IF YOU WANT to inspect a chip, there are various ways to do it, from cheap to expensive, easy to hard. If you want to peer inside the guts of a chip package, your options are more limited, but Scanning Acoustic Microscopy is one good way to do it.
Looking inside the package of a chip, or even between the layers of transistors is hard work. There are two main tools to use when doing it, X-rays/X-ray tomography or Scanning Acoustic Microscopy(SAM). Both have their uses, X-rays being potentially more precise, expensive, and tricky to operate while SAMs are fairly easy to use and, cheaper, but less precise. SAMs are also non-destructive, something that is not always true for X-rays.
The idea is pretty simple, you take the chip you want to look at, immerse it in water, and run a beam of high frequency sound over it. The echoes will paint a picture for you just like sonar. If you pick the frequency right you can visualize images down to 10 microns, small enough for most people’s needs. The best part is that it still works when you are done with the scan, so they can be used to inspect things on the production line.
Sonoscan Gen 5 SAM
At Semicon, a company called Sonoscan had a few of their SAMs on display for chip inspection. The one above is called the Gen 5, one of the higher end models. Sonoscan SAMs range from $220,000 lab models to the brand new AW300 which will move in, scan and process 2 300mm wafers at once.
Why would you need a SAM? if you have a high cost part, or something that needs precise tolerances or absolute sealing, SAMs can look at them after the part is fully manufactured without destroying it, and tell you what is in the guts. A good example of this is the chip pictured below, with slices taken at various depths showing everything from the top of the heat spreader to the layers of the substrate.
Four slices of a chip SAM style
Things like solar concentrators where the silicon to heatsink bond is critical to prevent localized overheating and burning out are one good example. MEMS devices are also a nice fit, the resolution of a SAM allows you to see what is in a small system without having to chop it up or put it in an electron microscope. You can look at bonds, seals, and wires through the packaging, and still have functional parts afterwards.
A good example of this is if you have a high end and very expensive graphics card, and you want to know why it doesn’t overclock well, why it died, or possibly something as simple as what is under the heat spreader. You can pry the heat spreader/cap off the chip, but that often kills the part, or use a SAM and still have a working GPU afterwards, assuming it was working when you started. There are lots of interesting investigations you can do with access to one of these microscopes.
Early GT200, mind the gap(s)
For example, you find all sorts of odd things when you look under the heatspreader of a large chip. The 65nm GT200 shown above is an early production part, the dark parts are the areas under the chip ‘lid’ that has lots of thermal interface material (TIM), aka goo or thermal paste. That stuff absorbs sound very well, as do most ‘gummy’ substances.
The gray spots are voids where the sound waves passed through with far less damping. The lighter the image, the less TIM, and the white areas in the center are places where there is no TIM, top to bottom. This can lead to hot spots, poor overclockability, burned chips, and in general early death, but as always is the case, specifics parts may vary. A lot.
Late GT200, the yellow lines are the die edge
Much later on, the 55nm versions of the GT200 showed that many of the problems with packaging were solved. The uniformity of the TIM shows little to no voids, gaps, or other technical ‘whoopsies’. Progress like this is satisfying as long as you don’t already own an early part.
So, armed with a SAM, you can do a lot of interesting investigation. With a tool like Sonoscan’s AW300, you can inspect wafers in an automated way 24/7, or do things one at a time in one of the smaller versions. In any case, a scanning acoustic microscope is a very useful tool to have when doing Q/A work.S|A