Thermal-acoustic scanning systems and methods

Systems and methods for identifying thermal response of a test structure. An example of the system includes a device that generates a excitation beam, a scanning device that scans the thermal excitation beam over a test structure, and a detector that detects acoustic waves produced scanning by the test structure. The system also includes a display device that generates and displays an image based on the detected acoustic waves. A second detector detects a reflection of the beam off of the test structure and the display device generates and displays an image based on the detected reflection.

FIELD OF THE INVENTION

This invention relates generally to laser scanning systems and methods.

BACKGROUND OF THE INVENTION

Lasers can be used to locally heat a test structure. Localized heating produces a temperature gradient, which, in turn, produces stress and strain in the heated area. If the heating varies with time, then the resultant stress and strain will also vary with time. Time varying stress and strain produce acoustic/sound waves.

Pulsed lasers have been utilized to produce a thermal impulse response in a test structure. The resultant acoustic pulse is then detected after propagation through the test structure. These techniques are utilized to determine the acoustic propagation properties of the test structure. The acoustic propagation properties can be utilized to identify subsurface flaws and defects. These techniques are equivalent to standard ultrasound techniques utilized for the same purpose. A uniform surface structure is desirable for these techniques in order to obtain a constant amplitude and phase of the laser generated acoustic pulse.

A continuous wave (CW) laser impinging onto a test structure will produce no acoustic waves (sound). Scanning of a CW laser will produce time dependent heating. However, a uniform surface structure is still unlikely to produce any acoustic waves. It has been determined experimentally that scanning of a CW laser over a non-uniform test structure will produce acoustic waves. In particular, test structures with multiple materials present, e.g. a printed circuit board with components soldered into place, will produce a series of “pops and pings”, i.e. a laser thermal-acoustic signal. These pops and pings are a result of the differential expansion and contraction causing a release of built up stress and strain of the differing materials as they are heated and cooled. In particular, components, e.g. vias, solder joints, etc., on the test structure will produce different acoustic signatures, depending on the quality of that component.

Therefore, there exists a need to identify thermal response to determine the quality of the test structure components.

BRIEF SUMMARY OF THE INVENTION

The present invention provides systems and methods for identifying thermal response of a test structure. An example of the system includes a device that generates a continuous wave (CW) thermal excitation beam, a scanning device that scans the thermal excitation beam over a test structure, and a detector that detects acoustic waves produced by the test structure. The system also includes a display device that generates and displays an image based on the detected acoustic waves.

In another aspect of the invention, a second detector detects a reflection of the beam off of the test structure and the display device generates and displays an image based on the detected reflection.

In still another aspect of the invention, the detector includes a piezoelectric detector.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1a block diagram of an example system20is shown. The system20senses properties of a test structure2. The test structure2can be any physical object, such as an integrated circuit, the package in which the integrated circuit is housed, a printed circuit board, composite materials, etc. In one embodiment, the system20locally heats the test structure2and then generates an image relating to the thermally induced mechanical stress/strain of the test structure2. The stress/strain is related to the quality of the test structure2.

The system20includes a thermal excitation source4, a scanning device8, an acoustic sensor12, a signal processor and display device16, and an imaging sensor18. The thermal excitation source4generates a continuous wave (cw) thermal excitation beam6. There are many options for the thermal excitation source4, including but not limited to:Optical, e.g. a laserElectron beamIon beamAcoustic

The thermal excitation beam6is directed towards the scanning device8. If the thermal excitation beam6is an optical thermal excitation beam, the scanning device8may include a scanning mirror assembly102coupled with a lens104for focusing the excitation beam6onto the test structure2. If the beam6is an electron or ion beam, the scanning device8may include equivalent electromagnetic beam deflectors and focusing elements. The scanning device8directs the excitation beam6on to the test structure2and scans the beam across the test structure2. Any assembly that produces this result is acceptable. Shown inFIG. 1is an arrow10that indicates a possible upwards scanning motion of the excitation beam6. Note that the test structure2may also be moved to perform the scanning function (e.g., a scanning table).

The acoustic sensor12is placed in good acoustic contact with the test structure2. A piezoelectric type ultrasonic transducer is an example of the acoustic sensor12. The Piezoelectric transducer may include a fluid or gel coupling to achieve good acoustic contact. The sensor12may also include a laser acoustic sensor which does not require direct contact with the test structure2. The acoustic sensor12picks up “pops and pings” that are produced by the test structure2as the beam6scans. The pops and pings are acoustic signals that propagate through the test structure2and can be picked up at any convenient point on the test structure2. Multiple acoustic sensors12may be utilized to enhance the sensing process, e.g. obtain phase information. Some acoustic sensor placement points may produce better signal to noise than others. Generally an operator would select a sensor placement point that optimizes the acoustic sensing process.

The acoustic sensor12produces an acoustic signal, which is sent to the signal processor and display device16. In one embodiment, the device16collects the output of the acoustic sensor12as a function of position of the thermal excitation beam6on the test structure2. In another embodiment, the device16displays an image of the thermal-acoustic response of the test structure2.

As a specific implementation in the form of a laser scanning imaging system, the thermal excitation source4is a laser. The laser beam propagates to the scanning device8. The laser beam passes through the scanning mirror assembly102, which deflects the laser beam at an angle versus time. The first lens104transforms the angular scan into a position scan on the test structure2. In another embodiment, the first lens104also focuses the laser beam onto the test structure2.

A sufficiently high power laser beam can heat the test structure2by tens to hundreds of degrees centigrade. This temperature rise induces stress/strain in the test structure2which in turn produces an acoustic response, which is picked up by the acoustic sensor12and displayed by the signal processor and display device16.

An additional feature of the laser scanning implementation, is the ability to collect a reflected light image simultaneously with the thermal-acoustic image through use of the reflected light sensor18. The thermal excitation beam6, when in the form of a laser beam is reflected back from the test structure2and recollected by the first lens104. The return beam (i.e., reflection) passes back through the scanning device8and redirected by a beam splitter202towards a second lens204, which focuses the return beam onto a detector206. Note that the second lens204is not strictly necessary in all laser-scanning configurations. The detector206produces a reflected light signal208that is proportional to the amount of the laser beam reflected back from the test structure2. The reflected light signal208is sent to the signal processor and display device16where an image of the test structure2can be generated and displayed in conjunction with a thermal-acoustic image. As both images are obtained simultaneously, they will be spatially correlated. The reflected light image can therefore be used for navigation and location purposes.

Although not necessary for basic operation, the thermal excitation beam6could be modulated to enhance signal-to-noise via lock-in or other synchronous detection techniques.