OSAT, assembly and test
OSAT stands for outsourced semiconductor assembly and test, the back-end stage where finished wafers are cut into individual chips, each is packaged into a protective body with electrical connections, and every chip is tested before shipping. It is a distinct kind of facility from a wafer fab, with different skills and cost structures. Dholera's chip strategy centres on the large Tata front-end fab, and a fuller ecosystem needs back-end capacity too, which is why the region is building a broader cluster.
What OSAT means
OSAT stands for outsourced semiconductor assembly and test. It describes the companies and facilities that handle the back end of chip making, the work that happens after the wafer leaves the fab. The word outsourced reflects the industry's structure, in which many chip designers and even some fabs send their finished wafers to specialist assembly and test providers rather than doing that work in house. An OSAT takes a completed wafer and turns it into shippable, packaged chips. This stage is essential, because a bare chip fresh from the fab is fragile and its connections are far too small to use directly. Assembly and test give the chip a robust body and confirm it works. While a fab is the glamorous, capital-heavy part of the industry, the back-end OSAT stage is just as necessary, and a country that wants a complete chip ecosystem needs both.
Assembly: giving the chip a body
Assembly is the packaging half of OSAT. It starts with dicing, cutting the finished wafer along its grid lines into individual chips, often called die. Each good die is then attached to a package, a protective structure that also carries the electrical connections. The die's tiny contact pads are linked to the package's leads or contacts, traditionally with fine wires in a step called wire bonding, or in more advanced packages by flipping the die and connecting it through small solder bumps. The die and its connections are usually sealed, for example encased in a moulding compound, to protect against moisture, dust and physical stress. The package provides connection points, such as pins, leads or balls, that are large and sturdy enough to solder onto a circuit board. In short, assembly transforms a delicate sliver of silicon into a durable component that the rest of the electronics world can actually handle and use.
Test: proving each chip works
Testing is the other half of OSAT, and it happens at more than one point. Chips are first tested while still on the wafer, using fine probes, so that faulty ones are not wasted on packaging. After assembly, the packaged chip is tested again as a finished component. Automated test equipment applies electrical signals and measures the responses, checking that the chip meets its specification. This final test is often run across a range of temperatures and voltages, because a chip has to perform reliably in real conditions, not just at room temperature. Some chips are also subjected to burn-in, running under stress for a period to expose parts that would fail early, so that only durable chips reach customers. Only chips that pass are shipped. Rigorous testing is what lets a carmaker or medical device maker trust that a chip will work for years, which is why test is treated as seriously as assembly.
Why OSAT is a separate business
OSAT is a different kind of operation from a wafer fab, which is why the two are usually separate facilities and often separate companies. A fab works at the nanometre scale inside an extreme cleanroom, forming transistors on whole wafers, and its costs are dominated by ultra-expensive process tools. An OSAT works at a larger physical scale, handling individual chips and packages, with cleanliness requirements that are strict but less extreme than a fab's. The skills, equipment and economics differ substantially. Historically, back-end assembly and test grew into a large specialist industry concentrated in several Asian countries, because it is labour and equipment intensive but not as capital heavy per unit as a leading-edge fab. For a country building chip capability, developing OSAT capacity is often a natural early step, since it is less costly than a fab while still adding real value and jobs to the supply chain.
Advanced packaging is changing OSAT
For a long time, packaging was seen as a simple protective step, but that has changed. As shrinking transistors at the leading edge has become harder and more expensive, engineers have turned to advanced packaging to keep improving performance. Instead of building one ever-denser chip, advanced packaging combines multiple chips, sometimes made on different nodes, into a single package, connecting them with very short, fast links. This can deliver much of the benefit of a newer node at lower cost and risk. Techniques include stacking chips vertically and placing several chips side by side on a shared base. Because of this, the back-end OSAT stage has become a source of genuine innovation and value, not just a finishing step. For a chip ecosystem like the one forming around Dholera, strength in advanced packaging is increasingly as strategic as front-end fabrication, and it is an area where new capacity can add outsized value.
OSAT in the Dholera ecosystem
Dholera's semiconductor strategy centres on the large Tata Electronics and PSMC front-end fab, a Rs 91,000 cr (about US$11B) plant targeting up to 50,000 wafers per month. A front-end fab produces finished wafers, and those wafers still need back-end assembly and test to become shippable chips, so a complete ecosystem needs OSAT capacity alongside fabs. India's wider semiconductor push includes back-end and packaging projects among its sanctioned units, and around the Dholera anchor a broader cluster is forming. 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. 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 front-end fabs and back-end assembly and test.
FAQ
What does OSAT stand for?
OSAT stands for outsourced semiconductor assembly and test. It is the back-end stage of chip making, where finished wafers are cut into individual chips, each is packaged into a protective body with electrical connections, and every chip is tested before shipping. It is separate from the front-end wafer fab.
How is OSAT different from a fab?
A fab performs front-end work, forming transistors and wiring on whole silicon wafers at the nanometre scale inside an extreme cleanroom. An OSAT performs back-end work, handling individual chips to package and test them. The two have different skills, equipment and cost structures, so they are usually separate facilities and often separate companies.
Why does packaging matter so much now?
As shrinking transistors at the leading edge has become harder and costlier, advanced packaging has become a key way to keep improving performance. It combines several chips into one package with short, fast links, delivering much of the benefit of a newer node at lower cost. This has turned the back-end stage into a source of real innovation.
Does Dholera have OSAT capacity?
Dholera's anchor is the large Tata front-end fab, which produces finished wafers. A complete ecosystem also needs back-end assembly and test capacity. India's wider semiconductor push includes packaging and back-end projects, and a broader cluster is forming, including two additional units cleared on 5 May 2026, one in Dholera and one in Surat.
Why is testing done more than once?
Chips are tested first on the wafer, using fine probes, so faulty ones are not wasted on packaging. After packaging, each chip is tested again as a finished component, often across a range of temperatures and voltages, to confirm it works in real conditions. Only chips that pass are shipped to customers.