Multilayer eddy current probe array for complete coverage of an inspection surface without mechanical scanning

An eddy current surface measurement array structure for complete coverage of an underlying inspection surface without requiring mechanical scanning is disclosed. A three-dimensional array of eddy current sense elements is organized as a plurality of layers of two-dimensional sub-arrays. The sub-arrays, although in different layers, are essentially identical in configuration, and are staggered such that the sense elements of one layer provide at least partial coverage of portions of the inspection surface not covered by the sense elements of another layer. As many staggered layers are included as are necessary to ensure that no "blind spots" remain, for complete coverage of the underlying inspection surface. The sense elements are disposed in a layered flexible structure fabricated employing high density interconnect fabrication techniques or other photolithographic techniques. Static (electronic) scanning is employed, by individual layer and by row and column within each layer, to form a two-dimensional image of the inspection surface.

BACKGROUND OF THE INVENTION 
The present invention relates generally to the inspection of components 
employing eddy current techniques and, more particularly, to a 
three-dimensional eddy current probe array which provides complete 
coverage of an inspection surface without mechanical scanning. 
Eddy current inspection is a commonly used technique for non-destructively 
detecting discontinuities or flaws in the surface of various components, 
for example, aircraft engine parts and aircraft skin surfaces. Very 
briefly, eddy current inspection is based on the principle of 
electromagnetic induction in which a drive coil is employed to induce eddy 
currents within the material under inspection, and secondary magnetic 
fields resulting from the eddy currents are detected by a sense coil, 
generating signals which are subsequently processed for the purpose of 
detecting flaws. 
Eddy current testing for flaws in conductive materials is typically done by 
mechanically scanning a single probe in two dimensions. For example, U.S. 
Pat. No. 5,345,514, entitled "Method for Inspecting Components Having 
Complex Geometric Shapes", describes methods for interpreting eddy current 
image data acquired by a single probe, particularly in the context of 
inspecting a high pressure turbine (HPT) disk dovetail slot. 
Although effective, the single probe scanning method is time consuming. 
Probe arrays have been developed to improve the scanning rate, as well as 
to increase flaw detection sensitivity. For example, General Electric High 
Density Interconnect (HDI) technology has been used to fabricate flexible 
eddy current probe arrays. In particular, Hedengren et al., U.S. Pat. No. 
5,389,876, entitled "Flexible Eddy Current Surface Measurement Array for 
Detecting Near Surface Flaws in a Conductor Part", the entire disclosure 
of which is hereby expressly incorporated by reference, discloses a hybrid 
method of both electronic and mechanical scanning employing an eddy 
current probe array comprising a plurality of spatially correlated eddy 
current probe elements disposed within a flexible interconnecting 
structure which may be employed to collect a discrete plurality of 
spatially correlated eddy current measurements for non-destructive near 
surface flaw detection. An array of such elements can, in a single 
uni-directional scan, accommodate inspecting an area covered by the active 
width of the array. Thus, the array is mechanically scanned in a direction 
perpendicular to a row of sense elements, and electronically scanned along 
the row. Such an array typically includes two staggered rows of sense 
elements to in effect interleave the tracks defined by the individual 
sense elements during the mechanical scan for more complete coverage of an 
inspection surface. 
Suitable electronics for acquiring data from a probe array such as is 
disclosed in U.S. Pat. No. 5,389,876 are disclosed in Young et al. U.S. 
Pat. No. 5,182,513, entitled "Method and Apparatus for a Multi-Channel 
Multi-Frequency Data Acquisition System for Nondestructive Eddy Current 
Inspection Testing", which patent is expressly incorporated by reference. 
HDI fabrication techniques which are advantageously employed in the 
fabrication of the flexible array structure of the above-incorporated U.S. 
Pat. No. 5,389,876, are disclosed in Eichelberger et al. application Ser. 
No. 07/865,786, filed Apr. 7, 1992, U.S. Pat. No. 5,452,182, entitled" 
Flexible High Density Interconnected Structure and Flexibly Interconnected 
System, the entire disclosure of which is hereby also expressly 
incorporated by reference, which is a continuation of application Ser. No. 
07/504,769, filed Apr. 5, 1990, now abandoned. 
By way of further background, as disclosed in Eichelberger et al. U.S. Pat. 
No. 4,783,695, and related patents and applications such as Ser. No. 
07/865,786, the high density interconnect structure developed by General 
Electric Company has previously offered many advantages in the compact 
assembly of electronic systems. For example, an electronic system such as 
a microcomputer which incorporates between thirty and fifty chips, or even 
more, can be fully assembled and interconnected on a single substrate 
which is two inches long by two inches wide by 50 mils thick. This 
structure is referred to herein as an "HDI structure", and the various 
previously-disclosed methods for fabricating HDI structures are referred 
to herein as "HDI fabrication techniques". 
Very briefly, in typical systems employing this high density interconnect 
structure, a ceramic substrate is provided, and individual cavities or one 
large cavity having appropriate depths at the intended locations of the 
various chips are prepared. Various components are placed in their desired 
locations within the cavities and adhered by means of a thermoplastic 
adhesive layer. 
A multi-layer high density interconnect (HDI) overcoat structure is then 
built up to electrically interconnect the components into an actual 
functioning system. To begin the HDI overcoat structure, a polyimide 
dielectric film, which may be Kapton.RTM. polyimide available from E. I. 
du Pont de Nemours Company, about 0.0005 to 0.003 inch (12.5 to 75 
microns) thick is pretreated to promote adhesion and coated on one side 
with ULTEM.RTM. polyetherimide resin or another thermoplastic and 
laminated across the top of the chips, other components and the substrate, 
with the ULTEM.RTM. resin serving as a thermoplastic adhesive to hold the 
Kapton.RTM. film in place. Exemplary lamination techniques are disclosed 
in Eichelberger et al. U.S. Pat. No. 4,933,042. 
The actual as-placed locations of the various components and contact pads 
thereon are determined, typically employing optical imaging techniques. 
Via holes are adaptively laser drilled in the Kapton.RTM. film and 
ULTEM.RTM. adhesive layers in alignment with the contact pads on the 
electronic components in their actual as-placed positions. Exemplary laser 
drilling techniques are disclosed in Eichelberger et al. U.S. Pat. Nos. 
4,714,516 and 4,894,115; and in Loughran et al. U.S. Pat. No. 4,764,485. 
A metallization layer is deposited over the Kapton.RTM. film layer and 
extends into the via holes to make electrical contact to the contact pads 
disposed thereunder. This metallization layer may be patterned to form 
individual conductors during the process of depositing it, or may be 
deposited as a continuous layer and then patterned using photoresist and 
etching. The photoresist is preferably exposed using a laser which, under 
program control, is scanned relative to the substrate to provide an 
accurately aligned conductor pattern at the end of the process. 
Exemplary techniques for patterning the metallization layer are disclosed 
in Wojnarowski et al. U.S. Pat. Nos. 4,780,177 and 4,842,677; and in 
Eichelberger et al. U.S. Pat. No. 4,835,704 which discloses an "Adaptive 
Lithography System to provide High Density Interconnect". Any misposition 
of the individual electronic components and their contact pads is 
compensated for by an adaptive laser lithography system as disclosed in 
U.S. Pat. No. 4,835,704. 
Typical such systems, being formed on a ceramic substrate, are not 
flexible. However, the above-incorporated Eichelberger et al. application 
Ser. No. 07/865,786 discloses techniques for making at least portions of 
the high density interconnect structure flexible. 
The eddy current inspection systems described herein up to this point 
employ either mechanical scanning (e.g. a single probe) or a hybrid method 
of both electronic and mechanical scanning (e.g. a probe array comprising 
two staggered rows). 
As noted above, scanning a single probe in two dimensions is a 
time-consuming process. Accordingly, a variety of static scanning 
approaches have been proposed in the literature, whereby a two-dimensional 
array of sense elements is placed in a stationary position, and scanning 
is accomplished by electronically switching between the elements. Examples 
of this approach are disclosed in the following literature references: 
Bert A. Auld, "Probe-Flaw Interactions with Eddy Current Array Probes", 
Review of Progress in Quantitative NDE 10, edited by D. O. Thompson and D. 
E. Chementi (Plenum Press, New York, 1991), pages 951-955; Yehuda D. 
Krampfner and Duane D. Johnson, "Flexible Substrate Eddy Current Coil 
Arrays", Review of Progress in Quantitative NDE 7, edited by D. O. 
Thompson and D. E. Chimenti (Plenum Press, New York, 1988), pages 471-478; 
and Mirek Macecek, "Advanced Eddy Current Array Defect Imaging", Review of 
Progress in Quantitative NDE 10, edited by D. O. Thompson and D. E. 
Chimenti (Plenum Press, New York 1991), pages 995-1002. 
A major drawback of static scanning, recognized for example in the 
above-referenced Krampfner and Johnson paper, is that complete coverage of 
the underlying inspection area is not achieved. 
Thus, the hybrid scanning approach referred to above offers an attractive 
compromise. One or more staggered rows of elements are mechanically 
scanned in a direction perpendicular to the rows, while the elements are 
electronically scanned along each row. Attractive trade offs are realized 
between electronic complexity and scanning time while, at the same time, 
providing the ability to selectively oversample data in one direction to 
enhance flaw detection capability. 
Nevertheless, there are applications where, due to obstructions or other 
considerations, any approach involving mechanical scanning is unsuitable 
or undesirable. 
Accordingly, there remains a need for an effective static scanning approach 
which provides complete coverage of the inspection surface. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide a probe array 
structure which can provide complete coverage of an underlying inspection 
surface without mechanical scanning. 
Briefly, in accordance with the present invention a three-dimensional array 
of eddy current sense elements is provided, organized as a plurality of 
layers of two-dimensional sub-arrays. Each of the sub-arrays may consist 
of a multilayer structure. The sub-arrays are staggered such that the 
sense elements of one layer provide at least partial coverage of portions 
of the inspection surface not covered by the sense elements of another 
layer. As many staggered layers are provided as are necessary to ensure 
that no "blind spots" remain, for complete coverage of the underlying 
inspection surface. For example, there may be three such layers each 
comprising a two-dimensional sub-array. Various element shapes may be 
employed. For example, hexagonal shapes permit dense packing with little 
cross coupling when overlapping in different layers. 
In some embodiments the sub-arrays in different layers are essentially 
identical in configuration. 
Alternatively, in the case of arrays for inspection of irregularly-shaped 
objects or along curved perimeters, the sub-arrays may comprise similar 
but not identical elements in a staggered overlapping arrangement. 
In addition, the use of similar but non-identical elements may be 
beneficial or even required in some instances for reducing cross-coupling 
between sub-arrays. A combination of larger and smaller elements may also 
be employed with dual frequencies; the larger elements are excited at the 
lower frequency. The elements also are positioned for minimal 
cross-coupling. 
There is at least one drive element which cooperates with all of the 
sub-arrays of sense elements in the different layers. Alternatively, for 
reduced cross talk, the sub-arrays in the different layers can be 
sequentially excited by separate drive elements. 
Preferably, the sense elements are disposed in a layered flexible structure 
fabricated employing the HDI fabrication techniques described in the 
various background patents referenced hereinabove. Thus, multiple layers 
can be fabricated as a continuous, multilayer structure. Alternatively, a 
plurality of double-sided flexible printed circuit board-like 
substructures can be fabricated and joined together, with appropriate 
offsets. Vias or anisotropic conductive adhesives provide electrical 
interconnection between substructures on different layers. 
In methods employing the probe array structures of the invention for 
inspection by static scanning, it is desirable to use a physical 
calibration or reference part. Without physical scanning, it is sometimes 
difficult to determine what test signals mean. The calibration or 
reference part is examined in parallel with a test part by switching a 
shared electronics package between the two. Signals thus acquired by 
scanning the two parts are processed by differential amplifiers which 
produce a difference signal; a potential defect is indicated when the 
difference signal exceeds a predetermined threshold. Alternatively, 
signals from the calibration or reference part can be obtained and stored 
as a reference signal data set for subsequent comparison with test 
signals, in a manner similar to a known technique for single probe 
inspections. 
Preferably, data resulting from separate layers after appropriate signal 
processing is formatted, such as by interlacing, to produce an image with 
improved spatial resolution

DETAILED DESCRIPTION OF THE INVENTION 
With reference to FIG. 1, an eddy current array 10, particularly intended 
for static scanning, comprises two two-dimensional sub-arrays 12 and 14 
arranged in layers. Sense elements of the sub-array 12 are designed 12', 
and sense elements of the sub-array 14 are designed 14'. 
The sub-arrays 12 and 14 are staggered such that the sense elements 14' of 
layer 14, for example, provide at least partial coverage of portions of an 
underlying inspection surface 16 not covered by the sense elements 12' of 
the layer 12. 
Some "blind" spots 18 do, however, remain in the arrangement of FIG. 1, not 
covered by either of the sub-arrays 12 and 14. 
FIG. 2 depicts an eddy current array 20 with the addition of a third 
subarray 22 comprising sense elements 22', with staggering of all three 
layers 12, 14 and 22 to eliminate all "blind" spots. Complete coverage of 
the underlying inspection surface 16 results throughout the active area of 
the array 20. 
The depiction of the individual sense elements 12' and 14' in FIG. 1 and 
elements 12', 14' and 22' in FIG. 2 is representative only and highly 
schematic. As is well known, there are a great many possible sense coil 
arrangements and particular modes of operation which may be employed, and 
the subject invention is not limited to any particular sense coil 
arrangement. In particular, sense coil arrangements which are themselves 
multilayer structures may be employed. In addition, there are a variety of 
drive coil arrangements which may be employed, which likewise are not 
disclosed herein in detail. A number of coil configuration examples are 
disclosed in the above-incorporated U. S. Pat. No. 5,389,876, as well as 
in Hedengren application Ser. No. 07/904,634, filed Jun. 26, 1992, 
entitled "Apparatus and Method for Compensating an Eddy Current Surface 
Inspection Array for Lift Off", now abandoned, the entire disclosure of 
which is also hereby expressly incorporated by reference. 
As disclosed in the above-incorporated applications, array elements may be 
designed for multiple functions, including lift off compensation 
irregularly shaped elements may be "tuned" to specific geometries. 
Elements of different sizes and/or multiple frequencies may be combined to 
enable different inspection resolution and penetration or to compensate 
for effects of geometry. All of these approaches, and others, can be 
combined with the multiple layer staggered arrays of the subject invention 
for complete coverage of the underlying inspection surface 16. 
Also, it will be appreciated that it is the sense elements which are 
staggered in accordance with the invention, and that a common drive 
element may serve a plurality of such sense elements. Moreover, a common 
drive element may serve the sense element of one or more layers. Separate 
drive elements may also serve each of the staggered sub-arrays. Again, a 
variety of specific geometries may be employed. 
FIG. 3 somewhat schematically depicts in cross section the two layer array 
of FIG. 1 incorporated within a layered flexible structure 30 fabricated 
employing high density interconnect techniques and FIG. 4 correspondingly 
somewhat schematically depicts in cross section the three-layer array of 
FIG. 2 incorporated within a layered flexible structure 40 fabricated 
employing high density interconnect techniques. In FIG. 3, a first layer 
of polymeric dielectric film 32 has formed on its upper surface the sense 
elements 12', in the exemplary form of rectangular coils. The coils 12' 
are formed on the layer 32 employing the metallization techniques 
described in the above-incorporated HDI patents. Suitable conductive runs 
and vias (not shown), all as described in the above-incorporated patents 
related to HDI fabrication techniques, are included to provide suitable 
electrical connections to the coils 12'. 
In FIG. 3, and again in accordance with HDI fabrication techniques, another 
dielectric layer 34 is provided to encapsulate the first subarray element 
layer 12 and to support the second subarray element layer 14, which in 
turn is protectively encapsulated within yet another dielectric layer 36. 
In the three-layer structure of FIGS. 2 and 4, the third sub-array element 
layer 22 is supported on the dielectric layer 36, and is in turn 
encapsulated within a dielectric layer 38. 
In the two sub-array element layer arrangement of FIGS. 1 and 3, the 
elements 12' and 14' overlap by halves leaving the "blind spots", as noted 
hereinabove. This overlapping by halves is represented in FIG. 3, where 
the element interval is the distance "x", and each element 12' of the 
layer 12 is offset a distance "x/2" with reference to elements 14' of the 
layer 14. 
In the three sub-array element layer arrangement of FIGS. 2 and 4, the 
elements 12', 14' and 22' overlap by thirds, and there are no blind spots. 
This overlapping by thirds is represented in FIG. 4 where, again, the 
element interval is the distance "x". Each element 14' of the layer 14 is 
offset a distance "x/3" with reference to the elements 22' of the layer 
22, and each element 12' of the layer 12 is offset a distance "2x/3" with 
reference to the elements 22' of the layer 22. 
Thus, in general, elements in different layers (three or more) are 
staggered so that no blind spots remain. For n element layers, this means 
that each successive row of elements is staggered by 1/n of the element 
interval of another row. It will be appreciated that the order of the rows 
with reference to the staggering arrangement may vary. 
The sensor arrays of the subject invention are connected to suitable 
electronics for static scanning and readout, such as is disclosed in Young 
et al. U.S. Pat. No. 5,182,513. 
In general, static scanning proceeds in a conventional manner, one layer at 
a time, along the rows and then along the columns of each layer. In the 
structure of FIG. 4, the individual layers 12, 14 and 22 are relatively 
closely spaced. For example, there may be a vertical distance in the order 
of 20 microns between layers. Accordingly, each layer is 
electromagnetically coupled to the underlying inspection surface 16 in a 
similar manner for comparable resolution. Nevertheless, there may be some 
difference in response, which is compensated for by providing a different 
gain within the associated electronics (not shown) depending upon which 
layer is being scanned. The response of each layer is thus normalized. 
When analyzing data from the different layers, it is additionally 
preferable to format, such as by interlacing, the data from the separate 
layers to produce an image with improved spatial resolution. 
In the probe arrays of the invention, either absolute or differential type 
array elements may be employed. Differential types are preferred for 
surface defect detection, while absolute type array elements are preferred 
for measuring characteristics such as coating thickness or material 
conductivity. 
FIG. 5, and the corresponding cross section of FIG. 6, illustrate a 
representative situation where probe sub-arrays in different layers 
comprise similar but not identical elements in a staggered overlapping 
arrangement. In FIG. 5, there is a flat inspection surface 50 with a 
structure 52 having a curved perimeter 54. Surface 50 and structure 52 
form a solid part or might be formed together by welding or other 
processes. There is a lower row of elements 56 shown in dash lines, and an 
upper row of elements 58 shown in solid lines in FIG. 5. Each of the 
elements 56 and 58 is a differential element, depicted by respective lines 
56' and 58' at the element centers. 
From this geometry, it will be appreciated that the elements 56 are 
positioned radially outwardly with reference to the elements 58, and 
accordingly cannot be of the same size. 
FIG. 6 somewhat schematically depicts in cross section the two layer array 
of FIG. 5 incorporated within a layered flexible structure 60 fabricated 
employing high density interconnect techniques. In FIG. 6, a first layer 
of polymeric dielectric film 62 has formed on its upper surface the sense 
elements 56'. The elements 56' are formed on the layer 62 employing the 
metallization techniques described in the above-incorporated HDI patents. 
Suitable conductive runs and vias (not shown), all as described in the 
above-incorporated patents related to HDI fabrication techniques, are 
included to provide suitable electrical connections to the elements 56'. 
In FIG. 6, and again in accordance with HDI fabrication techniques, another 
dielectric layer 64 is provided to encapsulate the first sub-array element 
layer 56' and to support the second sub-array element layer 58', which in 
turn is protectively encapsulated within yet another dielectric layer 66. 
FIGS. 7 and 8 illustrate a combination of relatively larger elements 70 
with relatively smaller elements 72 in a different layer. The first layer 
of dielectric film 74 is shown in FIG. 8. The use of similar but 
non-identical elements may be beneficial or even required in some 
instances for reducing cross-coupling between sub-arrays. In general, to 
reduce cross-coupling it is desirable to minimize or eliminate parallel 
overlapping conductors. Moreover, the combination of larger 70 and smaller 
72 elements facilitates the use of dual frequency scanning. 
FIG. 9 illustrates how hexagonal element shapes permit dense packing with 
little cross coupling when overlapping in different layers. Thus, in FIG. 
9 there is illustrated a lower layer of elements 76 and an upper layer of 
elements 78, offset by one-half with reference to each other. More than 2 
layers may be employed as required. 
As indicated by the dash lines 76' in FIG. 9, the individual elements 76 
and 78 may be differential elements, wound in opposite direction in the 
two layers. 
In each of these embodiments, at least one drive element cooperates with 
the sense arrays simultaneously, or by sequential excitation of the 
sub-arrays, into different layers for reduced cross talk. Response is 
normalized by appropriately modifying the gain with reference to the 
different layers, either by software or electronically. 
FIGS. 10 and 11 depict a constructional technique, as an alternative to HDI 
fabrication techniques, where a plurality of double-sided flexible printed 
circuit board-like substructures are fabricated and joined together, with 
appropriate offsets. 
More particularly, FIG. 10 depicts a single substructure 80 formed on a 
layer 82 of flexible double-sided printed circuit board material, such as 
Kapton, generally employing conventional printed circuit fabrication 
techniques. On the top side of the layer 82 are coil halves 84 spiraling 
clockwise inwardly, and on the bottom side of the layer 82, shown in dash 
lines, are coil halves spiraling clockwise outwardly. The coil halves 84 
and 86 are connected in the center of each resultant coil through a via 
88. 
FIG. 11 illustrates how three such substructures 90, 92 and 94 may be 
assembled to form a three-dimensional array organized as a plurality of 
sub-layers. Each of the layers in FIG. 11 is shown in cross section. The 
fabrication technique of FIGS. 10 and 11 provides a viable fabrication 
technique where it is not desired to employ the HDI-fabrication technique. 
The individual substructures 90, 92 and 94 are fabricated and joined 
together by gluing or other means, with appropriate layer-to-layer offsets 
or no offset if desired. The substructures 90, 92 and 94 can be separated 
by dielectric layers (not shown). The multiple layers can be joined 
electrically using anisotropic adhesives, which conduct only in a 
direction through a thin adhesive layer, and not laterally. Such an 
anisotropic adhesive may be formed by employing an epoxy resin filled with 
conductive spheres in the form of silver-plated glass balls, which balls 
do not touch each other, where the layer thickness corresponds to the 
diameter of the conductive spheres. Alternatively, the multiple layers 90, 
92 and 94 can be joined electrically using plated through hole technology 
common to the flexible printed circuit industry after the multiple layers 
are laminated together using conventional non-conducting adhesives. 
Finally, FIG. 12 depicts an inspection method employing probe array 
structures of the invention. When physical scanning is not employed, it is 
difficult to determine what test signals mean. Thus, in FIG. 12, there is 
a test component or part 100 and a calibration or reference part 102. A 
pair of nominally identical array structures 104 and 106 as described 
hereinabove are placed in position over the respective parts 100 and 102. 
A single electronics package 108 drives the two probe arrays 104 and 106, 
and their individual signal conditioning circuits 110 and 112 which are 
connected to respective outputs of the probe arrays 104 and 106. The 
outputs of the signal conditioning circuits 110 and 112 thus represent 
test and reference signals acquired by static scanning and are applied to 
inputs of a differential amplifier 114 which generates, on line 116, a 
difference signal reflecting difference between the test part 100 and the 
reference part 102. A potential defect is indicated when the difference 
signal exceeds a predetermined threshold. 
An alternative method, which is similar, is to employ an array structure as 
described hereinabove to acquire and store a reference signal data set, 
thus in effect establishing response curves ahead of time. Subsequently, 
when an actual part or component is inspected, either the same array 
structure or a nominally identical array structure is employed to acquire 
a test signal data set. The test signal data set is then compared with the 
previously-stored reference signal data set to determine differences 
between the component and the reference part, in effect comparing current 
test signals against stored signals. 
Eddy current array technology is applicable to a variety of applications 
such as inspection of engine components. For some applications, the hybrid 
method of both electronic and mechanical scanning may be appropriate. For 
other applications, static scanning employing arrays in accordance with 
the invention is most appropriate. 
While specific embodiments of the invention have been illustrated and 
described herein, it is realized that numerous modifications and changes 
will occur to those skilled in the art. It is therefore to be understood 
that the appended claims are intended to cover all such modifications and 
changes as fall within the true spirit and scope of the invention.