WO2005081000A1

WO2005081000A1 - Method and apparatus for inspecting a printed circuit board assembly

Info

Publication number: WO2005081000A1
Application number: PCT/CH2004/000096
Authority: WO
Inventors: Thomas Spring
Original assignee: ASETRONICS AG
Current assignee: ASETRONICS AG
Priority date: 2004-02-23
Filing date: 2004-02-23
Publication date: 2005-09-01
Anticipated expiration: 2006-08-23
Also published as: WO2005081000A8

Abstract

The inventive method and apparatus allow non-contact testing of electronic modules, in particular electronic circuit boards (10), by performing the steps of disposing at least one electromagnetic emission sensing probe or a probe array (1, 11-1, 11-2, ..., 1x-y) a short distance from an electronic module (10) under test; applying test signals to operate the circuit board while sensing electromagnetic emission from a region of the circuit board near the probe or the probe array (1, 11-1, 11-2, ..., 1x-y); receiving time domain signals provided by the probe or the probe array (1, 11-1, 11-2, ..., 1x-y); simultaneously or sequentially establishing frequency domain information of the sensed electromagnetic emission for n sub-ranges of a second frequency range that is contained in the audible frequency range; and selectively emitting audible signals that correspond to the frequency domain information in the second frequency range. The acoustical signature, resulting from said procedures, can be perceived by the operator of the apparatus and compared to a previously heard acoustical signature of an already proven electronic module in order to detect a malfunction of the electronic circuit board (10) under test.

Description

Method and apparatus for non-contact testing of electronic modules

This invention relates to a method and an apparatus for non- contact testing of electronic modules according to claim 1 and claim 9.

The present invention relates in particular to methods and apparatus for automated non-contact testing of electronic circuit boards .

As described in [1], U.S. Pat. No. 5,406,209, electronic circuit boards can conventionally be tested by physically contacting test points on the circuit board with electric probes to sense voltages at those test points. The sensed voltages for the circuit board under test can then be compared to calculated voltages or voltages sensed from a circuit board known to be operating properly to determine whether the circuit board under test is operating properly.

A further advanced testing apparatus allows automated contact testing of circuit boards. Such automated testing apparatus may have an array of probes arranged in a "bed-of-nails" configuration for simultaneously contacting several different test points on the circuit board under test.

Disadvantages of the described methods, such as limited process speed, requirement of space on the circuit boards for test points, as well as alignment and contact problems that may arise during test procedures, are described in [1] .

An apparatus, that avoids these problems by means of automated non-contact testing of electronic circuit boards, is described in [2], U.S. Pat. No. 5,006,788. The disclosed apparatus comprises a rectangular array of wire loop probes, a tuned

BESTATIGUNGSKOPIE receiver, and addressing circuitry for connecting individual probes of the array to the receiver. The apparatus further comprises a signal processor for processing signals detected by the probes, and a display for displaying the received signals as a two-dimensional map. Said apparatus is operated by placing the array of wire loop probes adjacent to an electronic circuit board and, with the electronic circuit board in operation, successively connecting individual probes of the probe array to the tuned receiver. Local magnetic fields caused by electromagnetic emission from the operating circuit board induce currents in the wire loop probes, and these currents are successively sensed by the tuned receiver. The signal processor assembles the successively sampled currents into a data file which is displayed as a two-dimensional map of electromagnetic emissions at the frequency to which the receiver is tuned. As further described in [1] , the apparatus disclosed, in [2] can be used to distinguish properly operating circuit boards from improperly operating circuit boards. The improperly operating circuit boards should have a different pattern of current flow leading to a different pattern of electromagnetic emissions as compared to properly operating circuit boards. Assuming that all properly operating circuit boards have a sufficiently similar pattern of electromagnetic emissions, the improperly operating circuit boards should be distinguishable as those circuit boards which do not have the pattern of electromagnetic emissions which is characteristic of properly operating circuit boards .

However, as still further detailed in [1] , the electromagnetic emissions of properly operating circuit boards do not always have a pattern of electromagnetic emissions which is distinct enough to permit reliable discrimination of properly and improperly operating circuit boards. In fact, the difference in electromagnetic emission maps at a specific frequency for two properly operating circuit boards is sometimes greater than the difference in emission maps for properly and improperly operating circuit boards so that improperly operating circuit boards cannot be readily distinguished from properly operating circuit boards in some cases.

In order to improve the solution disclosed in [2] , tine method for non-contact testing of electronic circuit boards disclosed in [1] comprises disposing at least one electromagnetic emission sensing probe a short distance from a circu.it board under test, operating the circuit board while sensing electromagnetic emission from a region of the circuit board near the probe, developing a time domain representation of the sensed electromagnetic emission, and comparing the time domain representation of the sensed electromagnetic emission to a time domain representation of electromagnetic emission of a circuit board known to be operating properly. According to [1] , the time domain representation of the sensed electromagnetic emission contains more information than the limited frequency domain representation provided by the method disclosed in [1] , since the time domain representation includes phase information which is not available in a frequency domain representation. Such phase information is often critical to proper assessment and diagnosis of circuit board operation.

However, the establishment and exploitation of the time domain representation of the sensed electromagnetic emission, in particular the exploitation of detected phase differences, requires not only high processing capabilities but also a sophisticated software solution that allows a correct interpretation of the time domain representation of trie sensed electromagnetic emission and the detected phase differences for individual units under test. Hence, efforts to create a test apparatus and in particular the software that correctly interprets phase differences and that allows adaptation to individual tests units are therefore considerable. Moreover, due to changes in the environment , e.g. changes of temperature and humidity, signals emzLtted by the test unit may drift, causing phase and frequency changes, which may not be correctly interpreted.

It would therefore be desirable to jprovide an improved method and an apparatus for testing electronic modules, in particular circuit boards .

It would be desirable in particular to provide a method that can be implemented with reduced effort, that allows detection of malfunctioning electronic modules with high probability and that allows fast adaptation to different electronic modules as well as to changes in the environment .

SUMMARY OF THE INVENTION

The above and other objects of t-he present invention are achieved by a method and an apparatus according to claim 1 and 9.

The inventive method and apparatus, allow non-contact testing of electronic modules, in particular electronic circuit boards, by performing the steps of

a) disposing at least one electroTnagnetic emission sensing probe or a probe array a short distance from an electronic module under test;

b) applying test signals to operate the electronic module while sensing electromagnetic emission from a region of the electronic module near the probe or the probe array;

c) receiving time domain signals provided by the probe or the probe array;

d) simultaneously or sequentially establishing frequency domain information of the sensed electromagnetic emission for a second frequency range that is contained in the audible frequency range; and

e) selectively emitting audible signals that correspond to the frequency domain information in the second frequency range.

Hence, the present invention allows establishing an acoustical signature of the electromagnetic emission of the electronic module or parts thereof. The acoustical signature can be perceived by the operator of the apparatus and compared to a previously heard acoustical signature of an already proven electronic module.

The operator of the apparatus, who has adapted his perception to the acoustical signature of proven electronic modules, can therefore easily detect fine differences in the acoustical signatures. Further he will be able to build up a large knowledge base of acoustical signatures of elements of the electronic modules. He will quickly learn how acoustical signatures of good or faulty electronic modules typically appear. Still further he will quickly develop experience that allows differentiating between changes of the acoustical signature that are caused by environmental influences and changes of the acoustical signature that are caused by malfunctions of the unit under test.

In production processes faulty boards can therefore easily be detected and redirected to a repair center. In the repair center an engineer may use an inventive apparatus like a stethoscope in order to check signals that should be present in certain locations of the board. E.g., the engineer will soon learn how a well operating bus system or clock generation system will sound like. Since produced boards that have been detected and routed to the repair center, normally show one of a few typical faults the engineer can quickly detect the fault by obtaining the acoustical signatures at the areas of interest. In order to facilitate fault detection he may use the module under test in parallel to a proven module, so that the sound emissions can be compared.

For use in production processes the apparatus preferably comprises a probe array with numerous probe elements that are capable to detect electromagnetic emissions. For use in a repair center an apparatus with a single probe, that may be guided manually, may be preferable.

In order to provide an acoustical signature, that corresponds to the electromagnetic emission of the electronic module, the audible signals are sequentially emitted, preferably separated by short intervals, in decreasing or increasing order with a signal duration of less than one second, preferably in the range of 100 ms, in order to provide a signal sequence or acoustical signature of a length of 1 to 5 seconds. In a preferred embodiment each signature is initiated with a signal that indicates the start of the sequence.

The transformation of the sensed signals from the time domain to the frequency domain representation, as well as the conversion of the frequency of the sensed signal from a first frequency range possibly via an intermediate frequency range to the second frequency range can be performed in different ways.

Frequency domain information of the signals originating from the electronic module can be established by means of a tuneable receiver, by means of band pass filters that correspond to the sub-ranges of the first or second frequency range, or by means of Fourier Transformation.

Frequency conversion can be performed in the time domain by means of programmable mixing stages, e.g. by frequency dividers as described in [3], U.S. Pat. 6,665,368 B2 , or after analog to digital conversion in digital signal processors. Fourier Transformation as described in [4] , EP 1 162 545 A2 and frequency conversion can be performed for example by means of a digital signal processor.

Hence, the frequency domain information provided for the n subranges in the first or second frequency range can be presented in various ways, e.g. by distinct time domain signals provided for the n sub-ranges of the first or second frequency range or by data relating to the frequencies and amplitudes of the received time domain signals.

As an alternative the time or frequency domain information established for all of the n sub-ranges of the first frequency range is integrated in order to create values that represent the intensity of the signals that have been emitted in said n sub-ranges of the first frequency range. Then, for each of the resulting values a corresponding audible frequency is selected or generated in the second frequency range, e.g. by means of a signal generator.

The resulting acoustical signatures may therefore be emitted in ascending, descending or, typically, in alternating frequency order .

In a preferred embodiment the inventive apparatus comprises a test unit controller that is used to operate the electronic module under test . The test sequences initiated by the test unit controller and the measurement procedures are preferably synchronised by means of the system controller, so that an optimal performance is achieved. If the probe is located above the memory bus system, then the test unit controller is preferably instructed to perform memory read and write operations. If a probe array is used, then the corresponding probe elements may selectively be activated.

However it is important to note that the inventive unit can also advantageously be applied without a special test unit. A test can already be performed when standard operation conditions are established on the electronic module.

The inventive apparatus is a valuable alternative to the test equipment described in [1] and [2] . However the equipment described in [1] and [2] may provide additional information to the operator of the inventive apparatus. Methods described in [1] and [2] may therefore be implemented in parallel to the inventive method. If a comparison of the time domain representation of the sensed electromagnetic emission with the time domain representation of electromagnetic emission of a circuit board known to be operating properly indicates a faulty board as described in [1] , then the operator of the inventive apparatus may rehear the acoustical signature to verify the previous judgement. In addition to the acoustical signature, as described in [2] , a memory map of the electromagnetic emission as a function of board position may be generated and displayed together with the circuit board layout so that regions of high emission level can be identified in the circuit. Documents [1] and [2] are therefore incorporated herein in their entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

Some of the objects and advantages of the present invention have been stated, others will appear when the following description is considered together with the accompanying drawings, in which:

Figure 1 shows an inventive apparatus with a probe array 1 mounted, moveable or fixed, above an electronic module 10 that is under test;

Figure 2 shows an inventive apparatus with a probe 1, that comprises a single probe element, manually held above the electronic module 10 under test; Figure 3 shows a block diagram of an inventive apparatus that comprises a tuneable receiver 3;

Figure 4 shows a block diagram of an inventive apparatus that comprises a receiver 3 followed by a module 31 that performs Fourier Transformation;

Figure 5 shows a block diagram of an inventive apparatus that comprises a receiver 3 followed by a transformer 32 that comprises band pass filters;

Figure 6 shows stages of the inventive apparatus that are designed for signal transforming and converting;

Figure 7 shows a time domain representation of signals sensed by the probe 1, 1-1 and different representations of these signals in the frequency domain;

Figure 8 shows an inventive apparatus that comprises an integrator 30; and

Figure 9 illustrates the functionalities of the inventive apparatus shown in figure 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows a block diagram of an inventive apparatus that comprises a probe array 1 with numerous probe elements lι-ι, l -

₂, ..., l_x-y arranged in x rows and y columns. The probe array 1 is mounted fixed or moveable above an electronic circuit board

10 that is under test. The electronic circuit board 10 is driven by a test unit controller 9 that runs test sequences, which allow the generation of a distinct acoustical signature.

During the test procedures the electrically conductive traces on the electronic circuit board 10 emit electromagnetic waves that are sensed by the probe elements li-i, lι-₂, — r lχ-_y of the probe array 1 which for that purpose is brought into the near vicinity of the electronic circuit board 10. A probe controller 2 is provided to selectively connect individual probe elements lχ-ι;, lι-₂; •»; lχ-_y to a receiver 3, which is in tuneable to n sub-ranges of a first frequency range that corresponds to the bandwidth of the electromagnetic waves emitted by the electronic circuit board 10. This first frequency range and the number of its sub-ranges are preferably adaptable to different kinds of test units which operate in different frequency ranges. It may even be necessary to use different probe arrays 1 for test units 10 that use different clock frequencies.

For a specific test condition, the receiver 3 is tuned to all sub-ranges of the first frequency range in order to establish frequency domain information for the electromagnetic waves emitted by the electronic circuit board 10. By this step, the electromagnetic waves emitted by the electronic circuit board 10 are split into signals with frequencies in the corresponding sub-ranges of the first frequency range. The signals provided for the sub-ranges of the first frequency range are in the time domain but provide frequency domain information for the electromagnetic waves emitted by the electronic circuit board 10. As shown in Figure 7a the electromagnetic waves are split into different signals s_Rι, ..., s_Rrι or signal mixtures for the sub-ranges f_Ri, ..., f_Rn of the first frequency range. The signals may further be processed in this mode or only after analog to digital conversion as symbolically shown by the dotted lines in figure 7b. As further shown in figure 7b by rectification of the scanned signals the complete frequency spectrum, i.e. the complete frequency domain information for the electromagnetic waves emitted by the electronic circuit board 10 could be established. As detailed below (see figures 3 to 5) , the required frequency domain information can also simultaneously be established by means of a Fourier Transformation module 31 or band filters 32. Subsequent to the receiver 3 a frequency converter is provided, which converts the established frequency domain information from the sub-ranges of the first frequency range to corresponding sub-ranges of a second frequency range that is contained in the audible frequency range . The techniques used for converting the frequency domain information depend on the mode in which the frequency domain information is present. If the frequency domain information is provided as a series of independent time domain signals then the frequency conversion may be achieved by mixing stages MX, filters as shown in figure 3. A frequency divider with .a high input frequency is described for example in [3] . The frequency conversion of digitised signals in the time or frequency domain may however be performed much simpler with mathematical algorithms. Hence, frequency conversion can easily be done by a signal processor. Since A/D conversion of signals in higher frequency ranges requires expensive components it may be advisable to use frequency dividers or frequency offset stages before the signals are digitised. Figure 3 shows for example the use of a controllable offset stage 33 that allows shifting of the frequencies of the input signals to the second or to an intermediate frequency range .

After the frequency conversion, the signals are amplified by an amplifier 5 and forwarded to a transducer 6, e.g. an electro- dynamical loudspeaker in such a way that an acoustical signature is presented to the operator of the inventive apparatus .

The audible signals are sequentially emitted, preferably separated by intervals, in ascending, descending or alternating frequency order with signal duration of less than one second, preferably in the range of 100 ms, in order to provide a signal sequence or acoustical signature of a length of 1 to 5 seconds. An acoustical signature consists for example of n audible sounds of length of 250 ms which are separated by intervals with a length of 100 ms . A digital signature may comprise sequences of sounds that alternate in ascending and descending order. The presentation of the signature, amplification or filtering of individual signals or frequency ranges may be adjustable by the operator to optimize perception of the signature. The emission of an acoustical signature can be initiated with a typical start signal that alerts the test engineer.

In the sub-ranges of the first or the second frequency range the frequency domain information is preferably digitized and stored in a memory unit 82. The stored acoustical signature can therefore be repeated and modified as required by the operator.

The operations and procedures performed by the inventive apparatus are controlled by a system controller 8 that comprises a processing unit 81, the above mentioned memory unit 82, as well as the required application programs 83, that is directly connected to an input device 7 and that is connected to the modules 2, 3, 4, 5, 6 and 9 of the inventive apparatus via a system bus 85. As symbolically shown in figure 2 the system controller 8 and further modules, in particular modules for processing digitised frequency domain information and for the amplification of signals in the second frequency range, can be implemented by the use of standard modules of a personal computer. The probe controller 2, the receiver 3 and an analog to digital converter ADC may be integrated on a single electronic circuit board that can be connected to the system bus of the personal computer 80. The test unit controller 9, which may be implemented in a separate personal computer or which may be realised as a separate electronic circuit board may be connected to an interface or directly to the system bus of the personal computer 80.

In order to detect the relevant emissions of the electronic circuit board 10 the emission activities initiated by the test unit controller 9 and the detection activities of the probe array 1 and the processing procedures in the further stages are preferably harmonised by the system controller 8. If the probe array 1 is large enough, thus covering the electronic circuit board 10 completely with its probe elements li-i;, lι-₂; •■•; lχ-y / then the system controller 8 will address the probe element l_x-_y that is located above the area of interest in order to detect the expected signals. Further the system controller 8 may tune the receiver to the frequency of the expected signals. However if the probe array 1 is to small for completely covering the electronic circuit board 10 then a mechanical device that holds the probe array 1 may be driven accordingly to transfer the probe array to the area of interest .

Figure 2 shows an inventive apparatus with a probe 1 that comprises a probe with a single probe element 1-1. This probe 1 is manually positioned above the areas of interest of the electronic circuit board 10. The probe 1 comprises preferably electrically insulating elements that can be positioned on the electronic circuit board 10. Since the probe is manually held it is preferably integrated together with the input device 7 in a common casing, so that the test operations can easily be controlled by the operator. The inventive apparatus can be applied like stethoscope for analysing the status of a test unit .

The detection, transformation, conversion and procedures to compose acoustical signatures can be performed in numerous advantageous ways .

Figure 3 shows a further block diagram of an inventive apparatus that comprises a tuneable receiver 3, that allows scanning of the frequency spectrum of the electromagnetic waves sensed by the probe 1. The electromagnetic waves, designated foAL are applied together with a tuning signal f_TuNE to a first mixing stage MX1. At the output of the mixing stage MX1 the difference frequency f_TOτAL - fτijNE = fSELECT is presented to a filter 1 which suppresses further mixer products . In the subsequent offset stage 33 or by the frequency converter 4 the frequency is converted to the second frequency range. With this embodiment of the inventive apparatus the signals can be converted and emitted without intermediate A/D and D/A conversion. The acoustical signature is determined by the tuning sequence applied in the receiver 3.

Figure 4 shows a block diagram of an inventive apparatus that comprises a receiver 3 and an A/D converter followed by a transformer 31 which performs a Fourier Transformation. The transformation of the electromagnetic waves sensed by the probe

1 is performed simultaneously for all sub-ranges of the first or second frequency range. The frequency conversion of the signals sensed by the probe 1 may be performed before or after the Fourier Transformation. Converting the frequency to the second frequency range before the Fourier Transformation allows use of less expensive signal processors but requires additional

RF-circuitry at the input stages. As described in [4], Fourier Transformation can be performed by means of a digital signal processor.

Figure 5 shows a block diagram of an inventive apparatus that comprises a receiver 3 followed by a transformer 32 that comprises band filters 32 which divide the frequency spectrum of the sensed electromagnetic waves into the sub-ranges of the first or second frequency range. The band filters 32 are followed by an A/D converter, a frequency converter 4 and a memory unit 82, in which the signals are stored either in the time or frequency domain. A selector 8, preferably the system controller, assembles the acoustical signature by reading the signals sequentially out of the memory unit 82.

Figure 6 shows the transformer and converter stages of the inventive apparatus of figure 4 in further detail. Electromagnetic waves sensed by the probe 1 are digitised by means of an A/D converter, before the Fourier Transformation is performed in the subsequent stage 31 in order to establishing frequency domain information for the sub-ranges SUB11, ..., SUBln of the first frequency range. Subsequently the established frequency domain information is processed by the frequency converter and forwarded to the sub-ranges SUB21, ..., SUB2n of the second frequency range .

Figure 7 shows, as described above, a time domain representation of signals sensed by the probe 1 and different representations of this signals in the frequency domain.

Figure 8 shows another preferred embodiment of an inventive apparatus . Subsequent to the receiver 3 of this apparatus is an integrator 30 provided that integrates frequency domain information, that is sequentially represented by scanned time domain signals, for all of the n sub-ranges of the first frequency range in order to create values v_x, ..., v_n that represent the intensity of the signals that have been emitted in said n sub-ranges of the first frequency range.

The individual values Vi, ..., v_n provided by the integrator 30 are sequentially, controlled by the system controller, forwarded to a signal generator 45 that provides for each of the values Vi, ..., v_n a corresponding audible frequency fi, ..., f_n of the second frequency range .

The acoustical signature provided with this apparatus comprises therefore frequencies that may sequentially follow in ascending, descending or, typically, in alternating order.

Since the frequency changes in these acoustical signatures are typically significantly higher, recognition of deviations that indicate a malfunction of the electronic module 10 may be easier. What has been described above is merely illustrative of the application of the principles of the present invention. Other implementations of the inventive method and apparatus can be realised by those skilled in the art without departing from the spirit and scope of protection of the present invention. Men skilled in the art may perform signal processing with known hardware or software components. Further techniques to process signals may be applied. Various control signals may be added to the acoustical signature to provide further information to the operator. One or more acoustical signatures with intermittent control signals may be composed for a test unit 10.

The inventive apparatus is a valuable alternative, but also a possible enhancement to the test equipment described in [1] and [2] . In addition to providing acoustical signatures, an inventive apparatus enhanced with solutions described in [1] and/or [2] may in addition compare electromagnetic signatures and/or display the established acoustical or electromagnetic signatures .

REFERENCES:

[1] U.S. Pat. No. 5,406,209 [2] U.S. Pat. No. 5,006,788 [3] U.S. Pat. 6,665,368 B2 [4] EP 1 162 545 A2

Claims

1. Method for non-contact testing of electronic modules (10), in particular electronic circuit boards, the method comprising: a) disposing at least one electromagnetic emission sensing probe or a probe array (1, li-i, lι- , .», l_x-_y) a short distance from an electronic module (10) under test; b) applying test signals and/or the required supply voltage to operate the electronic module (10) while sensing electromagnetic emission from a region of the electronic module (10) near the probe or the probe array (1, lι_χ , l .₂ , -, lχ-y) ; c) receiving time domain signals provided by the probe or the probe array (1, l_x-_x , l ._{2 l} ..., l_x__y) ; d) simultaneously or sequentially establishing frequency domain information of the sensed electromagnetic emission for a second frequency range that is contained in the audible frequency range; and e) selectively emitting audible signals that correspond to the frequency domain information in the second frequency range .

2. A method as defined in claim 1, wherein the step of frequency domain information comprises dl) simultaneously or sequentially establishing frequency domain information of the sensed electromagnetic emission for n sub-ranges of a first frequency range; and d2) converting frequency domain information of the first requency range into frequency domain information for n sub-ranges of the second frequency range.

3. A method as defined in claim 1 or 2 , wherein the frequency domain information provided for the n sub-ranges in the first or second frequency range is represented by distinct time domain signals or by data relating to the frequencies and amplitudes of the received time domain signals.

4. A method as defined in claim 1, 2 or 3, wherein the audible signals, that correspond to the electromagnetic emission of the electronic module (10) are established and sequentially emitted by a) selectively establishing frequency domain information for one of the n sub-ranges of the first frequency range, converting said frequency domain information into the corresponding sub-range of the second frequency range, emitting a corresponding audible signal and performing these steps for the further subranges of the first frequency range; or b) completely establishing frequency domain information for all n sub-ranges of the first frequency range, selecting frequency domain information from one of the n sub-ranges of the first frequency range, converting said frequency domain information into the corresponding sub-range of the second frequency range, emitting a corresponding audible signal and selecting equency domain information from the next of the n sub-ranges of the first frequency range; or c) completely establishing frequency domain information for all n sub-ranges of the first and, by frequency conversion, for to the second frequency range, selecting frequency domain information from one of the n sub-ranges of the second frequency range, emitting a corresponding audible signal and performing these steps for the further sub-ranges of the first frequency range and emitting a corresponding audible signal and selecting frequency domain information from the next of the n sub-ranges of the second frequency range.

5. A method as defined in one of the claims 1 to 4, a) wherein the time or frequency domain information for all of the n sub-ranges of the first frequency range is integrated in order to create values (v_l ..., v_n) that represent the intensity of the signals that have been emitted in said n sub-ranges of the first frequency range ; b) wherein for each of the values ( i, ..., v_n) a corresponding audible frequency (f_x, ..., f_n) is selected or generated in the second frequency range.

6. A method as defined in claim 4 or 5 , wherein the audible signals are sequentially emitted, preferably separated by intervals, in ascending or descending order with signal duration of less than one second, preferably in the range of 100 ms , in order to provide a signal sequence or acoustical signature of a length of 1 to 5 seconds.

7. A method as defined in one of the claims 1 to 6, wherein a) the electromagnetic signals sensed by the probe or the probe array (1, li-i, lι-₂/ —, lχ-_y) are forwarded to a tuneable receiver that is sequentially tuned to the sub-ranges of the first frequency range in order to sample corresponding frequency domain information, followed by converting the frequency domain information of the first frequency range to the second frequency range ; or b) the electromagnetic signals sensed by the probe or the probe array (1, lχ-ι, lι-2 — lχ-_y) are forwarded to band pass-filters that provide frequency domain information for the corresponding sub-ranges of the first frequency range, followed by converting the frequency domain information of the first frequency range to the second frequency range; or c) the electromagnetic signals sensed by the probe or the probe array (1, lχ-ι, I1-2/ —, lχ-_y) in the time domain are transformed in the first or second frequency range by means of a transformation method, such as Fast Fourier Transformation, into the frequency domain, in order to establish provide the frequency domain information for the n sub-ranges of the first or second frequency range.

8. A method as defined in one of the claims 1 to 7 , comprising the steps of storing, filtering, averaging and/or amplifying frequency domain information or signals provided in the first or second frequency range.

9. A method as defined in one of the claims 1 to 8, comprising the steps of successively disposing the probe or the probe array (1, li-i, lι-₂/ — lχ-_y) ; preferably synchronised to the sequence of test signals; in different locations, each location being a short distance from a corresponding region of the electronic module (10) ; and repeating application of the time dependent test signals to operate the electronic module (10) while successively sensing electromagnetic emission at each location of the probe or the probe array (1, li-i, I1-2, —/ lχ-_y) ; successively developing frequency domain representations of the sensed electromagnetic emission and establishing sequences auf audible signals with preferably alternatively ascending and descending frequency order.

10. Apparatus for non-contact testing of an electronic module (10) such as an electronic circuit board, comprising a) a probe or a probe array (1, lι_ι, l_ι-₂, -, l_x-_y) for non- contact sensing electromagnetic emission from at least one region of the electronic module (10) near the probe or the probe array (1, li-i, lι-₂, •■•/ lχ-_y) while test signals and/or supply voltage are applied to the electronic module (10) ; b) a receiver (3) designed for receiving and amplification of the signals supplied by the probe or the probe array

c) means (3, 8) for simultaneously or sequentially establishing frequency domain information of the sensed electromagnetic emission for a second frequency range that is contained in the audible frequency range; d) a system controller (8) designed for sequentially selecting frequency domain information from the second frequency range for emission purposes; and e) an amplifier (5) and a transducer (6) for emitting audible signals that correspond to the frequency domain information in the second frequency range.

11. Apparatus as defined in claim 10, comprising cl) means (3, 8) for simultaneously or sequentially establishing frequency domain information of the sensed electromagnetic emission for n sub-ranges of a first frequency range; and c2) a converter module (4) designed for converting frequency domain information from the first frequency range into frequency domain information for the n sub- ranges of the second frequency range.

12. Apparatus as defined in claim 10, comprising cl) means (3, 8) for simultaneously or sequentially establishing frequency domain information of the sensed electromagnetic emission for n sub-ranges of a first frequency range; and c2) an integrator (30) that is designed to integrate the frequency domain information for all of the n subranges of the first frequency range in order to create values (vi, ..., v_n) that represent the intensity of the signals that have been emitted in said n sub-ranges of the first frequency range; c3) a converter module (4) and/or a signal generator (25) designed to provide for each of the values (v_1; ..., v_n) a corresponding audible frequency ( f_{l r} ..., f_n) in the second frequency range.

13. Apparatus as defined in claim 10, 11 or 12, comprising at least one memory module (82) that is used to store the frequency domain information of the n sub-ranges of the first and/or second frequency range .

14. Apparatus as defined in one of the claims 10-13, wherein, in order to establish frequency domain information, a) the receiver (3) is tuneable to the n sub-ranges of the first frequency range; or b) the receiver (3) comprises band pass filters (32) that are tuned to the n sub-ranges of the first frequency range ; or c) the receiver (3) is followed by cl) a first transformation module (DFT/FFT) that is designed to perform a transformation of the signals from the time domain to the frequency domain c2) a second, transformation module that is designed to provide frequency domain information for each of the n sub-ranges of the second frequency range; and a c3) a third transformation module that is designed to ^• provide time domain signals relating to the frequency domain information for each of the n sub-ranges of the second frequency range.

15. Apparatus as defined in one of the claims 10-14, wherein the receiver (3) comprises at least one fixed or variable offset stage (33) designed to convert the frequency of the signals supplied by the probe or the probe array (1, Ix._± , lι-2 ■■•/ lχ-y) to the second frequency range or to an intermediate third frequency range.

16. Apparatus as defined in one of the claims 10-15, wherein an A/D Converter (ADC) is provided to digitize a) the signals supplied by the probe or the probe array (1, li-i, I1-2, .», lχ-_y) ; or b) the signals provided by the receiver (3) after an amplification stage; or c) the signals provided by the receiver (3) at the output of the band pass filters (32) ; or d) the signals provided by offset stage (33) ; and wherein a D/A Converter (DAC) is provided following the memory module (82) that is used to store the frequency domain information of the n sub-ranges of the first and/or second frequency range .

17. Apparatus as defined in one of the claims 10-16 comprising a) a signal processor (DSP) that is performing the tasks of the integrator module (30) and/or the converter module (4) ; b) a probe controller (2) that allows selectively connecting one or more probe units (li-i, lι-2/ —_/ lχ-_y) of the probe array (1) to the receiver (3) ; and/or c) a test unit controller (9) that is designed to apply test signals to the electronic module (10) .

18. Apparatus as defined in one of the claims 10-17, wherein the system controller (8) is designed to coordinate the application of the test signals, the selective connection of the probe units (li-i, I1-2, —/ lχ-_y) , the selection of tuning and offset frequencies in the receiver (3) and/or the selection of audible signals in order to provide an acoustical signature of the electromagnetic emission of the electronic module .

19. Apparatus as defined in one of the claims 10-18, comprising means to adjust the ratios for the frequency conversions from the first to the second frequency range and/or comprising means to selectively filter and/or amplify the audible signals relating to the n sub-ranges of the second frequency range.

20. Apparatus as defined in one of the claims 10-19, comprising at least one signal generator (45) that is designed to transform frequency domain information provided for the n sub-ranges of the second frequency range into time domain signals.