CAPACITORS
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An acoustic image of surface-mounted MLCCs.
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This article addresses a particular form of MLCC damage—hairline cracks induced by panel singulation during production—and outlines an acoustic technique for detecting it prior to product assembly.
Formation of Singulation Cracks
MLCCs that are cracked during the singulation of panels into individual printed circuit boards (PCBs) are especially worrisome because the damage is caused near the end of the production cycle, when the value of each board is high. These cracks occur when the board is flexed during singulation. Typically the cracks do not extend to the outer surface of the MLCC. Therefore, they are not likely to be detected optically.
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Figure 1. (click to enlarge)Internal cracks in MLCCs caused by singulation tend to be thin, vertical, and near the terminations.
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Some of these cracks may cause sufficient electrical disruption to be detected during final electrical tests and inspection, but many of them escape detection. In service, the cracks are likely to expand as a result of normal thermal cycling of the system. The number of plates affected by a crack will thus increase.
The frequency with which singulation cracks occur may be increasing as lead-free solders are introduced to replace traditional tin-lead alloy solders. Currently, lead-free solders are required for all electronic products sold in the European Union except for those products for which an exemption has been issued. The first stage of somewhat similar legislation went into effect in China on March 1, 2007. However, the initial list of products from which lead and other hazardous substances must have been removed has not yet been released.
Globally, the most widely used lead-free solders are alloys of tin, silver, and copper. Two of these alloys, Sn96.5Ag3.0Cu0.5, known as SAC 305, and Sn95.5Ag4.0Cu0.5, known as SAC 405, probably account for well over 90% of the lead-free solder currently in use.
One of the most significant differences between traditional tin-lead and the SAC solders is in their rigidity. Tin-lead is relatively plastic—when the PCB is flexed, it will absorb some of the stress. SAC solders, on the other hand, are much more rigid—when the PCB soldered with them is flexed, more of the stress must be borne by the capacitor. Although no hard evidence is yet available, it seems likely that the use of SAC solders may increase the incidence of singulation cracks in ceramic chip capacitors.
Detecting Cracks
The problem with a singulation-caused crack is that it is very difficult to detect if it does not have an electrical signature. It is not feasible to cross-section capacitors in a production environment, and the small, extremely thin cracks are impossible, or nearly impossible, to find with x-ray technology.
If a board that has previously exhibited few or no MLCC-related failures suddenly begins to sustain singulation cracks, the manufacturer is unlikely to become aware of the problem until field failures begin to occur. Even then, much work may be necessary before the problem can be definitively linked to the MLCCs, because the nature of the field failures may make diagnosis difficult. In some instances, the MLCC-related failures may mimic software problems. Diagnosis may of necessity be spread over a long time period, because singulation cracks grow at various rates. Therefore, field failures are likely to be intermittent and to occur over a substantial time span.
An efficient and nondestructive method for finding cracks in MLCCs is acoustic microimaging. Acoustic microscopes excel at locating and imaging cracks, delaminations, voids, and other gap-type internal defects within the body of a component. Every year, millions of MLCCs intended for high-reliability applications, including medical equipment, are imaged acoustically, either before or after surface mounting, in order to inspect for internal cracks, delaminations, and voids. Any of these defects can degrade the long-term performance of the system. Fortunately, they are all excellent reflectors of ultrasound.
Acoustic Imaging
Ultrasound is pulsed into an MLCC, which may be surface-mounted or loose, by a transducer that rapidly raster-scans the area of the MLCC while focusing on its interior. The speed of ultrasound within the body of the MLCC is very high; the round-trip for a pulse to be sent into the MLCC and the return echo signals to come back to the transducer takes just a few microseconds.
In most acoustic imaging, a time window, also called a gate, is created. This gate accepts return echo signals from the desired investigational depth and ignores echo signals returning from other depths. For many applications, the time window may be quite narrow. However, routine acoustic imaging of MLCCs uses a time window that extends from a depth just below the top surface of the capacitor to a depth just above the bottom surface of the capacitor—in other words, the entire bulk of the capacitor. The ultrasonic pulse is typically focused at the midpoint of the capacitor's thickness.
Because of the high speed of ultrasound within the capacitor, the transducer can perform its pulse-receive function several thousand times per second while raster-scanning the MLCC. What this means is that the transducer sends a pulse of ultrasound into the target and receives the return echoes from various depths within the sample. The time for the round-trip of the pulse from the transducer to an internal feature and back to the transducer is measured in nanoseconds. This measurement is sometimes used, when the acoustic velocity of the material is known, to determine the depth of a particular feature.
During the time it takes for the pulse of ultrasound to make its round-trip, the lateral motion of the transducer is very slight. The location of a single pulse-receive set can be thought of as one coordinate set in the x-y scanning of the sample. The return echo signal received at each spot becomes one pixel in the acoustic image that is displayed as scanning progresses. One acoustic image may contain data from thousands or millions of x-y coordinates.
An ultrasonic pulse sent down into a sample is reflected only by material interfaces. Homogeneous materials do not reflect it. The degree of reflection is determined by differences in the acoustic properties of the two materials at the interface; the greater the difference, the greater the amplitude of the reflection.
A person might very well think that an MLCC would be strongly reflective at all depths because of the large number of material interfaces that exist between electrode and dielectric. In practice, however, there are essentially no reflections. This is because the electrode material and the dielectric material are very similar acoustically, and because both capacitor materials are extremely thin. Subjected to pulsed ultrasound, a defect-free MLCC behaves much like a block of homogeneous material.
That explains why, in routine imaging, the ultrasonic pulse is gated just below the top surface of the capacitor and just above the bottom surface. This pattern of gating avoids the interface at the top surface of the capacitor and the interface at the bottom surface. It also limits imaging to the bulk of the capacitor. The alternating layers of dielectric material and electrodes constitute the gated depth, and these materials are essentially featureless. If no gap-type defect is present, the acoustic image of the bulk of the capacitor will typically consist of a featureless, fairly bright rectangle with a somewhat darker, but also featureless, termination at each end.
Imaging of Gap Defects
The material interfaces that reflect ultrasound most strongly are those between a solid and a gas. When a pulse of ultrasound from the transducer encounters a crack or another gap-type defect, it encounters the interface between the bulk solid material of the capacitor and the gas within the crack.
All materials have a characteristic acoustic impedance. This impedance is the product of the acoustic velocity—the speed of ultrasound through the material—and the density of the material. For solid materials, the acoustic impedance is so large that the measurement is generally expressed in megarayls (Mrayl). The ceramic dielectric in a capacitor typically has an acoustic impedance of about 31 Mrayl. But the acoustic impedance of air and other gases is measured not in units as large as megarayls or kilorayls but in rayls, at the level of the base unit. The acoustic impedance of air is about 410 rayl.
The two materials at the defect interface therefore have profoundly different acoustic impedances. Reflection of ultrasound from a material interface is determined by the formula
where z1 is the acoustic impedance of the solid material and z2 is the acoustic impedance of air.
The result is that virtually all of the ultrasound that strikes this interface is reflected to the transducer and detected as a very-high-amplitude return echo signal. In theory, a very small portion of the pulse striking the defect interface traverses the air-filled gap and, on the far side, encounters a second interface where the air meets the bulk material of the capacitor. In practice, reflection from the far side of the gap is too weak to reach the transducer.
Cracks and similar defects that are even modestly horizontal in orientation give a strong reflection that results in bright pixels in the acoustic image. The thickness of the gap is relatively unimportant. Research carried out for the author's company by an independent laboratory showed that a gap as thin as 0.01 µm produces near-total reflection, as do much thicker gaps. The high acoustic reflectivity of gap-type defects explains why, in some applications, MLCCs are imaged acoustically before surface-mounting in order to screen out capacitors having delaminations and similar anomalies.
Imaging of Singulation Cracks
The vertically oriented singulation cracks, however, present a special challenge for acoustic microimaging. First, these cracks are typically very thin and may represent a gap of 0.1 µm or less. Second, singulation cracks are typically vertical or nearly vertical. In terms of the physical area encountered by the pulsed ultrasound, there is almost nothing to reflect the wave, because the crack is like an extremely thin knife edge aligned in parallel with the direction of the ultrasound wave. But a newly developed technique makes it easy to image and identify singulation-related cracks, even though they are vertical and near the capacitor's terminations.
In the new technique, the scanning procedure is altered such that it produces an acoustic shadow of the crack, rather than an acoustic reflection. The shadow is much wider than the crack itself. Large-angle rays that ordinarily contribute to the focal spot are diverted when they encounter the vertical crack; this causes the wide shadow.
An engineer needing to know whether panel singulation has caused termination cracks in any of the ceramic chip capacitors can use a two-step procedure. To do this, the gate, and in some cases the focus, are moved from their normal positions. How they are moved varies with the thickness of the capacitor, the acoustic characteristics of the materials in the capacitor, and the frequency being used.
The first step is to acoustically image a given capacitor using the normal reflection mode. This step will easily image all nonvertical cracks anywhere in the body of the capacitor. It may also image cracks that are more vertical than horizontal but that have enough of a horizontal component to reflect ultrasound.
The next step is to image the capacitors on the board by means of the new technique. This technique will display vertical cracks anywhere in the capacitor as shadows—even those that were missed by normal reflection-mode imaging. Nearly all vertical cracks are adjacent to, and parallel to, the termination. Singulation, along with other events that flex the board and stress the capacitors similarly, appears to be the most common cause of vertical cracks.
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Figure 2. (click to enlarge)An acoustic image of three surface-mounted MLCCs generated using the technique that displays vertical cracks as wide acoustic shadows. Arrows indicated vertical singulation cracks in capacitors. Part of a surface-mounted IC also is visible.
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The acoustic image in Figure 2, of a portion of a printed wiring board that was inspected for vertical cracks in the capacitors, includes three ceramic chip capacitors as well as a portion of an integrated-circuit (IC) package. The capacitor at left in the figure has two vertical singulation cracks (indicated by arrows), one at each end of the capacitor adjacent to the termination. The dark acoustic shadows created by the cracks largely obscure the termination itself.
The capacitor at the center of the image has one vertical singulation crack adjacent to the bottom termination. There is no crack beside the termination at the top of this capacitor, nor is there any beside either of the two terminations on the capacitor at the right side of the image.
Problem-Solving Application
When the singulation of panels into individual boards is causing vertical cracks in capacitors in volume production, the problem may not be noticed until a history of field failures or erratic device performance has begun to accumulate. Even with field data in hand, it is difficult to conclude that the failures are being caused specifically by singulation cracks in ceramic chip capacitors.
Numerous other phenomena, such as moisture in IC packages or software problems, might generate the same types of failures. And even if it is known—from acoustic imaging of failed field systems, for example—that some ceramic chip capacitors do have cracks, this still does not prove that the cracks are generated during the singulation process. The following procedure to clarify diagnosis has been worked out, however.
The first step in this procedure is to ensure that vertical cracks are not present in the ceramic chip capacitors as received from the supplier. A quantity of loose capacitors is imaged acoustically, using both conventional imaging and the new technique, to screen out any devices that have vertical cracks or any other internal defects, including delaminations and voids. This imaging process additionally provides a good measure of the quality of the capacitors being delivered by the supplier.
Capacitors that pass this screening process are then used in production—that is, they are surface-mounted and subjected to reflow. After reflow, however, the panels are not singulated. Instead, the intact panels are imaged acoustically to determine whether cracks have appeared in the capacitors during the reflow process.
If no cracks are found during the acoustic imaging of the panels, the panels are then singulated. Care must be taken to reproduce the conditions that may have created cracks in the original product that experienced field failures.
Finally, the singulated boards are examined acoustically to determine whether the singulation process has caused vertical cracks in the capacitors. If vertical cracks do appear at this point, it can be concluded that the singulation process is placing undue stress on the capacitors and should be modified to avoid generating vertical cracks.
After the singulation process has been analyzed and modified, elements of the acoustic microimaging procedure can be used in future inspections to determine whether vertical cracks are being formed and to ensure that no MLCCs with cracks become components of medical devices.
Conclusion
Singulation cracks in MLCCs have been a form of component damage not generally detected until equipment performance degradation is analyzed. In the interest of both product quality and patient safety, a means of detecting vertically oriented cracks in these chip capacitors prior to assembling such defective components into a piece of medical equipment is highly desirable.
The standard method of acoustic microimaging has been adapted such that, through an adjustment in the scanning procedure, ultrasound pulses make these cracks visible. Equipment manufacturers now have a tool that should minimize the incidence of field equipment failures.
Tom Adams is a freelance writer, photographer, and consultant based in New Jersey. He can be reached at teadams@earthlink.net.
