How are chips shipped? Tape and reel, dry pack and moisture sensitivity levels

Most chips ship like this: tested chips go one by one into the small pockets of a plastic tape that is wound onto a reel, called tape and reel. Parts that are sensitive to moisture are baked dry first, then sealed in a moisture barrier bag with a pouch of desiccant and a humidity indicator card, called a dry pack. The bags are labeled, boxed and sent to the customer's assembly plant, where machines solder the chips onto circuit boards one by one.

Why not just put the chips in a bag and send them?

First, the parts on a circuit board are placed by machines, and each time a machine has to find the chip in the same spot, facing the same way. Second, the black plastic around a chip slowly takes on moisture, like a cracker left out of its packet; soldering it onto a board heats it to well over 200 °C, and the water it has soaked up causes trouble. So shipping has to look after two things at once: which way the chips face, and keeping them dry. For how a chip gets its package, see "What is chip packaging?"; this work is often done at a test-and-assembly house, see "What is an OSAT?".

How are chips shipped, step by step?

Step 1: ship only parts that passed test

• What happens: packaged chips go through final test first, and only the good ones move on (see "How are chips tested?").
• Why: once they're in the tape and sealed in a bag, there's no picking them out one by one.
• Without it: a bad chip is only found after it's soldered onto a board, and taking it off to replace it is a lot of work.

Step 2: one by one into tape and reel (or tubes, or trays)

• What happens: a plastic tape has a row of pockets pressed into it. Each pocket holds one chip, all facing the same way; a thin film is sealed over the top, and the tape is wound onto a reel, often a few hundred to a few thousand chips per reel. Some chips go into long plastic tubes instead; some larger ones sit in trays with a grid of pockets.
• Why: the customer's machine uses a row of small holes along the edge of the tape to pull it forward one pocket at a time, peels off the film, and picks up each chip to place it on the board. Because every chip faces the same way, the machine knows which side "pin 1", where the pin numbering starts, is on.
• Without it: if the chips face the wrong way, the machine places them anyway, and the board may not work or may even burn out.

Step 3: check how much it fears moisture

• What happens: every kind of plastic package that sits on the surface of a board has a moisture sensitivity level, MSL for short. It is measured during development: samples soak up water at a fixed temperature and humidity, go through an oven at soldering temperature a few times, and are then checked for cracks and layers coming apart, and whether they still work electrically (for other tests, see "What is reliability testing?").
• Why: size, thickness and materials change how sensitive a package is. The level decides whether it needs a dry pack and how long it can sit out once the bag is opened.
• Without it: the customer can only guess, and a wrong guess can mean cracks during soldering.

Step 4: bake the sensitive ones and seal them in a moisture barrier bag

• What happens: the least sensitive level can ship as it is. The others are usually baked dry first, to drive out the moisture they've already soaked up. Then the reel goes into a bag that moisture can't get through, with a pouch of desiccant and a humidity indicator card; some of the air is sometimes pumped out, and the opening is sealed shut with heat.
• Why: the desiccant soaks up the moisture left in the bag, like the little pouch in a packet of crackers. The humidity card has small dots, each for one humidity level; when the inside gets that damp, the dot changes color, most often from blue to pink, so whoever opens the bag can see at a glance whether it's still dry.
• Without it: the chips take on water day by day on the road and in the warehouse, and arrive at the customer already damp.

Step 5: label everything

• What happens: every reel, bag and box gets a label: part number (which chip it is), quantity, lot number and date. A moisture-sensitive bag also gets a caution label on the outside: the level, the bag seal date, how long you have to solder the parts after opening, the highest soldering temperature the part can take, and how to bake it if the time runs out.
• Why: the label tells the customer how long the parts can still sit out; if something goes wrong, the lot number tells you which batch it was.
• Without it: with one box empty, say the seal date, the customer can't work out how long the bag is still good for.

Step 6: box it and send it to the customer's assembly plant

• What happens: the bags are boxed and loaded on a truck. The customer checks the labels and the humidity card first, then loads the reels onto the machines (for how chips are soldered on, see "How are chips soldered onto circuit boards?").
• Why: the box protects against knocks and makes counting easy.
• Without it: a crushed box or a torn bag undoes all the work of keeping the parts dry.

Why do plastic packages fear moisture?

The plastic around most chips is a material called epoxy resin. The longer it sits out and the damper the air, the more water it takes in.

The trouble comes at soldering. The board and its chips go through a reflow oven and are heated until the solder melts, at over 200 °C. The water inside the package turns to steam all at once, expands enormously, and builds up a lot of pressure from the inside. In mild cases, the plastic is pushed away from the chip or the board inside and no longer sticks; this is delamination. In bad cases, the whole package cracks. It's a lot like popcorn: the water inside a corn kernel turns to steam when heated and bursts the shell, which is why it's called popcorning.

Where the layers are pushed apart, the wires or pads inside can be pulled away and the chip stops working on the spot; or it may work for now and fail later, once moisture creeps in through the gap. Lead-free solder needs a higher soldering temperature, so this calls for even more care.

Why is there a time limit after the bag is opened?

As soon as the bag is opened, the chips start taking in water again. So each level sets a time: once the bag is open, how long the parts can sit in ordinary factory air before they must be soldered. This is called floor life.

Moisture sensitivity level	Roughly how long after opening the bag
Level 1 (least sensitive)	No limit, and no dry pack needed
Level 2	A year, or a few weeks
Level 3	About a week
Levels 4 and 5	About one to three days
Level 6 (most sensitive)	Must be baked before every use

These times come from public industry standards and assume ordinary factory temperature and humidity; somewhere hotter or damper, the time is shorter. The "half levels" in between are grouped together here.

If the time runs out, or the humidity card has already changed color when the bag is opened, the parts are baked first: a few hours at a temperature a little hotter than boiling water, or a day or two for thick packages, and then the clock starts again. The plastic of the tape can't take that much heat, so sometimes the chips are moved into heat-proof trays to be baked, or baked at a much lower temperature for several days, or even weeks. Parts left over go back into a very dry cabinet, or are resealed in a bag with fresh desiccant.

If something goes wrong, how do you find which batch it came from?

The lot number on each reel leads back to the batch at the test-and-assembly house, and that house's records lead back to the lot of wafers at the fab, where MES recorded every step it went through (see "What is MES?"). So if one chip on a board has a problem, you can trace it all the way back: which other reels came from the same batch and where they went, stop them first, then look for the cause.

In fab24hr

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. Three of the four shifts touch on this article:

• The evening shift is chapter 12 of the main story, The life of a chip, "The Night Before Shipping". The last item on the pre-shipment checklist is the dry pack: the labels are checked bag by bag in the shipping area (by you, if you picked up the sample label earlier; otherwise by Mei-Hua from QA), and on one reel the seal date box is empty.
• On the morning shift, the customer sends back a sample that wouldn't power on after it was soldered onto a board. With Fang in the reliability lab, you find it was popcorning: a fine crack on the outside, delamination on the ultrasonic scan, and a sample bag that had been open for three weeks before the part went on a board, without being baked before the oven.
• On the late-night shift, the customer's line calls in the middle of the night: the chips in the tape don't seem to face the way their machine is set up for.

The test-and-assembly house, the chip Glowmere, the customer, the people and the numbers in the game are fictional; what tape and reel, dry packs and moisture sensitivity levels are, and why they're done this way, follow public, general sources. For the customer liaison's job, see "What does the customer liaison at a test-and-assembly house do?"; for other jargon, see Chip fab jargon in plain English.