How are chips soldered onto circuit boards? SMT and reflow soldering

Chips are soldered onto circuit boards with SMT, short for surface mount technology: solder paste is printed onto the board, a machine places the parts one by one, and the whole board goes through a reflow oven, where the solder melts and sets again, and the parts are soldered on. That melting step is called reflow soldering. Most of the tiny parts packed onto the boards in phones and computers got there this way.

What does a packaged chip solder with?

A packaged chip (see What is chip packaging?) has a grid of small solder balls underneath, or a row of metal leads around its edges (for the different shapes, see What do chip packages look like?). On the circuit board, facing every lead or ball, there's a small patch of metal called a solder pad. Soldering means letting solder join the two: current can flow through, and the part is held in place.

The older way is through-hole: a part's leads go through holes in the board and are soldered on the back. SMT goes back to the 1960s and became mainstream in the 1980s and 1990s. With no holes to go through, parts can be smaller, both sides of the board can carry them, and they can be packed closer together.

What happens on an SMT line, step by step?

Step 1: Print the solder paste

• What: Solder paste is very fine solder powder mixed with flux, and it's sticky. A thin steel stencil with small openings is laid on the board, each opening right over a pad; a squeegee drags the paste across and pushes it into the openings. When the stencil lifts off, each pad has a small dab of paste.
• Why: One pass prints thousands of spots across the whole board, like spray-painting through a cut-out stencil. The stencil is about a tenth of a millimeter thick, close to a sheet of paper.
• Otherwise: How good a joint is depends on how much solder it gets. Too little and it won't connect; too much and it spreads to the next pad.

Step 2: Check the paste

• What: Cameras check where each dab landed and how much there is; some machines also measure its height to work out the amount.
• Why: No parts are on yet, so a bad print is much easier to fix.
• Otherwise: Found only after soldering, it means taking parts off and soldering them again: more work, and the board can get damaged.

Step 3: Place the parts (pick and place)

• What: The parts come in small pockets on a long tape wound onto a reel (tape and reel; see How are chips shipped?). A pick-and-place machine's nozzles pick each part up by vacuum, a camera checks its position and direction, and it's set down on the paste. A fast machine places more than ten thousand parts an hour.
• Why: The parts are tiny and there are lots of them, too many to place quickly and accurately by hand. The sticky paste holds each part in place on the way to the next station.
• Otherwise: A part placed crooked or the wrong way round gets soldered on wrong. A chip has a mark showing pin 1; the machine picks each part the way it sits in the tape and places it the way it has been set up to, so the chips have to face the right way in the tape (see "How are chips shipped?"). Placed half a turn round, it may not work when powered on, or may even burn out.

Step 4: Reflow

• What: The board travels slowly on rails through a long reflow oven. The oven has several zones: first it warms up gently so the whole board reaches an even temperature; then it rises past the solder's melting point, to somewhere over 200 °C, for a short while; finally it cools, and the solder sets. One pass takes a few minutes, and the temperature along the way, drawn as a line, is called the temperature profile.
• Why: Parts come big and small, and big ones heat up slowly; heat everything at once and some solder melts while some hasn't yet. The solder is already on the pads, so heating it just makes it flow again, which is what "reflow" means. Melted solder pulls itself into a rounded shape, and as it does, it draws a slightly off-center small part back to the middle of its pads.
• Otherwise: Too cool or too short, and the joints are weak; too hot or too long, and the parts and the board get damaged and the joints turn brittle. Every kind of board needs its own profile.

Step 5: Check the joints

• What: Cameras photograph the whole board and compare it with how it should look: is every part there, is any crooked, is each joint the right shape. This is called automated optical inspection (AOI). Chips with solder balls underneath hide their joints, so they're X-rayed, which shows whether balls have run together, whether any are missing and whether there are bubbles inside.
• Why: Cameras are fast enough to check the whole board; X-rays are slower, so they're mostly kept for the places cameras can't see.
• Otherwise: A bad board that gets built into a product and fails there is harder and more expensive to track down.

Step 6: Through-hole parts and touch-up

• What: Some parts still go through holes, and they're soldered after SMT: the underside of the board passes over a wave of molten solder, or only those few spots are heated, and small numbers are soldered by hand.
• Why: Connectors that get plugged and unplugged a lot, and big, heavy parts, hold better with their leads through the board; heating only those spots spares the small parts already soldered another round of heat.
• Otherwise: A part that takes a lot of force, held only by small joints on the surface, comes loose more easily.

What are the common soldering problems?

• Too little solder, no connection: an opening was clogged or too little was printed, so the solder doesn't reach the lead and no current flows.
• Too much solder, joints run together: solder forms a little bridge between two leads, called a short; the closer the leads, the more likely it is.
• One end lifts up (tombstoning): a tiny resistor or capacitor sits with each end on a dab of paste. If the two ends melt one after the other, or have very different amounts, the end that melts first pulls the part over and the other end lifts, standing up like a tombstone. The smaller the part, the more likely it is.
• Damp parts pried apart in the oven: a plastic package soaks up moisture from the air. If it's left too long after its dry pack is opened and isn't baked dry before reflow, the moisture turns to steam and pries the package and the chip apart. This is called popcorning. So once the bag is opened, the parts have to be soldered within a set time, or baked dry first.

Where is SMT done, and how does it relate to wafer fabs and test-and-assembly houses?

From chip to product, there are roughly three kinds of factory:

1. Wafer fab: builds the circuits on the wafer, layer by layer. There's no SMT line here.
2. Test-and-assembly house: tests the wafers, cuts them up, packages the chips, tests them again and ships them on reels (see What is an OSAT?).
3. Assembly plant: solders the chips and other parts onto circuit boards with SMT and builds them into phones and computers. This is often handed to electronics contract manufacturers that assemble for other companies (see What does each part of the semiconductor supply chain do?).

Is SMT the same as flip chip, or as what happens inside an SiP?

All of them melt solder to join two things; the difference is what gets soldered, and onto what:

	Flip chip	SMT inside an SiP	SMT on a board (this article)
What's soldered	The bare chip, flipped over	Tiny resistors and capacitors	Packaged chips, modules and small parts
Soldered onto	The small board inside the package	The small board inside the module	The product's circuit board
Solder	Small bumps on the chip	Solder paste	Solder paste
Usually done at	Test-and-assembly house	Test-and-assembly house	Assembly plant

Flip chip happens inside the package, soldering the chip onto the package's small board (see What is flip chip?); SMT happens outside it, soldering the packaged chip onto a circuit board.

The SMT inside an SiP module is the same kind of work, done inside the module: print paste on the small board, place resistors and capacitors, go through a reflow oven (see What is SiP?). Once the module is packaged, the assembly plant solders the whole thing onto a circuit board, with SMT again.

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 to the SMT line: "Here the resistors and capacitors get soldered onto the board first. That's SMT. Then the chip is glued on. That's die attach." It's a small SMT line of its own.

In the codex's Machines list, the SMT line has "Take it apart" too, and pressing "See it work" shows you, step by step, the paste being squeegeed, the nozzle picking resistors and capacitors off the reel, and the pass through the reflow oven.

Soldering the modules onto circuit boards is the job of the customer's assembly plant. Tap "Clock in", choose "A day in the fab", tap "Test-and-assembly house" on the "Your role today" card and swipe to the customer liaison (the shift follows the time on your device). On the late-night shift, the customer's assembly plant calls: their machine will pick the chips off the reels and place them on boards, but the direction of pin 1 doesn't seem to match. On the morning shift, the customer sends back an engineering sample: the bag was opened three weeks before it went on the board, and it wasn't baked dry before the oven.

There's no assembly plant to walk into in the game; the test-and-assembly house, the chip Glowmere, the customer and the people are fictional. How SMT works comes from public, general sources, with temperatures and speeds given only as rough magnitudes. For other jargon, see Chip fab jargon in plain English.