Cu wire, hence Cu wire comes into picture. Such reliability concern should be greatly minimized with OPM with NiPdAu plating. Moreover, since Cu wire is much harder than Au wire, OPM will greatly reduce Cu bonding stress to bond pad underlying metallization. In view of the need for characterization of over-pad-metallization, a study was conducted to understand fine pitch Au and Cu wire bond integrity vs. different Ni thickness of NiPdAu bond pad on C90 Low k wafer technology to meet- - high temperature automotive requirement. C90 wafers were separately plated with NiPdAu on bond pads, each wafer with different Ni thickness at 1, 2 & 3um. Wafers with bare Al bond pad were used as control. After wafer dicing, dice were bonded on 24 × 24mm LQFP and wire bonding was done with 2N Au and 4N Cu wire respectively. The bonded ball size was 43um. In this study, in order to assess high temperature automotive reliability equivalent to 150 deg C for 5000hrs for an Al bond pad, wire bonded units were subjected to thermal aging at 225 deg C in nitrogen environment for 40hrs for Au wire, and 15hrs for Cu wire which were calculated based on the respective diffusion rate (i.e. Au in Al: 8.62E-161 cm²/sec at 150 deg. C, 1.09E-13 cm²/sec at 225 deg. C, Cu in Al: 1.37E-17 cm²/sec at 150 deg. C, 4.45E-15 cm²/sec at 225 deg. C). To assess ball bond integrity for each Ni plating thickness, key wire bond responses collected were ball shear and wire pull at TO and after thermal aging using Dage4000 shear/pull tool. To analyze any potential bond pad damage as well as to understand the intermetallic compound at the interface between bonded ball and bond pad after cross-sectioning, SEM and EDX were performed accordingly. The result of this study showed that OPM greatly helped in the improvement of ball bond integrity to meet high temperature automotive requirement, lum Ni thickness was found to have pad sinking effect, hence the preferred Ni plating thickness was minimum 2um. For cost saving purpose, the combination of Cu wire with 2um is recommended.