Self-Alignment of Active Si Bridge using Solder Joint Capillary Forces

This paper introduces and validates a novel 'Si bridge first' approach to IBM's DBHi packaging technology, which exploits the self-alignment capability of mass reflow (MR) to enable effective assembly of double-sided TSV-type Si bridges onto a substrate via solder joint surface tension. By understanding the relationship between geometry and capillary forces, we demonstrate that high-throughput MR assembly can replace thermocompression bonding (TCB). Fluidic simulations of vertical and lateral capillary forces as a function of solder thickness between metallized pillars defined optimal configurations for solder-on-pillar interconnections. A semi-additive manufacturing process supported these configurations. A design of experiments evaluated alignment effectiveness on Si bridges assembled on Si substrates. Results using a single 100 μm diameter pillar on each bridge corner demonstrated consistent alignment accuracy under 2 μm, achieving in some cases accuracies better than 1 μm, while mechanical profiling showed z-tilt variations below 1 μm. Comparisons of various corner pillar (1-4 bumps per corner) and full matrix designs confirmed that self-alignment capability is consistent. Controlled lateral offset experiments supported the ability of the configurations to accommodate industry standard flip chip pick and place tolerances. Thermo-mechanical finite element analysis (FEM) simulations compared TSV-type Si bridges with two-sided bonding to single-sided designs, both on organic substrates in a DBHi module configuration. Results showed no significant effect on Si chip, Si bridge and substrate warpage. Si chip and underfill stresses were also equivalent. However, corner stresses in the Si bridge were significantly reduced in the two-sided bonding design due to the presence of the bottom side solder interconnections. While further experiments are necessary to validate production scaling, this study highlights the potential of MR-based solder self-alignment for TSV-type bridge assemblies with high precision and throughput, positioning it as a viable alternative to TCB.