Wrap it Up
Eventually each chip needs to be wrapped up in a package so it can be handled and shipped without damage. Most chips are housed in plastic packages that look like little black houses. (Chips are universally black; it's traditional.) The bare silicon is not simply dipped in melted plastic or sprayed with a covering because that would damage the delicate device. Instead, they're packaged clamshell style, as shown in Figure 4.9. First the chip is laid in the lower half of a plastic package, usually on a spot of glue to hold it still; then the upper half of the package is cemented to the bottom. A hollow cavity in the middle leaves room for the chip and avoids crushing it.
Figure 4.9. This plastic package contains one silicon chip. The chip is buried inside the plastic for protection. The seam along the edge shows where the two parts of the plastic package were sealed together. The eight metal “legs” lead to tiny wires that connect to the chip's own wiring. Courtesy of National Semiconductor Corporation. Used with permission.
Finally, the outside of the package is marked with some lettering, one or two identifying part numbers, a date code that indicates when the chip was manufactured, and, if there's enough room, a company logo. The lettering is usually just stamped on with ink, but large chips are sometimes marked with a laser that burns the letters onto the outside of the package. This latter process is fascinating to watch and makes good promotional video footage for the companies that use it.
After the chips are packaged, they'll be completely tested once again. A few chips are damaged during packaging so it's important to make one last check to be sure everything's in order. If any chips fail at this point, they're just thrown in the trash.
A few chips never get packaged. Some very high-volume customers prefer to receive their chips “naked.” These are called known-good die (KGD). If a customer prefers to work with KGD it's usually because they're combining several bare chips together into something called a multichip module (MCM). MCMs are like chip apartment houses: several chips sharing a single package. An MCM package is comparatively large, about the size of a matchbox. Usually military contractors or aerospace companies build MCMs because they can pack the chips very densely together and shield the entire assembly against radiation, such as a spacecraft might encounter in orbit.