Below is a summary from an article by Calvin Lee, Director of Corporate R&D of ASE who is leading 3D IC packaging development.
For investors, the key point is that microbumps have a surprisingly long runway as packaging complexity increases. Their ability to scale within existing backend manufacturing flows makes them a lower-risk path for expanding advanced packaging capacity, while hybrid bonding remains concentrated in applications where its incremental performance justifies the substantially higher manufacturing requirements....'Microbumps and Hybrid Bonding both enable high-density die-to-die and die-to-wafer integration, but they impose very different requirements on the manufacturing process. Those differences are important because they help explain why microbumps remain the preferred solution for high-volume production today.👉Manufacturing maturity: Microbump processing builds on manufacturing infrastructure that OSATs and foundries already have in place: electroplating, resist patterning, UBM formation and reflow. The key challenge is continuing to push toward finer pitches, with 40–100µm already established and 20µm becoming an important target. This is largely an extension of existing processes rather than a fundamental change in the manufacturing environment.Hybrid bonding requires a much tighter process flow. Surface preparation, CMP, plasma activation, contamination control and sub-micron alignment all become critical. In practice, this introduces more frontend-like process requirements into the packaging flow, increasing both capital intensity and process complexity. That makes hybrid bonding considerably more difficult to deploy across a broad manufacturing base.👉Supply chain and process control: The microbump ecosystem is mature. Copper, nickel, solder alloys, UBM materials and the associated equipment are available from a broad supplier base, and the process is sufficiently tolerant to support multisourcing and production across different facilities.Hybrid bonding is more sensitive to relatively small variations in surface roughness, wafer flatness, oxide properties, contamination and bonding conditions. As a result, process recipes and equipment become more tightly coupled, making qualification, second sourcing and transferring production between sites more difficult. For an investor, this matters because it can translate into longer qualification cycles, higher switching costs and greater execution risk during capacity expansion.👉Alignment and yield:Microbumps also have a fundamental advantage in process tolerance. The solder-based connection provides some mechanical compliance during assembly and reflow, helping absorb modest alignment errors, wafer bow and thermal mismatch. That becomes increasingly valuable as package dimensions increase and heterogeneous dies are combined.Hybrid bonding removes much of that tolerance in exchange for substantially higher interconnect density. Direct copper-to-copper or oxide-to-oxide bonding requires extremely tight control of alignment and surface condition. Defects that would be tolerated in a conventional microbump flow can instead become yield limiters. The result is a narrower process window and a more demanding yield-learning curve, particularly as die sizes and stack heights increase.👉The OSAT perspective:This is where the distinction becomes particularly important. Microbumps already provide the density, bandwidth and power characteristics required for chiplets, HBM and large 2.5D packages, while remaining compatible with established backend processes such as RDL formation, known-good-die assembly and chip-on-wafer bonding.The technology is also continuing to scale. VIPack, for example, is targeting a progression from roughly 40µm toward 20µm pitch through improvements in the metallurgical stack rather than a wholesale change in the packaging architecture.Hybrid bonding will increasingly matter where interconnect density and electrical performance justify the additional process complexity, particularly for demanding 3D logic and future memory architectures. But for much of the market, microbumps offer a compelling trade-off: sufficient interconnect density, a mature equipment and materials ecosystem, broader manufacturing flexibility and a more forgiving yield profile.'Source: Chips & Wafers
