In this study, the heterogeneous integration of three mixed test chips is investigated and designed for AI hardware. An high bandwidth memory (HBM) test die, a die with a 75 and 55 µm mixed micro-bump solder joint pitch, and a smaller die at 150 µm solder joint pitch are all joined onto an organic substrate with 2 µm /2 µm line and space to achieve high-bandwidth interconnections. The organic substrate in this study is a 35 mm x 35 mm square with 4+1 5-2-5 layers and ENEPIG (electroless Ni/Pd/Au) surface finished pads. To enable the joints for mixed pitches across the substrate, both belt furnace reflow and thermo-compression bonding (TCB) processes were investigated. Additionally, the chip-side interconnection with varied copper content was also simultaneously investigated to fully optimize the chip join process. The joint quality was verified with cross-sectional analysis, 3D X-ray of the interconnect, and electrical testing. Selective cross-sectional analysis was used to study the geometry of the micro-bumps after the chip join, and it was confirmed that micro-bumps were joined without defects, even at different pitches within the same die. The underfill process was also optimized to reinforce the package and reduce the overall stress resulting from the substrate warpage and CTE mismatch. Reliability stressing revealed three fail modes associated with these high-density organic substrates. A complementary non-linear finite element analysis is performed to further characterize and design the allowable critical via configurations of the high-density substrate and of the critical solder joints subjected to aggressive thermal cycling tests. New substrate design groundrules are proposed to meet JEDEC reliability requirements that can enable AI hardware using high density organic substrates.