Inside an IC; using ultrasound to detect defects. Tom Adams.
Once, long ago, you had your first glimpse of the inside of a computer. Someone lifted the monitor, removed the cover, and pointed to shiny black plastic rectangles with short silvery legs sitting on a green board. "That's the memory," you were told. And you took it on faith.
The thing about integrated circuits is that, unless you design and manufacture the things, you do pretty much have to take them on faith. Either they work or they don't. You can't repair an IC.
When an IC does fail, "overheating" is likely to be cited as the cause. ICs generate heat, and that heat must be dissipated. That is why your computer has vents and maybe even a fan. But even with an active cooling system like a fan, some ICs self-destruct. This is what manufacturers call "premature failure in service," and it has a great deal to do with the way ICs are made. Physical Characteristics
The active part of the IC package is the chip itself, that precocious sliver of silicon. Also embedded in the black plastic package is the lead frame, a flat metal cut-out. In the center of the frame is a square called the "flag" from which radiate leads.
During manufacture, the chip is attached to the flag of the lead frame. This sounds innocent enough, but jut how the chip is attached has profound implications for the future of the chip.
After the chip is in place, very fine gold wires are installed to bridge the gap between the relatively huge legs of the lead frame (which are the connections of the chip to the outside world) and tiny bond pads on the chip itself.
Next, the outer ends of the legs are bent, and the whole thing is encased in an extrusion-molded package of black plastic. The purpose of the plastic is to protect the chip from the environment--especially from moisture. The finished product is known as an IC package, or more technically a 16- or 40-pin plastic DIP (dual in-line package).
When the completed IC package is installed in a computer and power begins to flow throught it, the chip begins to generate heat. The ordinary operating temperature of a chip is somewhere between 100 and 165 degrees C, depending on its particular use.
It is important to keep that temperature under control. As a rule of thumb, the life of the chip is cut in half for every 10 degrees Centigrade the temperature rises above normal. So the question becomes: how can heat be allowed to escape from the IC package? Assembly
Let's take a close look at the IC package and the way it is assembled. One of the most important steps is the attachment of the chip to the lead frame. Typically, on the production line, a row of lead frames moves along a track, stopping first for a dab of silver-filled epoxy to be placed on the flag at the center of the lead frame, and then for the chip to be dropped on by a pick-and-place arm.
A good part of the future of your computer is determined at the moment the arm places the chip on the lead frame. Why? We'll see shortly.
When the package IC is put into service, the chip heats up, as mentioned above. Unless heat is removed from the chip and the package around it, the chip will fall victim to a condition known as "thermal runaway," in which increasingly higher temperatures destroy the chip.
You might expect heat to escape upward from the chip. A small amount of heat does follow this route, even though the plastic in which the chip is encahsed is a mediocre conductor of heat. But the lead frame on which the chip rests is an excellent conductor. Even though there are many different kinds of plastic, of course, and different kinds of lead frames, too, the lead frame does offer the easiest route by which heat can escape.
But between the chip and the lead frame is the layer of epoxy that holds the chip in place. Sometimes solder is used instead of epoxy, but the problem is the same: both epoxy and solder are relatively poor conductors of heat. Heat from the chip will escape via the lead frame--but only if it can get to the lead frame.
This is why the attachment of the chip tot he lead frame is so important. The attachment material, whether it is solder or epoxy, needs to be as thin as possible (slightly more than a thousandth of an inch is typical) and as intact as possible. Several things can go wrong:
* The epoxy may crack as it shrinks and cures.
* Voids (actually bubbles) may form--especially in solder.
* The epoxy or solder may be separated (usually because of surface contamination) from the chip or the lead frame.
Any of these defects can keep heat from escaping from the chip. And once the chip is in place, defects are very difficult to detect and image with conventional detection technologies like x-ray. X-ray will spot a defect such as the complete absence of solder under part of a chip, but doesn't usually reveal gaps. Defect Detection
Detecting defects requires a new technology--specifically, acoustic micro-imaging. High frequency ultrasound is very sensitive to internal interfaces, and it is nondestructive. What happens is that the utlrasound is partly or entirely blocked by cracks, voids, separations, and the like.
The internal-feature photographs accompanying this article were made with the Scanning Laser Acoustic Microscope (SLAM, for short), made by Sonoscan, Inc. of Bensenville, IL. Properly termed acoustic amplitude micrographs, these photos show interior views of their subjects.
ICs (and many other objects) are opaque to light, but transparent to ultrasound. The SLAM transmits very high frequency upward through an IC, and a scanning laser "reads" the level of ultrasound that arrives at the top surface. A digital signal processor assigns a color to each level of ultrasound, and the full image is displayed on a color monitor.
All ICs, because they contain internal interfaces at the lead frame and chip, produce some sort of acoustic image. Ordinarily, the acoustic image of an IC with no unintended interfaces shows faint outlines of these features. The value of the SLAM is that it reveals unintended interfaces in an IC.
Virtually all IC manufacturers use electrical tests to weed out obviously bad ICs. Some also use a SLAM to image their ICs, either before or after they are encased in plastic. They look especially for anomalies in the bond layer. If a bond layer shows only a few tiny voids, it will probably pass, because the area of a void is what matters. Many tiny voids, a single large one, or a massive separation, will cause an IC to be rejected.
The SLAM also looks at the completed IC package. Here, bubbles in the molten plastic can form voids, and cracks can occur. Any defect in the plastic is potentially serious, because it can destroy the hermeticity of the package and allow moisture to reach teh chip.
This isn't meant to be a horro story. Its purpose isn't to make you run to your computer and peek inside to see whether your ICs are sprouting delaminations, creeping with contaminants, or approaching thermal runaway. Most ICs work and keep on working. Manufacturers have developed lead frame and package designs which largely prevent overheating.
Computer stores expect most IC failures to occur during the first few weeks after the sale; these are the ICs with gross internal defects, and their replacement is generally covered by warranty. Most of the ones that survive the initial burn-in period will remain hale and hearty for the life of the machine. But if someday you do run across a dead IC, you will have a good idea of what caused its demise.