Challenges and resolution of insulated PdCu wire bond for TBGA HVM robustness

Abstract

Insulated wire bond is getting more attention nowadays by packaging technologist as another potential option of cost saving after Cu wire bond conversion due to its flexibility in wire bond without the fear of wire short. The process offers greater flexibility in wire bond layout design which can lead to die size reduction and standardization of substrate/leadframe for cost saving. As the wire has the capability to touch each other without causing electrical short, this feature also has been adopted to generate or enhance the capacitance effect based on device requirement. Insulated wire bond process however has many challenges to overcome as compared to the bare wire bond process. With additional insulation organic wire coating, direct contact of 2 metals, wire and Al pad for IMC formation is not possible. Organic coating needs to be sufficiently removed for inter diffusion of 2 metals. The challenge can be easily observed during post bond formation with severe wire bond problems such as non stick on lead (NSOL) and short tail if the wire coating is not effectively eliminated through mechanical scrubbing. Ball bond process, with the existence of electrical flame off (EFO) to burn and evaporate organic coating before free air ball (FAB) formation, is a relatively easy process for insulated wire, but thicker insulation coating can result in pointed or irregular FAB formation resulting in non stick on pad problem in some severe cases. This study aims to define an optimum wire bond process and coating material recipe for Tape Ball Grid Array (TBGA) package with first ever attempt at using 18μm PdCu wire size to achieve high volume manufacturing (HVM) capability for 47μm pad pitch. TBGA is well known for its difficulty on post bond formation due to fine pitch lead finger on polyimide flex sitting on top of adhesive layers in substrate configuration. The characterization and optimization process involved 4 key factors: capillary, plasma cleaning, wire coating and wire bond parameters. Series of evaluation and DOEs were performed to establish optimized parameter window on each factor in terms of wire bond integrity and bondability. All optimized settings were later integrated and subjected to HVM run. From HVM verification, all wire bond quality requirements were fulfilled. Wire bond MTBA achieved more than 2hrs and successfully passed industrial level package reliability stressings with no electrical failures. In summary, the most challenging 18μm insulated Pd Cu wire bonding process on TBGA package can be made possible through a detailed process characterization and careful consideration of manufacturing performance.