What Is an Osat
An OSAT — short for Outsourced Semiconductor Assembly and Test — is a company that specializes in packaging finished silicon dies and testing completed chips, handling the back-end steps of chip manufacturing on behalf of foundries and chip designers who would rather not build and run that specialized capacity themselves.
An OSAT exists because packaging and testing are specialized enough jobs that many foundries and chip companies prefer to outsource them entirely. Front-end fabrication — actually manufacturing the transistors on a wafer — and back-end assembly and test require very different equipment, facilities, and expertise; rather than every foundry or fabless company building out its own packaging and testing lines, OSAT companies specialize purely in that back-end stage, serving many different chip customers at once.
| Stage | Who typically does it | What happens |
|---|---|---|
| Front-end fabrication | Foundry or IDM | Building transistors and circuits on the wafer |
| Dicing | OSAT | Cutting the finished wafer into individual dies |
| Packaging | OSAT | Encasing each die in a protective, connectable housing |
| Final test | OSAT | Verifying each finished chip functions correctly before shipment |
Final testing at an OSAT is the last checkpoint before a chip reaches a customer, catching defects that earlier testing stages might miss. Even after a die passes initial testing on the wafer, packaging itself can introduce new failure points, so OSATs run a final round of electrical and functional tests on the fully assembled chip — this final test step is what actually determines whether a specific finished chip is good enough to ship, separate from the wafer-level testing that happens earlier in the process.
The OSAT industry is geographically concentrated, which has made it a subject of growing supply-chain attention alongside front-end fabrication. A relatively small number of large OSAT companies handle a significant share of the world's chip packaging and testing volume, meaning that just as foundry capacity is often discussed as a chokepoint, back-end assembly and test capacity is increasingly recognized as its own important link in the overall chip supply chain, worth watching just as closely as fabrication itself.
As chip packaging has grown more advanced, some OSATs have taken on responsibilities that go well beyond simple assembly. Rather than just placing a die in a basic housing, leading OSATs now perform sophisticated advanced packaging work — stacking multiple dies, connecting chiplets together — meaning some OSAT companies now compete directly with foundries themselves in offering high-value advanced packaging services, not just traditional back-end assembly.
Read the OSAT through a specialization lens: just as fabless companies let chip designers focus purely on design, OSATs let foundries and chip companies focus purely on fabrication, trusting a dedicated back-end specialist to handle the equally demanding, equally specialized work of packaging and testing every finished chip.