What is an OSAT, and what happens in a test-and-assembly house?

In one sentence: OSAT stands for outsourced semiconductor assembly and test, and the factory that does it is often called a test-and-assembly house. After the wafer fab, wafers are sent there to be tested, cut apart, packaged and tested again, and only then are the chips ready to be soldered onto a circuit board.

What do the four letters mean?

• O (outsourced): the work is done for other companies. The chips belong to the customer who sent them.
• S (semiconductor): the product is chips.
• A (assembly): putting each chip into a package and bringing its connections out to leads or solder balls that can be soldered. In English "assembly" and "packaging" are often used for the same work.
• T (test): checking that every chip is good.

You will also see ATMP (assembly, test, marking and packaging), which describes the same stretch of work.

Why outsource it?

Making a chip has two halves. Building the circuits on the wafer is the front end, done in a wafer fab. Testing the chips on the wafer, cutting them apart and packaging them is the back end.

Not every chip company does both halves itself. Many companies only design chips and have no factory of their own: a foundry makes their wafers and a test-and-assembly house packages and tests them. Some companies that do own fabs also send part of their back-end work out. A company that specializes in assembly and test serves many customers at once and has the tools and experience for many kinds of packages, so one tool can work for many companies. It is the same idea as a foundry; see The news says "foundry". What is it?

Which stations does a chip pass through?

There are many kinds of packages, and the steps differ. A common flow looks roughly like this:

1. Wafer sort (CP): before the wafer is cut, probes test each chip, and the bad ones are recorded so no money is spent packaging them. Sometimes this happens at a dedicated test house.
2. Backgrinding: the wafer is ground thinner from the back.
3. Dicing: the wafer is cut into single chips.
4. Die attach: each good chip is glued onto a substrate or a lead frame.
5. Wire bonding: metal wires finer than a hair connect the chip's pads to the outside. Another method is flip chip: the chip is turned upside down and connected directly through tiny bumps.
6. Molding: plastic is molded around the whole thing.
7. Marking: the part number and lot number are printed on the package.
8. Final test (FT): every packaged part is tested again, because packaging can damage a chip and some checks only work on a packaged part.
9. Packing and shipping: parts go one by one into tape & reel, and moisture-sensitive ones get a dry pack.

How testing works is in How are chips tested?; why packaging is needed at all is in What is chip packaging?; and wire bonding in detail is in What is wire bonding?

How is it different from a wafer fab?

	Wafer fab (front end)	Test-and-assembly house (back end)
What you handle	Whole wafers, counted in lots and wafers	Single chips, often counted in thousands or tens of thousands
Main work	Building circuits on the wafer layer by layer	Test, cut, mount, wire, seal, test again
How clean	Usually the strictest cleanroom classes	Clean too, but most areas are usually less strict than the front end
Changeovers	One process runs for a long time	Many customers and package types, so products and fixtures change often
Tools	Litho, etch and thin-film process tools	Dicing saws, wire bonders, molding presses, testers, handlers

How cleanroom classes work is in How clean is a cleanroom?

What jobs are there?

• Test engineer: writes and maintains test programs, looks after the testers and probe cards, reads every chip's result and decides where the limits go.
• Assembly process engineer: owns the conditions at each station (dicing, die attach, wire bonding, molding) and finds which station is to blame when yield drops.
• Equipment engineer: maintains and repairs the wire bonders, dicing saws and testers.
• Quality and reliability engineer: checks before shipping, runs reliability tests on samples, analyzes parts that customers send back.
• Customer liaison: keeps the customer up to date on progress, specs and problems; when the customer asks "can it ship tomorrow?", this is who they ask first.
• Production control and technicians: schedule the lots at each station, run the tools and respond first when something goes wrong.

Common backgrounds: test leans toward electrical and electronics (EE, ECE), assembly process toward mechanical, materials and chemical engineering, and the customer liaison is often an engineer who likes working with people. Where each degree goes in the industry is in Which degrees work in a chip fab?

Why is it in the news so often now?

Packaging used to be seen as "wrapping it up at the end". Now, to make chips faster and more power-efficient, more and more products put several chips side by side, or even stack them, inside one package, so the package has become part of the chip's performance and assembly and test show up in the news more often. See The news says "advanced packaging". What is it?

In fab24hr

Act three of the main story, The life of a chip, takes place at a test-and-assembly house. In chapter 10 you are the test engineer, reading the bins and deciding the standby current limit; in chapter 11 you are the assembly process engineer, and a wire bond pull test has gone wrong, so you decide whether to stop the tool; in chapter 12 you are the customer liaison on the night before shipping, and the customer asks whether it can ship tomorrow. In A day in the fab you can also spend a whole day as the test engineer, the assembly process engineer or the customer liaison. The test-and-assembly house, its tools and its people are fictional; the flow and the jobs follow public, general sources, and every factory and every package type does it differently.