What is SiP? Chips and small parts packed into one tiny module

SiP stands for system in package: a chip and the small parts it needs, such as resistors and capacitors and often other chips too, are mounted together on a tiny board and sealed in plastic as one small module. From the outside it looks like an ordinary chip. Inside, it's a small system.

Why is a chip surrounded by tiny parts?

Open up an electronic device, and next to the black chips on the circuit board you'll often see a crowd of parts as small as sesame seeds. Most of them are resistors and capacitors.

• Resistor: limits how much current flows, like pinching a water hose.
• Capacitor: stores a little charge and hands it over the moment it's needed, like a small bucket of water kept close by. When the chip suddenly draws a lot of power, the voltage doesn't sag all at once.

The chip needs them to work steadily. Why not build them into the chip? Some can be. But a capacitor that stores more charge takes up a big patch of the chip, and space on a chip is expensive. So they're often separate small parts. A product also often has more than one chip.

What does SiP put together?

Ordinary packaging usually puts one chip in one case (see "What is chip packaging, and why does a finished chip still need a package?"). The company making the product then solders the chip and dozens of small parts onto its circuit board, one by one.

SiP gets that done inside the package: the chips and small parts are mounted on a tiny board, connected, and sealed together. The product maker receives just one part.

It's a bit like a lunch box: the same dishes, already packed into one small box, ready to eat.

There is no single definition everyone agrees on. A strict one says the package must hold two or more different chips, and common SiPs do hold several chips plus resistors and capacitors, some with an antenna or a sensor as well. The "heterogeneous integration" in the news means packaging separately made, different parts together, and SiP is a common kind of it.

Why pack things into an SiP?

• Small, saves space: parts that would be spread over a circuit board can sit much closer inside the module and shrink into one. Earbuds, watches, hearing aids and other small things packed with features need this most.
• Shorter paths: the closer a capacitor is to the chip, the faster it can top up the power; signals travel shorter distances too.
• Ready to use: the whole module is tested before it ships, and the product maker has far fewer parts to solder.

How is SiP different from SoC?

SoC stands for system on chip: computing, memory, wireless and other functions are all designed into one chip. SiP puts separately made chips and parts into one package.

Think of it this way: an SoC puts every shop in one big building; an SiP lines up a few small shops, each built on its own, on one plot of land and puts a wall around them.

	SoC (system on chip)	SiP (system in package)
What's where	Everything on one chip	Several things in one package
How it's made	All in the same process	Each chip can be made the way that suits it
Size	Smallest	Bigger than an SoC, much smaller than parts spread over a board
Paths between chips	Shortest, inside the chip	Longer than inside a chip, shorter than on a board
Making a new one	Designed from scratch, with new photomasks: slow and expensive, but cheapest per chip in very large volumes	Can be built from chips already made and tested, usually faster

Why not make everything an SoC? Some things are hard to make in the same way as the computing circuits, such as circuits that handle wireless signals and some memories and sensors, and larger capacitors don't fit. Those are made separately and packaged together.

The two aren't either-or. Many SiPs hold an SoC inside, along with the other chips and small parts it needs.

How is SiP different from advanced packaging?

The advanced packaging in the news places a few chips side by side on an interposer or stacks them, so close and so densely connected that they work like one big chip (see "The news says "advanced packaging". What is it?"). What it cares about is that the roads between chips are short and wide, which AI chips need most.

SiP cares about fitting a small system in: chips, resistors, capacitors, sometimes an antenna, all in one small module. The connections mostly use mature methods: SMT, wire bonding, or flip chip, where the chip is turned upside down and soldered straight onto the board.

The two terms overlap: some SiPs stack chips too, and some people count advanced packaging as a kind of SiP. One way to remember it: advanced packaging makes a few chips act like one chip; SiP makes a small system act like one part.

How is an SiP made?

Take an SiP that connects its chip with wire bonding. Roughly, with SMT and die attach in the order the game's assembly line uses:

1. Test, then cut: every chip on the wafer is first tested with fine needles (wafer sort), then the wafer is thinned from the back and cut apart (dicing).
2. SMT: solder paste is printed onto small metal patches on the board, tiny resistors and capacitors are set on top, and the board goes through a long oven. The paste melts and sets again, and the parts are soldered in place.
3. Die attach: the good chips are picked up one by one and glued onto the board. Chips that failed wafer sort are not picked up.
4. Wire bonding: metal wire finer than a hair connects the pads on the chip to the board, one wire at a time (details in "What is wire bonding, and how does a chip connect to the outside?"). Some SiPs skip the wires and use flip chip.
5. Molding: a whole board holding many modules goes into a mold and is sealed in plastic, then X-rayed to check that no wires were pushed out of place.
6. Cut apart: the molded board is cut into individual modules.
7. Final test: each whole module is tested again. Some problems only show up after packaging.
8. Pack and ship: the modules go into tape & reel. The plastic absorbs moisture and can crack when it's heated while being soldered onto a circuit board, so the modules are baked dry and put in a dry pack.

What happens if one part inside is bad?

SiP has one weakness: if any one part inside is bad, the whole module can't be used, and the good parts are wasted with it. Once it's sealed, it's also hard to open up and repair.

Try some made-up numbers. A module holds 4 chips, and each has a 90% chance of being good. The chance that all 4 are good is 0.9 × 0.9 × 0.9 × 0.9, only about two-thirds.

So every chip is tested first, and only good ones go in. That's also why chips get wafer sort before the wafer is cut (see "How are chips tested, and how is wafer sort (CP) different from final test (FT)?").

In fab24hr

In chapter 11 of the main story, The life of a chip, "The Ball Lifted", you are the assembly process engineer at a test-and-assembly house. Shu-Hui, the line leader, walks you from the dicing saw to the SMT line: "Glowmere goes into SiP modules: a small board with the chip on it, plus a few resistors and capacitors." SMT comes first, then die attach. At the row of wire bonders, two wires in WB-03's pull test come out wrong, and whether the machine stops is your call. In chapter 13, "Into His Hands", the new hearing aid with Glowmere inside reaches Mr. Shen.

Tap "Clock in", choose "A day in the fab", tap "Test-and-assembly house" on the "Your role today" card and swipe to the assembly process engineer. You get one shift by the time on your device; all four are on this assembly line, and the afternoon one is chapter 11.

The Glowmere module is simpler than most: one chip plus a few resistors and capacitors. By the strict definition, an SiP needs two or more different chips; the game uses the broader name. The test-and-assembly house, the chip, the people and the machine numbers in the game are fictional; what SiP is and how it's made follow public, general sources, and "90%" is a made-up number. For other jargon, see Chip fab jargon in plain English.