OSAT (chip assembly and test) is the work of cutting finished wafers into separate chips, sealing each chip in a package and testing it. OSAT stands for outsourced semiconductor assembly and test, because the plants are often run by specialist companies, not by the chip's designer or its fab. In India the work is also called ATMP: assembly, testing, marking and packaging. It is the first step India is taking in making chips at home.
A wafer comes from the fab as a thin disc of silicon, usually 300 millimetres across, holding hundreds or thousands of identical chips. Each chip on the wafer is called a die. A bare die cannot be used as it is. It is fragile, and its connection points are far too small to solder onto a circuit board. A package solves both problems. It is the small black case seen on any circuit board. It protects the die and carries its connections out to larger metal pins or balls.
Take, for example, a lot of wafers at an assembly and test plant like the one Tata Electronics is building at Jagiroad in Assam. Each wafer holds 900 power-management chips for phones. First comes wafer test. Fine probes touch each die and check that it works. On one wafer, 846 dies pass, and the failures are marked on a digital map of the wafer. Next, a saw with a diamond blade cuts the wafer into separate dies. A machine picks up each good die and fixes it onto a substrate, a small board that forms the base of the package. A wire-bonding machine then joins the die to the substrate with gold or copper wires thinner than a hair. In other packages, the die is turned face down and joined directly through tiny solder bumps. Next, the die is sealed in hard plastic, a step called moulding. Each package is marked with a code that identifies its wafer and lot. Finally, every package is tested again, at hot and cold temperatures. Here, 6 of the 846 fail and are discarded. The other 840 are packed on reels and shipped to a phone assembly plant.
Why test twice? Packaging a bad die wastes the package. Shipping a bad chip can stop a customer's production line. The share of good chips at each stage is the yield, and the plant tracks it every day.
Packaging used to be the simple end of chip making. Today parts of it are among the hardest. High-bandwidth memory (HBM) stacks eight or more memory dies on top of each other, joined by thousands of vertical connections. Placing such stacks right beside an AI processor, inside one package, is called advanced packaging.
India is building several such plants under the India Semiconductor Mission. Micron Technology is building one at Sanand in Gujarat, for memory chips. For now, their wafers come from fabs abroad. Later, some will come from the fab at Dholera, described on the page on semiconductor fabs in India. Every semiconductor foundry needs such plants after it. The final test machines are automated, but engineers set up and check their tests with oscilloscopes and test instruments.