Energy Dispersive Spectroscopy is like getting a fingerprint on your electronic component failure root cause.

So you’re having a problem with a printed circuit board assembly (PCBA). You’ve done all you can to narrow down the failure site, but you’re at the limit of the capabilities your equipment has available to you. What do you do now?

You see it, there’s something on your assembly that shouldn’t be there. Maybe it’s only one return that has the problem. If it’s a household product, you can probably ignore it. But if it’s in a critical market (aerospace, medical, automotive, etc.), your customer wants to know what it is, and how you’re going to prevent it from occurring again.

Or it could be that you’ve seen several of these failures recently and you need to know the level of your company’s exposure/risk or your customer is demanding immediate containment and a long-term fix.

Diagnosing the Problem Using Energy Dispersive Spectroscopy

With EDS (Energy Dispersive Spectroscopy) you can determine if the contaminant is organic or not, and what elements are present. This in itself can often help point to the source of the problem, but it’s an organic material, you should continue with the analysis using FTIR (Fourier Transform InfraRed).

By submitting the sample to a spectrum of the IR band and monitoring the frequencies that are absorbed and those that are reflected, a “fingerprint” of the sample can be obtained.

This technique has been around for decades and a vast library of “fingerprints” has been collected. An electronic failure analysis lab will have access to this library and can match your sample’s fingerprint to it. Thereby, determining its chemical composition.

Suppose it comes back as a flux, but your company uses three different types of fluxes. By submitting samples of the three fluxes to an electronic failure analysis company, their fingerprints can be compared to the original sample and the culprit identified.

You now possess the data you need to resolve the issue successfully. As a bonus, any good failure analysis report will provide the logical step-by-step details to support your conclusion, leading your customer to the same conclusion you arrived at.

Using a well-reputed third-party lab gives your customer confidence in the analysis because the data is unbiased.

Microelectronics X-Ray imaging allows an analyst to see the inner workings of a device without disturbing its physical integrity.

What is Microelectronics X-Ray Imaging

Most modern electronic devices are packaged as proverbial “black boxes.” It is nearly impossible to tell what is happening inside a device by looking at the outside packaging. What’s more, many devices are designed to be virtually impossible to open without causing irreversible changes to the product.

These types of electronic devices pose a unique problem for a microelectronics failure analysis lab – without being able to see the functional pieces of a device, it is nearly impossible to find a failing component or signal.

X-Ray Imaging Can See Without Destroying

While there are a plethora of destructive techniques available, allowing the analyst access to the “guts” of an electronic device, these techniques often carry with them a certain level of risk; destructively opening an integrated circuit or another assembly can, in very rare cases, induce damage.

To help prove beyond reasonable doubt that any damage an analyst finds was pre-existing and not created during the course of the analysis, a non-destructive way of looking inside the black box is necessary. X-Ray imaging lends itself perfectly to this application, penetrating the shroud surrounding most devices with ease.

X-Ray Imaging for Failure Analysis

The x-ray imaging systems used for failure analysis work in much the same way as those used for medical procedures, albeit at a much lower power level. By using an x-ray source and detector, an analyst can study the internal structure of a device to look for defects in the same way a doctor might study an x-ray to look for fractured bones.

Depending on the type of device and the reported failure condition, microelectronic x-ray imaging may be used to look for many different things. When studying an integrated circuit, for example, the x-ray can easily reveal problems with bond wires or flip-chip bumps, often showing open-circuit or short-circuit conditions and eliminating the need to open the package at all. Indeed, in some cases – for example, in the case of adjacent bond wires touching due to wire sweep during packaging – traditional decapsulation of the device can remove any evidence of the failure altogether!

X-ray imaging can also be useful for failure analysis of printed circuit assemblies. Since most modern circuit boards use multiple layers of conductive traces to route signals from point to point, it is not always possible to visually trace the electrical path between components. Since the x-ray can reveal all layers of a board simultaneously, following a signal and pinpointing a failure site is much more straightforward. Furthermore, some defects that may not be evident on visual inspection, like improper via drilling or component misregistration, can be identified much more readily with x-ray imaging.

Summary

Non-destructive testing (NDT) – gathering data about a sample without causing any irreversible harm or change – is one of the most important steps of failure analysis. By allowing an analyst to study the internal machinations of a sample without disturbing its physical integrity, x-ray imaging is an integral part of the NDT process.

In this post, you will learn how an electronics failure analysis lab uses a FIB for Wafer Lot Acceptance during design verification.

In the current era of System-on-Chip (SoC) designs with 10 and 11 metal layers, copper metallizations, exotic dielectric materials, and the use of area pads scattered across the entire die area of circuit design, FIB provides an ideal diagnostic aid.

Using a FIB for Wafer Lot Acceptance

FIB (Focused Ion Beam) technology has certainly come a long way since its introduction in 1975. I recall very well the first encounter I had with the technology as a young ASIC designer in the late 80s. It seemed the most magical thing I had ever encountered: the ability to rework semiconductor devices, not only by being able to cut metallization lines (to correct shorts, for example, as had been done previously on a mechanical probe station), but also to add new conductive paths. FIB literally provided a designer the ability to add what are essentially blue wires to correct bugs in a design, as could be done with a board-level product. FIB truly opened a whole new world.

A Little FIB History

In the early days, FIB machines were cantankerous and required a tremendous level of skill and dedication to keep alive. The handful of good operators that existed was highly sought after and comprised something of a brotherhood of alchemists. The chamber size on the early machines was small, the ability to image was limited and the control of the beam for cutting and deposition was somewhat crude, with manual control over the beam’s raster pattern provided by physical potentiometers.

I spent many hours in the company of one of those alchemists staring at the flickering green phosphor screen on an early Seiko FIB machine, looking for the telltale image bloom and screen washout that would occur when cutting through interlayer dielectrics and into the next conductive layer. And lo and behold: after a few hours of work, the prototype IC that was stillborn due to an error that I had made sprang to life. Magic!

With later generations of FIB hardware, it became possible to integrate voltage-contrast microscopy with the milling and deposition process, and the integration of tester hardware allowed devices to be actually operated upon while running test vectors under normal operating conditions. Further development allowed the integration of the physical design database for the device into the navigation process, even allowing a specific node to be identified by name from a netlist, navigated to via the design database, imaged via voltage contrast, and then altered via FIB cuts or depositions- all in a single action.

This proved to be the designer’s best secret weapon for rapid debug and prototype bringup.

More than one design manager was heard to denigrate the benefits of FIB, stating that it made it “too easy to recover from mistakes that should never have been made in the first place”. However, FIB unquestionably saved the bacon of many a fallible designer, and its use has become commonplace.

Benefits of Using a FIB

The capabilities of modern FIB machines utterly overshadow the primitive capabilities I so revered from the 80s. In the current era of System-on-Chip (SoC) designs with 10 and 11 metal layers, copper metallization, exotic dielectric materials, and the use of area pads scattered across the entire die area of a design, FIB provides an ideal diagnostic aid. It is even now possible to perform “backside FIB”, which involves milling into the die from the substrate side (as opposed to the top metal/passivation side). This allows the operator to avoid having to cut through multiple metal layers and complex, dense routing structures, and approach active devices from below.

Taken as a whole, these capabilities have proven to be a major boon for electronic failure analysis processes. FA professionals can use the surgical precision afforded by the ion beam milling process to selectively strip back layers of overburden to reach and image very fine structures suspected of causing yield problems, infant mortality, or electromigration issues.

Exotic technologies such as Silicon-on-Insulator or III-V semiconductors pose little difficulty for modern FIB hardware. Similarly, advanced three-dimensional technologies such as FinFET or GAA (Gate All Around, or nanowire) transistor designs are handled quite well by modern FIB machines. Imaging and milling deposition resolutions have comfortably kept pace with technology steppings down to critical dimensions on the order of tens of nanometers, and the ion beam milling process is very compatible with fragile 3D structures. The technology provides debugging and diagnostic tools that were utterly unimaginable just a decade ago.

More Reasons We Love Using the FIB

Insight Analytical Labs has built a sizable practice around its state-of-the-art FEI Dual-Beam Field Emission Scanning Electron Microscope (FESEM), which provides FESEM functionality combined with a high-resolution FIB capability in a single vacuum chamber. This unit allows IAL to section and image devices with resolutions down to 5nm. It can be used to prepare samples for transmission electron microscopy (TEM), and also incorporates a scanning TEM detector, allowing the capture of much higher resolution images than possible with SEM alone.

Further, the FESEM chamber is large enough to accept any packaged IC as well as wafers up to 6”, and its internal toolset allows many routine operations to take place entirely inside the chamber without breaking the vacuum. It can perform many of the sample preparation, sectioning, and imaging tasks required for Wafer Lot Acceptance or other Failure Analysis tasks in a single pump-down, saving a tremendous amount of time and expense.

Summary

Gone are the old pots on the front panel for beam control, replaced with a comprehensive software-driven user interface. However, I suspect that the operators still have a bit of the alchemist about them: notwithstanding any possible secret handshakes, they are still the wizards of debugging, bring up, and failure analysis in the brave new nanometer world. If it is small, expensive, exotic, and not functioning as it should: FIB can help.