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

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. 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. The result of this study showed that OPM greatly helped in the improvement of ball bond integrity to meet high temperature automotive requirement, 1um 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.