2.5D IC Micro-Bump Materials Characterization and IMCs Evolution Under Reliability Stress Conditions

High density I/O (Input/Output) with close proximity communication between dies in 2.5D heterogeneous integration of disparate ICs (Integrated Circuits) requires substantially miniaturized interconnect. Micro-bump consists of down sized Cu pillar and solder is the chosen interconnect technology for die stacking volume manufacturing. The interconnect solder bump height is shortened to less than 20µm due to significantly reduced Cu pillar diameter. In order to achieve high manufacturing yield and long term reliability of the micro-bump solder joint, a new BOM (Build of Materials) is required by optimizing Ni (Nickel), Cu (Copper), Au (Gold) and Pd (Palladium) composition within SnAg solder. The optimization can be achieved by studying the micro bump solder joint IMC (Inter-Metallic Compound) growth, and solder joint void formation due to thermal aging and thermal mechanical stress during manufacturing and reliability stress. SnAg solder alloy interaction with different composition of Ni, Cu, Au and Pd at the two terminals of the solder joint showed varied effects during die stacking, flip chip assembly, test and reliability stress tests. Different reliability performance can be observed from HTST (High Temperature Storage Test) and TCT (Thermal Cycle Test). Three different Ni, Cu, Au and Pd compositions available to high volume manufacturing environment were studied and impact to IMC, terminal metals and solder joint integrity are the three critical measures for yield, final HTST and TCT reliability. Post die stacking and assembly, three different types of solders showed significantly different IMC thickness. IMC post HTST showed the opposite growth rate compared to the initial IMC thickness post assembly. For HTST, "SnAg-3" leg showed the best solder morphology with full IMC conversion, the other two solders produced large amount of solder voids with partial IMC conversion. For TCT, SnAg-3 passed the test without void formation, cracks or delamination. SnAg-1 solder showed micro-cracks at the early TCT cycle with voids growth and crack propagation after extended TCT, SnAg-2 showed healthy joint at the early TCT read point but produced micro-cracks and voids towards end of life. SnAg-3 composition was implemented for the first high volume 2.5D interposer product in AMD Radeon R9 Fury series Graphics cards.