Effect of Bumped Wafer Shelf Life on Yield, Quality and Reliability of Flip Chip BGA Packages

Flip Chip BGA (FCBGA) packages have attained great attention in today's semiconductor industry as high density, high performance area array interconnects offering superior electrical, thermal and mechanical performance. Wafer bumping process is the backbone of flip chip packaging enabling array interconnects using miniaturized solder joints, commonly known as 'solder bumps', which are attached to the wafer pads using a multi layered under-bump-metallization (UBM). This solder joint structure could be weakened due to aging effect impacting long term product reliability. Hence an effort was taken to assess the impact of shelf life of bumped wafers on overall FCBGA package reliability through series of systematic experiments. Impact of aging of wafer storage tape on assembly manufacturability was also studied independently to establish optimal shelf life for the same.

Detailed Risk Analysis was conducted identifying potential risk factors which were studied during the experiments. Assembly yield, various qualitative factors like oxide growth on bumps and intermetallics growth in resultant solder joints, and JEDEC standard reliability tests which simulate long term product reliability, were used as key response variables in these experiments. Various state-of the art analytical methods - X-ray Photoelectron Spectroscopy (XPS), Oxide Depth Profiling, Scanning Electron Microscope (SEM), Energy Dispersive X-Ray Spectrometer (EDX) etc., were used to assess the impact of aging of solder bumps on the key response variables.

Bumped Wafer shelf life showed minimal impact on yield and quality of the FCBGA packages. An impact was observed on package reliability in terms of temperature cycling performance on large die FCBGA packages. A significant impact of wafer storage tape shelf life on package manufacturability was identified. The results justified the need for a control specification for both bumped wafer shelf life, and storage tape shelf life to mitigate quality and reliability risk on FCBGA packages.