Non-destructive acoustic micro imaging of package seals by Tom Adams

Seals, which close a medical or pharmaceutical package, are designed to maintain its contents in a sterile condition until use. Imperfections in the seals can allow bacteria or other contaminants to reach the contents, with potentially dangerous consequences. The technology of acoustic micro imaging, which has become widely used in the microelectronics industry, has recently demonstrated its ability to image and analyze seal integrity non-destructively.

In microelectronics, acoustic micro imaging is employed to look for defects in integrated circuit packages, the familiar black ‘computer chips’ found in computers and innumerable other systems.

Medical and pharmaceutical packages have more in common with integrated circuit packages than merely the term ‘package.’ The materials frequently used - various polymers and metals - are similar. So is the layered structure.

Acoustic micro imaging is probably not suitable for high-speed production lines in the packaging of medical devices and pharmaceuticals, but it is extremely useful in the design and prototyping of packages and in batch-sampling of production. It is, however, used in microelectronics production lines, where throughput speeds are less.

High speed reflection of ultrasound In the commonly used transmission mode, acoustic micro imaging employs an ultrasonic transducer, which scans the package while alternately pulsing ultrasound into the package and receiving the return echoes.

The rapid scanning motion of the transducer is possible because the speed of ultrasound in both directions is very high - around 3000 metres per second in plastics, for example. Ultrasound is pulsed into the package, reflected, and travels back to the transducer in a few microseconds. While scanning, the transducer switches between pulsing and

receiving several thousand times per second. Ultrasound will not travel through air, other gasses, or a vacuum, so the transducer is coupled to the sample by a liquid, usually water. The sample may be placed in a tank, or a tiny ‘waterfall’ nozzle, which rides on the transducer, may be used to create a small water jet, which greatly reduces the exposure of the sample to water.

A package might, for example, consist of a polymer layer sandwiched between layers of metallic foil. Ultrasound will travel downward through the first layer - the metal foil - without sending back any return echoes. But when ultrasound reaches the interface between the metal foil and the polymer below, a portion of the ultrasound is reflected back to the transducer and is collected. The remaining ultrasound continues to travel downward, where it will be reflected by deeper interfaces.

Gaps generate strong echoes These events occur only if the metal foil and the polymer are well bonded to each other. If there is a gap - a delamination, disbond, or void - between the two layers, all of the ultrasound is reflected back to the transducer. Since the transducer is scanning the package at high speed, it rapidly collects the very strong echoes sent back by gap-type defects. In the display acoustic image, gap-type defects will be visible in the highest contrast, and well bonded internal features will be visible in medium contrast.

The ultrasonic frequencies used cover a wide range. Transducers pulsing at the very low frequency of 10 MHz are used when a package is relatively thick, because lower frequencies provide better penetration.

Transducers above 200 MHz are used when penetration is not critical and when a high- resolution image is needed, because resolution improves as frequencies move up.

Gap-type defects containing air or another gas at any depth will reflect all of the ultrasound. If the bulk of a polymer layer, for example, contains voids, the voids will reflect the ultrasound even if the boundaries of the polymer layer are well bonded to other layers. This is why paper materials cannot usually be penetrated by ultrasound; the paper itself contains thousands of tiny voids. But ultrasound can usually penetrate a material which is bonded to paper.

Voids and channels Voids and other anomalies in the seal of a medical or pharmaceutical package may not be a threat unless they can form a channel by which bacteria or contaminants can breach the protection which the package is designed to provide. A significant channel may be very narrow. At high frequencies, ultrasound images features measure as little as several microns. Even at lower frequencies, significant features are likely to be noticed because ultrasound is capable of detecting features which are too small to be resolved.

Figure 1 is the reflection-mode acoustic image of a blister pack constructed of two polymer 1 ayers. In reflection-mode imaging, the display image can be made using only those return echoes from a specified depth in the sample. Echoes from other depths are ignored. This technique, called electronic gating, was used to make this image; gating was on the seal between

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the two polymers. The large black circles are the blisters. Grey areas of the seal are well bonded, but the irregular white areas, which reflected all of the ultrasound, are delaminations between the two layers. Some of the delaminations form channels running from one blister to the next.

Finding gaps at any depth Acoustic micro imaging is most frequently used in the reflection mode, but a second technique, called Thru-Scan, has been found to be very useful when imaging medical and pharmaceutical packages. Thru-Scan, and the reflection-mode acoustic microscope itself, were developed by Sonoscan Inc (Elk Grove Village, Illinois, USA).

In reflection-mode imaging, a portion of the ultrasound, after travelling through various interfaces, exits from the bottom of the package. Thru-Scan places a second transducer below the package. This transducer, scanning the bottom of the package while the upper transducer collects reflected ultrasound, collects ultrasound, which exits the bottom surface.

Since ultrasound pulsed by the upper transducer will not travel through gap-type defects, any delamination, void, or other gap will produce an acoustic shadow in the resulting Thru-Scan image. Thru-Scan is a quick, very effective method of determining whether a defect exists at any depth within a package. In imaging medical and drug packages, Thru-Scan is often used first; if a defect is identified, reflection-mode imaging is then used to gate to a specific depth and examine the fine details of the defect.

The same blister pack, but imaged by the Thru-Scan technique, is shown in Figure 2. Black areas are acoustic shadows representing gaps which prevented ultrasound from reaching the collecting transducer below the blister pack. The blisters themselves are black, as are the channels connecting one blister to another. In addition, there are irregular gaps which extend from both rhe inner and outer edges of the sealed area. This image displays less detail in the bonded internal interface, but it makes significant defects much more conspicuous.

A portion of the seal of a two-layer foil package is shown in Figure 3. Imaging was performed by the Thru-Scan method. The bright feature is the seal; dark areas are unbonded regions on either side of the seal.

There are no outright breaks in the seal, but the width of the seal varies, and in a few places channels lead partway into the seal.

Figure 4 shows a similar seal, here imaged in the reflection mode. Two anomalies have been induced in the seal. One, in the upper half of the image, nearly breaks through the seal; the second, in the bottom half, breaks completely through. The interior of the package (the large grey area) is filled with air, as are the two breaks in the seal.

Imaging very small defects Even very narrow channels may constitute a pathway by which bacteria or contaminants can enter a package. How narrow a channel can be imaged by acoustic micro imaging depends in part on the frequency of the transducer used.

Low frequency transducers (10 MHz, for example) penetrate relatively thick materials, but have relatively low resolution. The highest frequency transducers (above 200 MHz) penetrate less, but permit very high resolution. For best resolution, the highest frequency which will penetrate to the depth of interest is generally used.

The long development of acoustic micro imaging in the semiconductor industry has already yielded techniques capable of imaging very small features. Semiconductor packages are, very roughly, similar in overall thickness to the seals of many medical or pharmaceutical packages, and frequently consist of a larger number of layers of dissimilar materials that medical or pharmaceutical package seals.

Application in semiconductor packages A frequent use of acoustic micro imaging in semiconductor packages is the imaging of multiple voids in the bonding of a solder connection where the diameter of the bond is around 0.004 inch (100 microns); the individual voids in this case are on the order of a few tens of microns. Investigations into medical and food package seals have imaged channel leakers with widths in the 1 to 10 micron range. In all of these instances, the channel is a gap-type defect which reflects (or, in Thru-Scan mode, blocks) all of the ultrasound; some semiconductor features, however, consist not of gaps but of features which are out of place. Such features, which represent well bonded internal interfaces, are routinely imaged.

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Acoustic micro imaging versus X-rays Acoustic micro imaging and x-ray imaging methods, while superficially similar in that they depict internal features non-destructively, are fundamentally very different and overlap only slightly in their applications. X-ray can image an internal void (to pick one example) if the void is thick enough. Too thin a void - or disbond or delamination - makes it very difficult for x-ray to detect the small change in intensity. Very high frequency ultrasound, however, detects voids and other gap-type defects no matter what the thickness. Some years ago scanning electron micrographs of sectioned samples demonstrated that ultrasound was completely reflected from gaps as thin as 0.1 micron. New research at S onoscan has recently suggested that ultrasound is completely reflected by gaps as thin as several hundred Angstroms.

Transmitter Acoustic Image

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Several methods - dye penetrants, vacuum testing, destructive analysis - are used to determine the existence of anomalies in the seals of drug and medical packages. Generally these methods can establish the existence of a channel or a leak, but they are less successful at characterizing the location and structure of an anomaly.

Advantages of acoustic micro imaging

The advantage of acoustic micro imaging is that it images the internal anomaly non- destructively, both in the Thru-Scan mode and in the more analytical reflection mode. After acoustic imaging, the package remains intact and is available for other types of analysis.

Because the imaging process is both rich in data and non-destructive, acoustic micro imaging is ideally suited for two environments _ setting up new production lines and batch sampling from existing lines.

In characterizing new production lines, the technology rapidly identifies and analyzes sealing anomalies. In batch sampling of seals from existing lines, it serves as a monitoring system which finds potentially troublesome defects in seals.

Contact: Steve Martell, technical manager, Sonoscan Inc, 2149 East Pratt Blvd. Elk Grove Village IL 60007. USA. Tel: +I 847 437 6400; Fax: +l 847 437 1 mail: smartelIQsonoscan.com.

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