Who makes OSATs taking CoWoS back-end and overflow for AI data centers

Advanced packaging capacity does not expand smoothly. When demand for 2.5D packaging runs ahead of what the primary foundry line can absorb, the back-end steps — die attach, molding, substrate assembly, and final test — get pushed out to outsourced assembly and test houses, which is what this part covers. The OSAT is not doing the interposer work that defines the package; it is taking the stages either side of it, and taking the overflow when the queue for the constrained middle step is long. That makes this the release valve on the tightest process in Layer 1. It matters to an AI build in a way that is easy to miss: an accelerator can be blocked by packaging capacity long after the wafers exist, and the OSAT bench is where extra throughput comes from first, because qualifying a new assembly line is faster than building a new interposer line. The companies here are the large Taiwanese and US assembly and test houses. What to look for is qualification rather than nameplate capacity — an OSAT can only take work its customer has certified it to do, so an announcement of new floor space says less than a statement that a specific package on a specific accelerator has been dual-sourced. Test capacity is worth watching separately from assembly; the two constrain independently.