Monday, 20 February 2012 17:47
One of the cornerstones of non-destructive failure analysis of packaged integrated circuits, allowing an analyst a relatively simple way of examining the structural integrity of a device, is Scanning Acoustic Microscopy (SAM). By using tightly focused pulses of ultrasonic waves and analyzing the sound reflected by and transmitted through a sample, it is possible to create a detailed, accurate image of a packaged semiconductor device, showing any pockets of air or delamination that may contribute to early-life failure. SAM has been an invaluable tool in performing analysis on the types of parts that have traditionally been the most prevalent in the industry - plastic encapsulated, wirebonded ICs. Though the industry may be shifting away from these types of devices in favor of packaging technologies like flip-chip ball grid arrays (FCBGAs) due to the more efficient use of bonding space and potential for increased thermal compensation, the SAM is not obsolete; indeed, with a few changes, SAM can provide invaluable data on these cutting-edge technologies.
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To someone unfamiliar with failure analysis of integrated circuits, it can be extremely difficult to imagine how any sort of meaningful data can be produced from a non-functioning piece of electronics – especially when the problem description is often phrased in nebulous terms, sprinkled heavily with empty words like “broken” and “defective”. Yet, in many cases, a good analyst can turn these imprecise terms into a finely honed insight into a particular defect or device. One may ask how this is possible, given the extreme complexity of modern semiconductor devices. In IAL’s case, the answer lies in a well-planned IC failure analysis lab flow that takes a device from initial observations to final reporting.
One of the most challenging cases of PCB failure analysis is the search for an open circuit. Navigating the maze of metal interconnects with probes and an ohmmeter is time-consuming and, frustratingly, often ends without bearing fruit when an analyst encounters a component like a ball-grid array (BGA), with concealed connections that prevent further probing. At this point, the analyst is stuck; removing the component by desoldering would remove any evidence of an open circuit, and a blind cross section has low odds of success unless the component has large numbers of open solder joints. In such occasions, dye penetrant testing can be used to detect any solder defects, revealing broken or non-wetted joints at the expense of further testability.
Generally speaking, most discussion of electronics failure analysis is geared towards finding silicon-based integrated circuit defects. The reason for this is fairly straightforward; silicon is, by far, the most prevalent semiconductor used to create modern electronics, and therefore has the lion’s share of defects associated with it. In some cases, however, silicon circuits are simply insufficient for a given application – especially when extremely high frequency applications are considered. In these cases, it is much more common to use a III-V semiconductor like gallium arsenide (usually referred to as GaAs). Though the high frequency performance of III-V devices may be much greater than their silicon counterparts, their unique construction poses some difficult challenges for a failure analyst hoping to dig into their inner workings.
Performing a detailed failure analysis on electronic circuits requires a wide variety of tools, many of which are targeted at isolating a defect to a single point in the labyrinthine network of metal and polysilicon that make up an integrated circuit. The vast majority of these tools require the failing device to be electrically biased in its failing condition, at which point data is gathered about the part’s condition – thermal measurements are taken, light emitted from the circuit is gathered, and so on. Often, these tools are sufficient to find a failure; some defects, however, do not appear as readily under these methods of investigation. In these cases, it is often necessary to use a different class of tool, which uses an outside stimulus to create a change on the device, then measures the device’s reaction.
PCB failure analysis can be a daunting task in even the most ideal of cases. Modern printed circuit boards are densely-packed, multilayer rat’s nests of copper interconnects, integrated circuits, and discrete components. Isolating a single defect - which may often be a single splash of solder, misregistered via, or cracked copper trace - is an arduous process, requiring hours of probing and isolation to finally narrow down the point of failure. This process is taxing, to say the least; however, the problem is often compounded when the device to be analyzed is no more than a twisted, blackened hunk of burnt PCB material.
One of the benefits of a thorough failure analysis is the ability to properly classify a given IC defect, identifying its most likely origin and determining what caused it. With this data, a manufacturer can determine the proper course of action necessary to respond to the failure. If the defect arose from improper use, then the manufacturer can provide feedback to their customer, letting them know that they may have an inherent design flaw; on the other hand, if the defect is found to be related to the manufacturing process, it becomes necessary to evaluate the potential impact on other product manufactured during the same time frame.
One of the most pivotal points of any IC failure analysis is the process of electrical characterization. In order to correctly understand a failure and choose the proper course of action to find its root cause, it is vital to understand the failure’s electrical signature; for example, analysis of a short circuit will follow a far different path than an FA targeting an open circuit. Since it is is so crucial to properly understand the electrical characteristics of a failure, a good FA lab will have a comprehensive semiconductor test program in place that can handle a wide variety of devices.
One of the most critical points of any failure analysis is the decapsulation step. Decapsulation is the point where non-destructive analysis ends and more risky operations begin – the die is removed from its protective plastic shell to allow an analyst access to the complex circuitry within. Usually, decapsulation is performed using wet-etch procedures, dissolving the plastic encapsulant material of an IC package with any of a variety of different acids or solvents. The downside of this approach, of course, is that working with these potentially hazardous chemicals necessitates some serious safety measures like fume hoods and other types of personal protective equipment. Furthermore, the chemicals most often used for decapsulation, though relatively common, are still not cheap, and can amount to a significant expense depending on the number and type of parts that must be decapsulated. Most importantly, the chemical decapsulation process can often disrupt the failure on the part; in some cases, like when working with GaAs or some other III-V semiconductors, the decapsulation chemicals can even dissolve the integrated circuit completely! Fortunately, there is an experimental alternative to chemical decapsulation: laser decapsulation is one of the most promising new technologies on the horizon.
One of the most powerful tools at a failure analyst’s disposal for non-destructively studying the integrity of a component’s packaging is scanning acoustic microscopy. By using ultrasonic waves, the scanning acoustic microscope can detect cracks, air gaps, or delamination with relative ease. There is one caveat to the results from scanning acoustic microscopy, however; in many cases, seeing is believing, and an acoustic image does not necessarily quench the burning desire to view the defect directly. Many manufacturers requesting acoustic imaging services may call the results of a test into question (especially if the result is not one they find favorable); in these cases, it may be necessary to provide another, more tangible piece of evidence.
Oftentimes, discussion of failure analysis services for semiconductor devices tends to focus on the most complex of devices – microprocessors with millions of transistors, intricately designed printed circuit boards, or the fantastically precise silicon sensors called MEMS (micro-electro-mechanical systems). The reality of the industry, however, is that the vast majority of electronic components are far more simple, running the gamut from passive discretes like resistors and capacitors to simple active components like light-emitting diodes (LEDs) and power transistors. Almost every television remote control system, for example, uses a combination of LEDs and photodetectors to allow a user to channel surf. Despite their relative ubiquity, these types of components are just as susceptible to failure as any other – and, therefore, failure analysis can be just as useful in their improvement.
Modern consumer electronics are constantly subjected to all types of environmental abuse. They may operate in humid climates, with plenty of ambient moisture that can collect on sensitive circuits. Dust and other particulates can be sucked in by air intakes, introducing any number of organic contaminants onto a device. There is also the omnipresent danger of sugary, carbonated beverages – one of the most diabolical nemeses of electronics in the home, especially a home populated with children (or clumsy adults). All of these things can cause an electronic device to malfunction; fortunately, Auger spectroscopy can help an analyst determine whether these factors are truly the root cause of a given problem.
The failure analysis blog we run at IAL has been up for a little over a year now, during which time we’ve covered a handful of different topics pertaining to the services we offer and how we can be of benefit to our customers. We’ve striven to provide a no-nonsense, plain English description of what failure analysis is, as well as straightforward explanations of our equipment – how it works, what it is best suited for, and what its limitations may be. In doing so, we hope to make the general operation of our lab accessible to the widest audience possible, from the most experienced engineer to someone who may only have a passing interest in electronics FA.
Often, failing systems are so complex that it can be difficult to find a good starting point. A circuit board may be hundreds of square inches of densely packed discrete components, integrated circuits, and wiring; a schematic view may be so intricate as to require several feet of paper to print out. In these cases, electronic component failure analysis gains a whole new aspect of complexity; an analyst must be able to isolate the failing component amongst a plethora of other devices. At first glance, this may seem to be a Herculean task – devising a test program to analyze all the thousands of different components on a board is no easy feat. Fortunately, with the right approach, such an endeavor is not necessary.
Like any problem-solving endeavor, failure analysis can often be stopped cold by a seemingly insurmountable obstacle; electrical isolation techniques fail, deprocessing proves too challenging, or (worst of all) the failure mode simply disappears, vanishing into thin air like a magician’s cheap parlor trick. Any of these situations will give the dreaded result of “No Fault Found”, which essentially means that no valuable information was gleaned from the analysis. Encountering a stymie like one of these in the course of an analysis can elicit wailing and gnashing of teeth from even the most intrepid engineers; at times like these, it is often necessary to enlist the help of external failure analysis services to provide a fresh look at the problem.
The process of initially undergoing RoHS certification can be a daunting one. Every piece of a product – from the largest circuit board down to the smallest wire – must be accounted for, to ensure that any of the named hazardous substances are kept to an absolute minimum. For many companies, developing the capability to inspect a product to ensure it meets these stringent requirements entails a huge cost due to equipment purchases and additional hiring and training of dedicated personnel. For smaller startups, these costs may be prohibitive; however, the use of a failure analysis lab instead of onsite capabilities for RoHS certification can help to alleviate some of this cost.
One of the fundamental truths of electronics is thus: all devices generate heat to some degree. Some heat emitted by devices is normal – after all, millions of transistors, switching off and on millions of times a second, will consume sizeable amounts of power and therefore produce a considerable amount of heat. However, there are certain types of defects that increase power consumption, thereby increasing the amount of heat given off by a device. While this additional heat is immaterial to the engineering team responsible for the design and production of a device, it can provide a useful avenue for isolating the item that they’re truly interested in – the defect itself. Using thermal emission microscopy, slight differences in temperature can be turned into useful data about a device, enabling a failure analyst to drive to the heart of a defect and determine the root cause of failure.