We use a bewildering variety of electronic components every day. Most of these depend critically upon the smooth operation of integrated circuits in one form or the other. Whether we're talking about our powerful personal computers or the humble grinder in our kitchen, ICs are at the heart of their operations.
What's truly amazing is the resilience of these devices. Most of the time, the weakest link in their lifespan is the breakdown of some physical component. The actual chips themselves rarely if ever malfunction which is quite a miracle when you think of the millions of electronic packages churned out by factories every year.
Failure analysis of defective chips is at the heart of this low rate of error. A plethora of techniques are used to probe and isolate flaws - the lessons from which are traced right back to their origin, sometimes to the design process itself!
The choice of failure analysis technique depends on the type of the fault we suspect in the electronic package. Each procedure specializes in the detection of a unique category of flaw and a process that fits the unveiling of one might be completely unsuitable for another. For example, failure analysis of electronic defects is most easily and efficiently taken care of by infrared thermography techniques due to the heat signature of various types of malfunctions. Similarly, those problems that can be verified visually by a close examination of surface features are best uncovered by a scanning electronic microscope.
What do you do however, when you suspect a structural defect buried deep within the chip that may be too tiny to meet the eye? For sure, we have procedures like X-Ray analysis, but this doesn't give the detailed level of information that we're looking for. For this kind of flaw, we need to use acoustics.
Sound waves are unique in that they utilize the substance of the chip itself to propagate unlike say a heat wave when we use Lock-in thermography. We also possess the capability to modulate the frequency of sound waves to a high degree of accuracy. This means that any variations in the material easily show up using the right type of acoustic equipment.
This also has the advantage of maintaining the integrity of the chip without destroying it. As a Non Destructive Technique (NDT), acoustic based failure analysis is an indispensable too in the arsenal of engineers looking to obtain the best confirmation of a flaw before they commit themselves to a destructive procedure that will render the chip useless for future testing.
One type of defect that lends itself to detection via acoustics is voids. These are tiny inconsistencies in the semi conducting material that resemble "holes" or gaps deep within the integrated circuit. Due to the extreme precision required for a chip to function smoothly, these voids cause all kinds of malfunctions. If they're tiny, they don't show up using x-ray scans and we need to use acoustics to pin them down.
Another type of defect frequently found in chips is delamination which is the improper binding of a die to the material. Microscopic cracks are another flaw that is optimally detected using sound waves.
Like other waves, sound is reflected when it hits any kind of change in the propagation material. By measuring these reflections, we can create a "map" of the area under observation to a high degree of accuracy. It's a more sophisticated version of sonar which is used in applications as diverse as shipping to getting pictures of unborn babies!
Due to the extreme precision required, the experiment needs to be set up very carefully. Air itself is a pretty poor conductor of sound waves at the frequencies we need, not to mention it has quite a few impurities floating around in it like dust. Liquids do a much better job which is why our hearing is greatly enhanced when we're underwater. The chip needs to be submerged in a liquid or alcohol for the sound waves to propagate optimally. This somewhat limits the application of acoustic techniques for failure analysis to those items that will not suffer irreversible damage by such contact.
Generating the sound wave at the desired frequency can be easily accomplished by inducing a vibration based on input alternating current. Since we have a great deal of control over the latter, we have no problems obtaining the high sound frequencies needed.
Acoustic techniques are just one example of the various kinds of innovative procedures used by failure analysis engineers to detect faulty chips and improve the performance of the various kinds of electronic circuits we use every day. It's in large part due to their unstinting efforts that we have the high degree of confidence in our electronic components that we demonstrate every day.