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Fab versus OSAT
A fab and an OSAT are the two halves of chip making. A fab performs front-end work, building transistors and wiring onto whole silicon wafers, while an OSAT performs back-end work, cutting the finished wafer into individual chips, packaging each one and testing it. They differ in scale, cleanliness, equipment and cost. Dholera's semiconductor strategy centres on the large Tata front-end fab, and a complete ecosystem also needs back-end assembly and test capacity.
Last checked 2026-08-11 · every figure below carries its source
Two stages, one journey
Turning silicon into a working chip is a relay race with two main legs. The first leg happens in a fab, short for fabrication plant, where circuits are built onto a whole wafer of silicon. The second leg happens in an OSAT, short for outsourced semiconductor assembly and test, where the finished wafer is cut into individual chips, each is packaged and each is tested. The handoff between them is the point at which the wafer is diced into separate chips. Both legs are essential. A fab produces finished wafers that are not yet usable products, and an OSAT cannot make a chip from scratch, only finish one that a fab has already built. Confusing the two is common, so it helps to remember that a fab works on whole wafers at the nanometre scale, while an OSAT works on individual chips at a larger, more mechanical scale. A country that wants a full chip industry needs both.
What the fab does
The fab handles front-end fabrication, the creation of the circuit itself. It takes a blank, polished silicon wafer and builds the transistors and the metal wiring that connects them, through hundreds of repeated steps of deposition, lithography, etching, doping and polishing, all inside an extreme cleanroom. The features it creates are measured in nanometres, so the work demands the most sophisticated equipment in the industry, especially the lithography systems that print circuit patterns. This is the most capital-intensive part of the chain. The Dholera fab, a Tata Electronics and PSMC project, carries a reported investment of Rs 91,000 cr (about US$11B) and targets up to 50,000 wafers per month on 300 mm wafers, at mature and specialty nodes across a reported 28 to 110 nm range. When the fab finishes, the wafer holds a grid of complete, tested-in-place chips, ready to be diced and sent onward.
What the OSAT does
The OSAT handles back-end assembly and test, the work that turns finished wafers into shippable components. It begins by dicing the wafer into individual chips. Each good chip is then attached to a package, a protective structure that also provides larger, sturdier electrical connections, using techniques such as wire bonding or flip-chip bumps. The package is sealed to protect the fragile silicon. After packaging, every chip is tested as a finished component, often across a range of temperatures and voltages, so that only reliable chips reach customers. The OSAT works at a larger physical scale than a fab and with strict but less extreme cleanliness requirements. Its economics are different too, being less dominated by ultra-expensive tools and more by handling, packaging materials and test equipment. Advanced packaging has made the OSAT stage a growing source of innovation, since combining several chips in one package can boost performance without a newer node.
Key differences side by side
The clearest way to separate the two is by what they act on and how. A fab acts on whole wafers, an OSAT on individual chips. A fab creates the circuit, an OSAT protects and connects it. A fab works at nanometre precision, an OSAT at the scale of packages and connections you can nearly see. A fab needs the most extreme cleanroom and the most expensive process tools, especially lithography, while an OSAT needs strict cleanliness and specialist packaging and test equipment, but at lower cost per unit. A fab's output is a finished wafer, an OSAT's output is a packaged, tested chip ready to ship. Because of these differences, the two are usually separate facilities and often separate companies, and building each requires different skills and investment. Both, however, sit on the same supply chain, and a weakness in either one limits a country's overall chip capability.
Why a country needs both
A fab without back-end assembly and test cannot ship finished chips, and an OSAT without a fab has no wafers to finish. For a country building semiconductor capability, this means a genuine ecosystem needs both, along with suppliers of materials, gases, chemicals, equipment and skilled people. In practice, many countries start with back-end assembly and test, because it is less capital intensive per unit than a leading-edge fab, then add front-end fabrication as skills and demand grow. India's approach spans both. The Dholera fab is the large front-end anchor, and India's wider semiconductor push includes back-end and packaging projects among its sanctioned units. This balance matters, because a fab is only as valuable as the finishing and testing that turn its wafers into products customers can buy. Growing both halves in step is how a chip industry becomes self-sufficient rather than dependent on sending wafers abroad to be finished.
How the split plays out at Dholera
At Dholera, the headline is the front-end fab, the Tata Electronics and PSMC plant targeting up to 50,000 wafers per month, with first trial silicon targeted around December 2026 and commercial-scale production reported for about mid-2028. That fab will produce finished wafers, which still require back-end assembly and test to become shippable chips. A broader cluster is forming to fill out the ecosystem. Two additional semiconductor units were cleared on 5 May 2026, one in Dholera and one in Surat, worth over Rs 3,900 cr combined and more than 2,200 skilled jobs. Tata Electronics also signed a lithography partnership with ASML, reported around 17 May 2026, strengthening the front-end side. For context that should never be attributed to Dholera alone, sanctioned semiconductor projects across India now number about 12 with cumulative investment near Rs 1.64 lakh crore, spanning both fabs and back-end assembly and test across the country.