What is chip packaging, and why does a finished chip still need a package?

Packaging means putting a chip that was made on a wafer into a case that protects it and lets it connect to a circuit board. Open up a phone or a computer, and the small black squares on the circuit board are usually not the chips themselves but packaged chips: the real chip is inside, often much smaller than its case.

What does a chip look like when it's first made?

Chips are made on a wafer, a thin, round slice of silicon. Hundreds or thousands of identical chips sit on it in a grid, like the squares of a chessboard. Each one is a small square of silicon, anywhere from smaller than a grain of rice to about the size of a fingernail, or even bigger.

That little square is thin. A wafer starts out less than a millimeter thick, about the same as a credit card. Before packaging, it is ground down from the back, often to about the thickness of a sheet of paper or even thinner. And silicon is like glass: hard, but brittle. Knock an edge and a piece can chip off.

Can a bare chip go straight into a phone?

No. A bare chip has at least three problems:

• You can't touch it: it is thin and brittle, so a drop or a squeeze cracks it. The circuits on its surface are very fine metal and thin films; dust, moisture or the sweat from your fingers will corrode them over time. Some circuits are even affected by light.
• It can't connect: a chip gets power and sends signals through small metal patches on its surface called pads. The pads are so small you need a microscope to see them clearly, and they are packed close together. Solder joints on a circuit board can't be made that fine, so the chip can't be soldered on directly.
• It can't get rid of heat: a chip heats up when it works. When your phone feels warm after a while, the chips are one of the reasons. A tiny square of silicon has nowhere to send that heat, and if it gets too hot it makes errors or even breaks.

What three jobs does a package do?

A package solves all three problems at once:

1. Protection: a layer of hard plastic (sometimes ceramic or metal) seals the chip in, keeping out knocks, dust, dirt and light.
2. Connection: the fine, closely spaced pads on the chip are wired one by one to metal pins you can see on the outside of the case, or to rows of tiny solder balls underneath. Pins and balls are much bigger than the pads and spaced further apart, so they can be soldered to a circuit board.
3. Cooling: heat flows from the chip through the case and the pins into the circuit board, where it spreads out. A chip that uses a lot of power can also have a heat sink or a fan on top of its case.

The second job is a bit like an adapter: one side matches the tiny pads on the chip, the other matches the much bigger solder pads on the board, and the package handles everything in between.

What are the steps of packaging?

There are many ways to package a chip. In one of the most common, the steps go roughly like this:

1. Dicing: the wafer is ground thin from the back and stuck onto a sheet of tape, then a thin blade studded with diamond grit cuts along the narrow gaps left between the chips. The tape keeps the cut chips from scattering.
2. Die attach: a machine picks up the good chips one by one and glues each onto a tiny board, or onto a metal frame called a lead frame. Chips that use a lot of power are attached with metal solder instead, so heat can flow out through the bottom of the chip.
3. Wire bonding: metal wires finer than a hair connect the pads on the chip, one by one, to the board or frame. The machine melts the tip of the wire into a tiny ball, presses it onto a pad and bonds them with heat, pressure and ultrasound, then draws the wire to the other end and presses it down there. It's a lot like sewing with very fine thread, except a machine makes every stitch.
4. Molding: the chip and its wires go into a mold, and plastic softened by heat is pressed in to cover everything, then hardens in the heated mold. The wires are so fine that the flowing plastic can push them out of place, so the parts are checked with X-rays afterwards. The part type and lot number are printed on the case, and finally the parts are cut apart or trimmed into single pieces.
5. Final test: the packaged chip is tested again. A wire may not be connected, and some characteristics change once a chip is packaged, so it gets tested once more, and only good parts ship.

Some chips skip the wires: small bumps are built on the chip's pads, the chip is turned upside down, and the bumps are soldered straight onto the board. This is called flip chip. Either way, the package does the same three jobs.

What does a test-and-assembly house do?

Making a chip comes in two big stages. Building the circuits on the wafer, layer by layer, is the wafer fab's job, usually called the front end (what each area of a fab does). Testing the finished wafers, cutting them apart, packaging the chips and testing them again is usually called the back end, or simply packaging and testing.

Some companies do both stages themselves. Many chips, though, go to a plant that specializes in packaging and testing, called a test-and-assembly house.

When wafers arrive at a test-and-assembly house, the first step usually isn't cutting but testing. Before the wafer is cut, fine needles touch the pads of every chip to measure it, and the bad ones are recorded (wafer sort). This saves money: weed out the bad chips first, and nobody pays to package them. Once packaged and given their final test, parts are often placed in the small pockets of a long strip of tape and wound onto a reel (tape and reel), then sent to the customer's factory, where machines place them on circuit boards one at a time.

What is the "advanced packaging" in the news?

Most of the above is about one chip packaged into one part. The "advanced packaging" you see in the news these days means putting several chips side by side or stacking them, and packaging them together. How it's done, and why it comes up so often lately, will get an article of its own.

In fab24hr

In fab24hr's main story, The life of a chip, act three takes place at a test-and-assembly house. In chapter 10, "Standby Current", you are the test engineer and decide where to draw the limit for one test. In chapter 11, "The Ball Lifted", you are the assembly process engineer, and two wires in the wire bonder's pull test come out wrong. In chapter 12, "The Night Before Shipping", you are the customer liaison and decide whether to run a sample first or ship. In "A day in the fab", tap "Switch roles" and pick "Test-and-assembly house" to play any of these jobs at any time of day. The assembly process engineer's four shifts each guard one station: boards waiting to be wire bonded late at night, the dicing saw's blade in the morning, the pull test in the afternoon, and wires pushed out of place by molding in the evening.

The chip in the game, "Glowmere", is a fictional hearing-aid chip packaged as a small SiP module: tiny resistors and capacitors sit next to the chip on a small board. The test-and-assembly house, the people and the numbers are fictional; what packaging does and why it's needed follow public, general sources. For other jargon, see Chip fab jargon in plain English.