NDT applications of scanning acoustic microscopy

Nigel J. Burton, BSc(Eng), CEng

Indexing terms: Nondestructive testing, Instrumentation and measuring science, Microscopes, Acoustics

Abstract: The scanning acoustic microscope isbecoming an important new tool for nondestruc-tive testing. The paper concentrates on the currentapplication of the technique and recent develop-ments in instrumentation relevant to the NDTfield: an area not well covered in the literature.

1 Introduction

The scanning acoustic microscope is becoming an impor-tant new tool for nondestructive testing (NDT). Theunderlying research and development has been welldocumented in a number of excellent papers [1-4]including one in this journal [5] and will not be reviewedhere. A useful introduction to high-frequency acousticmicroscopy is now available [6] although this does notcover any of the biological applications that are possible.

A brief but necessary introduction to the principles ofoperation is followed by a closer look at the currentdevelopments in instrumentation. A number of new butrepresentative images show the range of potential appli-cation of the commercial instruments at present avail-able.

At first sight the scanning acoustic microscope is nomore than a conventional C-scan ultrasonic imagingsystem, the novel aspects of the instrumentation beingthat imaging is the normal mode of operation (ratherthan flaw detection and gating) and that the frequenciesused are higher. However this view ignores the realprogress in understanding made as a result of research inthis field.

The frequencies used initially were in the 1 GHzregion, producing resolution comparable to that ofoptical microscopy but with a different contrast mecha-nism. The wide angle lenses, typically 50-60° half aper-ture, used for surface imaging revealed a hithertounidentified contrast mechanism due to Rayleigh waves[7]. Analysis of this contrast mechanism [8-10] has ledto a number of new results and the potential of quantitat-ive materials characterisation. However there are seriousdifficulties to be overcome before extensive quantitativematerials analysis becomes feasible. Most of these can besolved by (sufficient investment in) the appropriatemicrowave, RF and digital signal processing. The expenseinvolved however is likely to considerably delay thesedevelopments.

Paper 5170A (S6) first received 14th February and in revised form 8thMay 1986The author was formerly with VG Semicon Ltd, The Birches IndustrialEstate, Imberhorne Lane, East Grinstead, Sussex RH19 1XZ, UnitedKingdom, and is now with Robert Fleming Securities, 25 CopthallAvenue, London EC2R 7DR, United Kingdom

Thus important new practical and theoretical resultshave arisen from research in this field which are notsimple extrapolations of previous work. Some of theseare:

(a) very high frequencies of operation [11](b) V(z) or AMS technique, allowing measurement of

Rayleigh velocity [12](c) improved understanding and modelling of surface

wave behaviour and imaging around cracks [13](d) derivation of materials' parameters on a micro-

scopic scale [14](e) improved understanding of aberrations in interior

imaging [15].

There are important actual and potential applications ofhigh-frequency acoustic microscopy, many of which willbecome more widespread as a result of the introductionof commercially available instruments. Use of the tech-nique, however, is expanding more rapidly with the lowerfrequencies, in the 50 MHz region. In this field, which isof the most immediate relevance to NDT, it is the abilityof the acoustic microscope to produce an image belowthe surface of optically opaque materials that is the mostimportant. Applications are concentrated at the lowerend of the frequency range between 30 and 100 MHz.This region is of course what has hitherto been regardedas very high frequency by the existing ultrasonic NDTcommunity, and thus we are seeing a convergence of theapplications and instrumentation between two fields.

2 Principles of operation

A focused acoustic beam is produced by electrically excit-ing a piezoelectric transducer that illuminates a single-surface lens. The instrument is normally operated in apulsed reflection mode as illustrated in Fig. 1, thus alsousing the transmitting lens as a receiver. The lensassembly is mechanically scanned over the sample in araster fashion, and image data are recorded point bypoint. Transmission mode imaging, although the firstmode demonstrated and indeed the first to have beenattempted commercially, is currently of limited practicalvalue owing to limitations on sample thickness andgeometry.

A unique feature of the acoustic microscope is that thelens is generally designed to produce a diffraction-limitedfocus [14]. The resolution is further improved by usingthe transmit and receive lens in the confocal mode. Thus,with a crrectly designed lens, the acoustic microscopeproduces better resolution than other acoustic methodsat any given frequency.

Lenses used for high-resolution subsurface imagingtypically have a quartz lens rod, with piezoelectric quartzor lithium niobate transducers. No matching layer is

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 3, MARCH 1987 283

required on the lens surface. The half aperture is approx-imately 45° or more for surface imaging, and 10° for sub-surface imaging. This provides the optimum subsurfacefocus for most materials. The radius is usually 10-20 mm.

inputo —

impedancematchingnetwork

RF pulse

to pulsedetector andimage store

echo fromspecimen

transducer

lens

lens rodmechanicallyscanned overspecimen

glass "hlUmatching layer(as appropriate)

ity. Other parameters such as density and impedance canbe derived in principle, but the instrumentation com-mercially available is not sufficiently developed to realisethis potential. The V{z) phenomenon only occurs with

specimen surface

Fig. 1 Schematic diagram of the acoustic microscope lens config-uration

The radius of a 1 GHz lens however is typically only80 /mi.

Pulses of energy reflected back from interfaces withinthe sample (and from the surface) are collected by thelens before returning to the transducer where they areconverted back into an electrical signal. After suitableelectronic amplification, detection and processing, theamplitude of these returning pulses, together with theexact time elapsed from the original signal, indicate themagnitude and depth of physical changes in the speci-men. It is also possible to make use of the phase informa-tion in the received signal [17].

The lens assembly, which is usually mounted on astandard optical microscope for ease of alignment, ismechanically scanned relative to the specimen in a rasterenabling a complete two-dimensional image to be digi-tally stored and displayed on a monitor screen. Imagemagnification is simply the ratio of the display size to thescan size and may typically vary from 1 x to 10000 xdepending on the application. Some applications involv-ing larger samples require a scanning frame covering over300 mm in each direction in place of the microscope-based scanner, as illustrated in Fig. 2.

The acoustic microscope image shows contrast varia-tions that are directly related to changes in mechanicalproperties such as density, elasticity, and viscosity. Dis-continuities, voids, cracks and delaminations are mostreadily detected. Individual materials can be quantitat-ively analysed and identified using the V{z) technique,which provides a highly characteristic signature of thesample [7] not available with other acoustic methods.

The V(z) technique is based on the interferencebetween two acoustic beams, the first being the on-axisdirect reflection. The second component is caused by thebeam arriving at the sample at the Rayleigh angle. Thisexcites a Rayleigh surface wave which reradiates into thewater and is collected by the lens. Interference betweenthe two components produces a transducer output thatchanges in amplitude and phase as the sample is defo-cused. The amplitude maxima and minima are regularlyspaced and can be used to readily derive Rayleigh veloc-

Fig. 2 Large area scan frame suitable for low-frequency imaging oflarger components up to 300 x 300 mm

wide angle lenses that cover the Rayleigh angle, typicallyabove 30°. This would explain why the effect had notbeen noticed previously.

Table 1 summarises the resolution and depth per-formance at typical operating frequencies. The depth of

Table 1: Resolution and penetration at typical operatingfrequencies

Frequency,MHz

3050

100500

10002000

wavelengthin water./ym

503015

31.50.75

resolutionsurface

352010

210.5

, /yminterior

70-17540-10020-504-102-51-3

depthpenetration./ym(typicalfigures)

0 to 70000 to 50000 to 20000to200to100 to 5

penetration is sample dependent and more difficult topredict than the resolution; the figures shown are indica-tions only and have not been proved at the higher fre-quencies. In general, homogeneous media produce lowerscattering and attenuation than composite or granularmaterials. Most media are anisotropic and the effect ofthis is more severe at high frequencies.

In theory the penetration depth at the higher fre-quencies could be increased in some low-loss materials.However, suitable lenses have not been produced evenfor research purposes.

Details smaller than the theoretical resolution canrealistically be detected. This applies particularly tocracks and delaminations. These would only need to betypically 50 nm wide or thick, respectively, to be seeneven with the 50 MHz microscope, this being made pos-sible by Rayleigh wave interactions [10].

In NDT the reliability and repeatability of detection isof paramount importance. A recent study analyses theperformance of the acoustic microscope for surface,volume, and bond inspection [18]. Further detailsshowing the application of Rayleigh wave imaging to

284 IEE PROCEEDINGS, Vol. 134, Pt. A, No. 3, MARCH 1987

surface inspection for NDT are also described. Thiswould appear to be a promising area for new develop-ment which is now being pursued at VG Semicon Ltdand elsewhere.

3 Instrumentation, automation and data display

Fig. 3 shows the ASM 100 acoustic microscope from VGSemicon Ltd; currently the most widely used acoustic

Fig. 3 ASM 100 acoustic microscope

Control panel and keyboard (left), basic microscope (centre), computer anddisplay monitors (right), colour monitor not shown here

microscope. This system covers the range 30 MHz to2 GHz. Similar microscopes have been announced andeither are or shortly will be available from a number ofother manufacturers including Leitz (800 MHz to 2 GHz)and Olympus (100 MHz to 1 GHz). Several other Japa-nese manufacturers are known to be developing similarinstruments. Thus all the commercial systems cover thehigh-frequency range around 1 GHz for examination ofsurface and near-surface detail, with similar specificationsin this range in terms of frequency, scan size and imagingtime. The ASM 100 instrument extends the frequencyrange down to below 50 MHz giving 20-100 nmresolution, depending on sample material, at depths of upto 10 mm.

The instrument illustrated features computer controlof stage motion and scanning, data acquisition andstorage, image processing and analysis. The computercontrol is designed to make the instrument easy to use bynew and unskilled operators, although clearly interpreta-tion of the data, particularly with the wide angle lensesnormally associated with high-frequency imaging, canoften require significant experience. Computer controlallows semiautomated inspection of specific samples, andin one instance several microscopes are being integratedinto a large network of automated test equipment forquality control, with online transfer of test data.

3.1 UltrasonicsThe major elements of the ultrasonic system can differwidely. The simplest system used in the low-frequencyregime (below 100 MHz) is similar to the standardpulser/receiver used in ultrasonic NDT, although withimproved bandwidth. The operating frequency is deter-mined by the characteristics of the lens-transducerassembly. The computer interface to this is straightfor-ward, and thus it is possible to replace this part of the

system with a number of other industry standard equiva-lents. This allows wider applicability and the ability tocompare results from different instrumentation. Furtherdevelopments, such as pulse compression techniques[19], can be integrated into the system.

The high-frequency system, in its simplest form, useshigh-speed pin diode switches at the output (typically1 W) of an amplifier to generate a toneburst of knownwidth and frequency, usually close to 1 GHz, whichexcites the transducer. The receiver can be as simple asan amplifier followed by a simple diode detector.Improvements can be made to this system, but little prac-tical advantage is to be gained for current applications.In many respects developments in the instrumentationavailable for high-frequency imaging have advancedbeyond the general level of the applications being investi-gated.

3.2 Developments in instrumentationIt was noted earlier that there is a convergence betweenacoustic microscopy and the more conventional ultra-sonic NDT techniques. The low-frequency microscope,when used for interior imaging, can be improved byadopting some of the more usual techniques of ultrasonictest instrumentation. However in each case an improvedperformance has been achieved.

(i) Interface gating; whereby the sample gate timing isrelative to another reflection rather than to the transmit-ted pulse. This allows the instrument to cope with sometilted or nonplanar samples while focusing at an approx-imately constant depth. This facility has been extended toallow surface imaging on similar sample geometries.

(ii) Normally the acoustic microscope uses a narrowgate, sampling at a fixed instant. This can sample nar-rower pulses than the variable-width gate used on manyultrasonic pulsers can, although it requires more skill touse. Added flexibility is provided by a peak-hold circuitwhich captures the largest peak during a gate period.Although this is not new the implementation used, whichrequires a minimum of two pulses per sample point, pro-vides better performance than existing systems andallows further features to be added.

(iii) The circuit used for peak holding, which operatesup to 100 MHz, is designed to allow the addition of aB-scan facility in conjunction with the computeriseddata-capture system.

In Section 2 it was mentioned that large scan framescould be used. Clearly under computer control there isno fundamental limit to the scan sizes used. With adigital framestore 256 x 256 is a usual choice for framesize; although our system can display 512 x 512 pixelimages it is found that 256 x 256 images are generallyadequate and more practical for data storage. However,once the scan size is increased beyond the point where

scan length = (number of pixels x resolution)/2

then undersampling occurs. For a 50 MHz surfaceimaging system where the resolution is approximately25 //m this would correspond to a scan length of 6.4 mmfor a 256 pixel wide framestore. Thus for many samplesthis is not just a theoretical problem but an actualproblem. In a fundamental sense the problem cannot besolved. However a useful development in instrumentationrelevant to NDT applications has been made.

In this mode of operation the data-capture system andthe pulser/receiver, which normally operate asynchro-nously, are linked. The computer indicates when a newpixel is to be stored. The largest signal recorded by the

1EE PROCEEDINGS, Vol. 134, Pt. A, No. 3, MARCH 1987 285

gating circuit since the last pixel was stored is used as thedata. The gating circuit is then reset. Thus the defectdetection performance is determined by the resolutionsize rather than the pixel size, but only in one dimension.This clearly would not solve the resolution problem forC-scan imaging but for most NDT applications it is thelargest signal, usually indicating a flaw, that is of interest.If necessary smaller regions of interest can then be re-scanned in more detail. This approach has also beenimplemented independently elsewhere [20].

The conventional flaw-detector function can also beprovided by the data-capture system. The approach is tostore only pixels where the data level is above a thresh-old. In this case the position (X, Y) or position and data(X, Y, V) can be stored. The latter allows a margin oferror in the setting of the flaw gate that can be alteredafter data capture.

The low-frequency acoustic microscope is limited in itsspeed of operation by mechanical scanning. Howeverwith a pulse repetition frequency (PRF) of approximately10 kHz for the low-frequency system it is clear that thescope for increasing speed is limited. The maximumlinear speed allowable can be calculated as follows:

speed (max) = resolution/2 x PRF

In our example at 50 MHz, this becomes

speed (max) = 25 ;im/2 x 10 kHz

= 125 mm/second

The stepper motors currently used on the ASM 100provide linear speeds of 25 mm/s and the performance isthus already within a factor of 5 of the theoretical fastestscan at this resolution for surface imaging. It would ofcourse be possible to increase the linear speed by approx-imately this factor by using servomotors in place of thestepper motors. However, for subsurface imaging theresolution at 50 MHz may well be closer to say 75 fim,and so in this case a factor of 15 improvement in speedcould still be made. For this reason we have developed afaster linear scanner for scanning the lens in one axis overa distance of up to 35 mm, allowing a reduced frame timeof approximately 60 s for small frame sizes.

3.3 AutomationSoftware control allows several modes of operation to beprovided with the same instrument: an approach usedelsewhere [21]. Thus for a research environment the fullflexibility of the software is available to the user.However in a routine test environment it is important tomake the user interface simple by providing differentoptions or a modified subset of the available features;perhaps allowing the operator to initialise the systemwith a customised scan for each sample and then toobserve the resulting data in the original or processedforms. Automation of a system can lead to significantincreases in productivity, even in a research environment.Once the instrument is set up the user can concentrate onthe experimental results rather than pay constant atten-tion to the instrument. Total automation is also possible,although this does not yet extend to sample handling.

3.4 Data displayThe ability to process data, and this can be concurrentwith data capture, allows the output to appear in what-ever form is suitable for a specific application. The pro-cessing can be done interactively if required. This is animportant feature because it is not easy to produce

general criteria for visual interpretation of experimentalimages where no prior knowledge of the data exists. Thedynamic range of the data stored, 48 dB, exceeds thatwhich can be readily assimilated simultaneously by thehuman eye. It is often the case that the interesting dataare contained in the details of an image, it then becomesnecessary to use some relevant image enhancementscheme.

Experience has shown that even a limited image pro-cessing capability greatly enhances the ease of use of themicroscope. The most widely useful facilities have beenfound to be contrast enhancement, edge enhancementand pseudocolouring. The use of pseudocolour [22] doesnot introduce new data, but the technique can make thedata that are available more apparent to the viewer and,therefore, more easily comprehensible. It is an aid to per-ception that is of particular value for the highlighting ofregions where subtle changes in brightness, not easilyobservable in monochrome, have taken place. Theincreased convenience of digital storage is also of greatbenefit. Often details are not noted during experiments,or results are poorly photographed. The ability toexamine and reproduce data without repeating experi-ments should not be underestimated.

Although the digital image processing facility is impor-tant for research environments where manipulationallows extra information to be presented, it can also beuseful in routine use. In this case it is often necessary toreduce the amount of data available to aid interpretation,or to present them in a standardised format. This willbecome increasingly important as relevant national andinternational standards, such as JIA48, become estab-lished, although few such standards currently exist.

4 Practical applications results

A wide variety of practical applications have been investi-gated recently at VG Semicon Ltd. Further applications,including biological, are being examined elsewhere [23,24].

The images presented here are chosen as typical of theresults obtained using the technique. All these imageshave been taken using a reflection-mode acoustic micro-scope at a frequency around 50 MHz, except Fig. 4 whichwas taken at 1.15 GHz. The maximum depth penetrationcurrently demonstrated with this instrument at 50 MHzis 10 mm. Greater depths are expected with new lenses.

Fig. 4 1 GHz image, 100 \an wide of a gold finder grid deposited onglass

286 IEE PROCEEDINGS, Vol. 134, Pt. A, No. 3, MARCH 1987

4.1 High - frequency imagingFig. 4 shows a 1 GHz image, 100 ̂ m wide, of a goldfinder grid deposited on glass. This exhibits many smallbubbles where the gold film has not adhered to the glassthat was not fully cleaned before deposition. This couldnot be seen optically. In this case the data is relativelyeasy to interpret. However in a more realistic samplewith significant topography and features comparable insize to the wavelength, such as an integrated circuit, thedata are often far harder to interpret, and imaging arte-facts can dominate. This is largely due to the V(z) pheno-mena, where the contrast mechanism is dominated bysurface wave propagation. However the contrast mechan-ism that produces these artefacts can be used to advan-tage when investigating cracks, as these can be detectedeven when their size is much less than a wavelength [6].This feature is now being exploited in materials investiga-tion, particularly for ceramics.

4.2 Low-frequency imagingMany electronics assemblies and packages, such ashybrid integrated circuit packages, have multiple layers,each of which may contain voids. It is generally impor-tant to discover which layer contains the defects for cor-rective action to be taken during manufacture. X-raytechniques are often not suitable in such applications, notonly because the depth information is absent, but alsobecause device damage can occur. Furthermore there area number of package types specifically designed to beX-ray resistant for various critical applications.

Fig. 5 shows an acoustic image of part of a hybridcircuit. In this case it is not possible to see the voids inthe solder bond underneath the central device usingX-rays without device damage. However the acousticimage clearly shows voids in the bond. Further imagingof this sample at different depths shows that the electrodefoil is unevenly bonded to the substrate in many areas.

Fig. 5 Acoustic image of part of a hybrid circuit 15 mm wide

The ability of the acoustic microscope to discriminatebetween specific layers is illustrated in Figs. 6b, c and dwhich are images at three different depths of the pin gridarray integrated circuit package shown in Fig. 6a. In thiscase the sample is made of a fine ceramic and each layeris 1 mm thick.

Die attach imaging is a well established application ofthe acoustic microscope, suitable for all commercialpackage types. The results are well documented and willnot be repeated here. In some cases it would be possibleto use alternative acoustic methods such as the scanninglaser acoustic microscope (SLAM). This system has

Fig. 6 Pin grid array

a Pin grid array integrated circuit packageb, c and d Illustrations of the ability of the acoustic microscope to discriminatebetween specific layers: images at three different depths

IEE PROCEEDINGS, Vol. 134, Pt. A, No. 3, MARCH 1987 287

advantages in terms of speed of operation for suitableapplications, but is limited to thin samples and cannotproduce an image inside samples such as ceramic ICpackages where there is a void above the die. In someapplications, such as routine testing of ceramic-chipcapacitors, the lack of depth discrimination can actuallybe an advantage [24].

Fig. 7 shows an image through a ceramic substrateused for surface mounting of components in the elec-tronics industry. The electrodes of the temperature sensor(the interdigital structure on the left) are unevenlybonded. A coating over some of the electrodes can beseen at the bottom. Surface contamination can also beseen between some of the bond pads to the right. Nor-mally the area to the left of these pads is covered by adevice package and cannot be adequately studied nonde-structively by established techniques with the com-ponents in situ.

Fig. 7 Image, 12 mm wide, through a ceramic substrate used forsurface mounting of components in the electronics industry

The acoustic microscope can be applied to the nonde-structive testing of adhesive bonds, although no strengthtest is available for compressed but disbonded interfaces.Fig. 8 shows an epoxy bond between two aluminiumalloy sheets, each 4 mm thick. The hexagonal mesh,visible where the surrounding bond quality is good, is apolymer carrier. The bond structure is markedly unevenin the centre of the image, where PTFE surface contami-nation has been deliberately introduced. A long termstudy of this application area is being conducted toestablish quantitative results in this field [26].

Carbon fibre laminates are posing new problems forNDT. Fig. 9 shows a delamination between severalcrossed layers at a nominal depth of 1.5 mm. This is anapplication well suited to low-frequency acoustic micro-scopy. Further representative results can be found else-where [27].

Fig. 8 Image, 15 mm wide, of an epoxy adhesive bond between two4 mm thick sheets of aluminium alloy

Fig. 9 Image, 25 mm wide, of a delamination between crossed layers ina carbon fibre compositeDelamination between layers, due to impact damage, can be clearly seen. It is alsopossible to see several fibres that show unusual contrast running diagonallybetween the sites of major damage across the centre of the image. The microscopewas focused at an approximate depth of 1.5 mm

5 Conclusions

Practical applications of acoustic microscopy as a high-resolution ultrasonic inspection system are expandingrapidly, some are demonstrated above. Significant inter-est in applying acoustic microscopy to testing structuralceramics and new materials is being shown, and this isexpected to be an important applications area. Improve-ments in the sensitivity, resolution and depth penetrationof commercial instrumentation is expected to extend therange of use. Features useful in NDT have been added tothe low-frequency instrument and further applications,particularly at high frequency and including quantitativematerials analysis, can be expected to emerge in the nearfuture as developments in available instrumentation andgeneral awareness become more widespread.

6 Acknowledgments

The author would like to thank Dr. D.M. Thaker forproviding some of the images in this paper, Dean Mannwho helped to develop and implemented most of therecent improvements in instrumentation and also Shin-roku Tsukamoto of Marubun Corp., Tokyo for provid-ing the results shown in Fig. 6. Thanks are also due toProf. E.A. Ash for his continued support.

7 References

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6 BRIGGS, G.A.D.: 'An introduction to scanning acoustic micro-scopy' in 'Royal Microscopy Society Handbook 12' (Oxford Uni-versity Press, 1985)

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15 PINO, F., SINCLAIR, D.A., and ASH, E.A.: 'Scanning acousticmicroscopy of solid objects using aspheric lenses'. Proceedings of11th International Conference on acoustical imaging, Monterey,USA, 1981

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18 GILMORE, R.S., TAM, K.C., and HOWARD, D.R.: 'Acousticmicroscopy from 10 to 100 MHz for industrial applications'.Corporate Research and Development Report 86CRD015, GeneralElectric, Schenectady, New York, USA, 1986

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20 CROCKER, R.L., and GRAY, N.: 'Gated peak detector specifi-cation', Fulmer Research Institute Ltd., Stoke Poges, Slough, Berks,UK, 1986

21 ELSLEY, R.K.: 'The digital ultrasonic instrument' in THOMPSON,D.O., and CHIMENTI, D.E. (Eds.): 'Review of progress in quanti-tative nondestructive evaluation 2' (Plenum, New York, 1983),pp. 1487-1500

22 BURTON, N.J., PINO, F., SIVAPRAKASAPILLAI, P., andNIKOONAHAD, M.: 'Digital image processing for scanning acous-tic microscopy', IEEE Trans. Sonics & Ultrason., 1984, 31, pp. 279-286

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25 SEMMENS, J.E., KESSLER, L.W., and EWELL, G.J.: 'Non-destructive inspection of encapsulated capacitors: flaw growthdetection of parts submitted to 85% Humidity, 85°C in ProceedingsInternational Symposium on Test and failure analysis, Long Beach,USA, 21st-25th October 1985

26 THAKER, D.M., and BURTON, N.J.: 'Ultrasonic exploration ofadhesive bonds by acoustic microscopy'. Proceedings of the Interna-tional Conference on Adhesive and bonding technology, City Uni-versity, London, UK, 1986

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IEE PROCEEDINGS, Vol. 134, Pt. A, No. 3, MARCH 1987 289