Thermoelastic Regime Research Articles

Nondestructive Evaluation of Poor-Wetted Lead-Free Solder Bumps in Ball Grid Array Packages Using Laser Ultrasound and Interferometric Technique

A laser ultrasound and interferometer inspection system has been successfully applied to detect solder bump defects, including missing, misaligned, open, and cracked solder bumps in flip chips, land grid array packages and chip capacitors. This system uses a pulsed Nd:YAG laser to induce ultrasound in the chip packages in the thermoelastic regime; it then measures the transient out-of-plane displacement response on the package surface using a laser interferometer. The amplitudes of the out-of-plane displacement responses are usually in the order of nanometer. The quality of solder bumps is evaluated by analyzing the transient responses. In this paper, this system is used to evaluate the quality of lead-free solder bumps in ball grid array (BGA) packages; specifically, BGA packages with poor wetted solder bumps are used as test vehicles. Poor wetting not only decreases the mechanical strength of interconnection at the interface between the solder bumps and substrate, but also deteriorates electrical and thermal performance because of the unreliable connections between the package and the printed circuit board (PCB). Causes of poor wetting vary from the solder materials themselves to the manufacturing process. Here, poor wetting of solder bumps were intentionally created by using an improper reflow profile. The transient out-of-plane displacement signal responses from these packages are compared to a hybrid reference signal generated by signal responses from defect-free samples. Packages with poor wetted solder bumps are distinguished from the normal packages by using the correlation coefficient signal analysis method. Then, laser ultrasound inspection results are also compared with results from continuity test, X-ray inspection and scanning acoustic microscope (SAM). Finally, the cross-section images are further used to confirm the existence of the poor wetting in samples with unusual correlation coefficient. It can be concluded that this laser-ultrasound system is capable of identifying the presence of poor wetting in BGA packages.

Evaluation of Lead-Free Solder Bump Voiding Ball Grid Array Packages Using Laser Ultrasound and Interferometric Technique

Solder joint voids are usually formed by the entrapped gas bubbles during the reflow process and are common in all surface mount applications. It is a controversial issue on the reliability of the solder joint; however, the consensus is that voiding is only acceptable at low contents, whereas excessive voiding affects mechanical properties, and decreases strength, ductility, and fatigue life of the interconnections. X-ray is the most widely used technique to evaluate the voids, including the size and occurrence frequency. In this paper, a laser ultrasound and interferometer inspection system is used to inspect the voids in lead-free solder bumps in ball grid array packages. This system uses a pulsed Nd:YAG laser to induce ultrasound in the chip packages in the thermoelastic regime; and laser interferometer is used to measure the transient out-of-plane displacement response of the package surface to the laser irradiation. The quality of solder bumps is evaluated by analyzing the transient responses. In this paper, voids are intentionally created by adding the volatile flux during the assembly process. Through controlling the volume of flux dip, three different levels of voiding are proposed: 1) void free; 2) relatively low; and 3) relatively high. The presence of voids in the solder bumps is first verified using a 2-D X-ray technique. Meanwhile, the built-in image-processing software in X-ray tool measures the void fraction to quantify the level of voiding. A scanning acoustic microscope technique is also used to evaluate the void in the solder bumps. Then the laser ultrasound inspection system is used to evaluate the voids in these samples. Through comparing the vibration responses from the chip with solder bump voids and void-free chips, it is found that the laser ultrasound inspection system is capable of differentiating the chips with relatively high voiding from the void-free chip whereas the relatively low voiding is below the resolution of the inspection system. Finally, a further comparison between the void-free and voided solder bumps is carried out by the destructive cross-sectional technique. The comparisons among these different evaluation methods on solder bump voids will be presented in this paper.

Crack Detection in Single-Crystalline Silicon Wafer Using Laser Generated Lamb Wave

In the semiconductor industry, with increasing requirements for high performance, high capacity, high reliability, and compact components, the crack has been one of the most critical issues in accordance with the growing requirement of the wafer-thinning in recent years. Previous researchers presented the crack detection on the silicon wafers with the air-coupled ultrasonic method successfully. However, the high impedance mismatching will be the problem in the industrial field. In this paper, in order to detect the crack, we propose a laser generated Lamb wave method which is not only noncontact, but also reliable for the measurement. The laser-ultrasonic generator and the laser-interferometer are used as a transmitter and a receiver, respectively. We firstly verified the identification of S0 and A0 lamb wave modes and then conducted the crack detection under the thermoelastic regime. The experimental results showed that S0 and A0 modes of lamb wave were clearly generated and detected, and in the case of the crack detection, the estimated crack size by 6 dB drop method was almost equal to the actual crack size. So, the proposed method is expected to make it possible to detect the crack in the silicon wafer in the industrial fields.

Examination of the epicentral waveform for laser ultrasound in the melting regime

A laser ultrasonic source just below the ablation regime is examined by recording an epicentral waveform in a high purity tungsten sample. Using pulse energy as a parameter, a slight delay in the shear wave arrival time is observed upon transition to the melting regime. This phenomenon is attributed to a change in character of the ultrasonic source. In the thermoelastic regime, shear waves are generated by mode conversion at the sample surface of longitudinal waves emanating from subsurface sources. Just above the melting threshold, a molten pool forms in the center of the generation volume. Shear waves are not supported by the molten pool. As a result, shear waves generated from off-axis thermoelastic sources are weighted more heavily. This results in a delay of the shear wave arrival time.

Effect of Laser Pulse Fatigue on the Mechanical Characteristics of a CFRP Plate

A laser ultrasonic based nondestructive evaluation (NDE) technique has been widely used in aerospace industries for inspecting parts and structures made of composite materials. The thermoelastic regime is used for the ultrasonic generation, so no plasma is formed on the surface of composite structure. Generally, the service lifetime for an aircraft could be more than 25 years. Thus, the composite structures of the aircraft could be susceptible to laser pulse fatigue damage caused by the laser pulse energy of a laser ultrasonic generator in the long-term periodic maintenance inspection. In this paper, the effect of laser pulse fatigue on the mechanical characteristics of a carbon-fiber-reinforced polymer (CFRP) plate (USN175BX Carbon UD preprag) with the stacking sequence of [0/45/-45/90]s is investigated to verify the reliability of the use of a laser ultrasonic based NDE technique on the CFRP plate specimen inspection. A high-speed laser ultrasonic scanning system (400mm/s at the intervals of 0.4 mm) was setup to perform repeat scanning of 1300 times on a CFRP plate specimen with the scanning area of 70 mm x 60 mm. These repeat scanning times were set in consideration of the periodic maintenance inspection scheduled to be 1 time/week x 52 weeks/year x 25 years. A 532nm Q-switched continuous wave laser (QL) was used and set at the laser pulse energy levels of 0.6 mJ and 1.2 mJ. Lamb wave assessment based on pitch-catch method was proposed in this paper to monitor the mechanical characteristics of a composite specimen. In each completion of 100 times repeat scanning, the Young’s modulus of the scanning area was evaluated based on the group velocity of S0 Lamb wave mode. In addition, the surface condition of the scanning area was investigated by using a microscope.

Thermoelastic laser generated ultrasound using a ring source

A pulsed Nd : YAG laser is focused to a sharp ring on an aluminium sample in the thermoelastic regime. This is achieved by firstly expanding the collimated laser beam before passing it through a bi-convex lens and an axicon lens. The addition of the extra convex lens provides a significant focusing improvement. On thick samples, this shape of acoustic source generates simultaneously a surface Rayleigh wave and bulk waves. The surface waves generated by the ring source travel outwards from the ring and also inwards to focus at a point in the centre of the ring. Large amplitude Rayleigh waves are thus generated at the centre of the ring. The absolute, out-of-plane displacement caused by the acoustic waves travelling along a sample surface is measured using a Michelson interferometer with a bandwidth of approximately 80 MHz. The experimental results have been correlated with those obtained by numerical modelling to show good agreement and less successfully with finite element modelling. The ring acoustic source has various applications for defect detection and measurement, and material characterization.

An experimental and theoretical approach to the study of the photoacoustic signal produced by cancer cells

The distinctive spectral absorption characteristics of cancer cells make photoacoustic techniques useful for detection in vitro and in vivo. Here we report on our evaluation of the photoacoustic signal produced by a series of monolayers of different cell lines in vitro. Only the melanoma cell line HS936 produced a detectable photoacoustic signal in which amplitude was dependent on the number of cells. This finding appears to be related to the amount of melanin available in these cells. Other cell lines (i.e. HL60, SK-Mel-1, T47D, Hela, HT29 and PC12) exhibited values similar to a precursor of melanin (tyrosinase), but failed to produce sufficient melanin to generate a photoacoustic signal that could be distinguished from background noise. To better understand this phenomenon, we determined a formula for the time-domain photoacoustic wave equation for a monolayer of cells in a non-viscous fluid on the thermoelastic regime. The theoretical results showed that the amplitude and profile of the photoacoustic signal generated by a cell monolayer depended upon the number and distribution of the cells and the location of the point of detection. These findings help to provide a better understanding of the factors involved in the generation of a photoacoustic signal produced by different cells in vitro and in vivo.

Laser ultrasonic technique for evaluating solder bump defects in flip chip packages using modal and signal analysis methods

Current techniques for nondestructive quality evaluation of solder bumps in microelectronic packages have their own limitations and are unsuitable for in-line inspection because of high cost and low throughput. In order to meet requirements of quality inspection of electronic packages, a nondestructive and noncontact solder bump inspection system is being developed using laser ultrasound and interferometer techniques. This system uses a pulsed Nd:YAG laser to induce ultrasound in electronic packages in the thermoelastic regime; it then measures the transient out-of-plane displacement responses with an ultrasonic arrival on the surface of the packages using laser interferometer technique. This paper presents a systematic study of defect detection in flip chip solder bumps using a combined modal and signal analysis method. The correlation between the finite element (FE) modal analysis and wavelet analysis of laser ultrasound signals has been studied. With the help of FE modal analysis, the modes of flip chip packages sensitive to open bumps were predicted for further signal processing before conducting any experiments. The decomposed signal components sensitive to specific defects were extracted and analyzed with discrete wavelet transform. The results show that wavelet analysis increases the measurement sensitivity of the flip chip solder bump inspection compared with existing methods.

Defect Detection of Flip Chip Solder Bumps With Wavelet Analysis of Laser Ultrasound Signals

Microelectronics packaging technology has evolved from through-hole and bulk configuration to surface-mount and small-profile ones. In surface mount packaging, such as flip chips, chip scale packages, and ball grid arrays, chips/packages are attached to the substrates/printed wiring board (PWB) using solder bump interconnections. Solder bumps hidden between the chips/packages and the substrate/board are no longer visible for inspection. A novel solder bump inspection system has been developed using laser ultrasound and interferometer techniques. This system has been successfully applied to detect solder bump defects including missing, misaligned, open, and cracked solder bumps in flip chip packages, chip scale packages and land grid arrays. The system uses a pulsed Nd:YAG laser to induce ultrasound in the thermoelastic regime and the transient out-of-plane displacement response in nanometer scale on the package surface is measured using the interferometer technique. In this paper, wavelet analysis of laser ultrasound signals is presented and compared to previous signal processing methods, such as error ratio and correlation coefficient. The results show that wavelet analysis increases measurement sensitivity for inspecting solder bumps in electronic packages. Laser ultrasound inspection results are also compared to X-ray results. In particular, this paper discusses defect detection for a 6.35 mm × 6.35 mm × 0.6 mm PB18 flip chip package and flip chip package (?SiMAF?) with 24 lead-free solder bumps. These two types of flip chip specimens are both nonunderfilled.

Bulk waves excited by a laser line pulse in a two-layered cylinder

The acoustic displacements excited by a laser line source under either the ablation or thermoelastic regime are calculated for a two-layered cylinder made of homogeneous and isotropic materials. Two samples made by welding tin in two different copper tubes are considered. Experimental displacements are observed by the laser ultrasonic technique, and corresponding theoretical waveforms are calculated. Good agreement is found in the arrival time, shape, and relative amplitude of various longitudinal and shear bulk waves propagating in the two-layered cylinders, and the waves reflected by the interface are further identified by the ray trajectory analysis. The longitudinal wave reflected at the interface may be used for the nondestructive evaluation of the interface quality.

Finite element modeling of the interaction of laser-generated ultrasound with a surface-breaking notch in an elastic plate

The interaction of surface acoustic waves generated by laser line source in the thermoelastic regime with surface notches are investigated. The finite element method is used to establish the model of the transient displacement field for surface notches with various depths and orientation. The magnitude of the signal enhancement in the near field and the mechanism by which this occurs are explained. The positions of notches were evaluated by the reflected Rayleigh wave. The depths and orientations of the notches were also determined using a shear wave that was generated through mode conversion of a surface acoustic wave at the notch tip. The results agree with previously published experimental measurements.

Thermomechanical Reliability Study of Flip Chip Solder Bumps: Using Laser Ultrasound Technique and Finite Element Method

Current techniques for nondestructive quality evaluation of solder bumps in electronic packages are either incapable of detecting solder bump cracks, or unsuitable for in-line inspection due to high cost and low throughput. As an alternative, a solder bump inspection system is being developed at Georgia Institute of Technology using laser ultrasound and interferometric techniques. This system uses a pulsed Nd:YAG laser to induce ultrasound in electronic packages in the thermoelastic regime; it then measures the transient out-of-plane displacement responses on the package surfaces using laser interferometric technique. The quality of solder bumps in electronic packages is evaluated by analyzing the transient responses. This paper presents a systematic study on thermomechanical reliability of flip chip solder bumps using laser ultrasound-interferometric inspection technique and finite element (FE) method. The correlation between the failure parameter extracted from FE simulation for evaluating solder bump reliability and quality degradation characterization of solder bumps through noncontact, nondestructive laser ultrasound testing has also been investigated. Accelerated thermal cycling (ATC) tests were performed in two phases on flip chip package (FCP) test vehicles with 63Sn37Pb solder bumps. In phase I, four boards were thermal cycled up to 70 cycles. Every ten cycles, the boards were taken out of the chamber and FCPs on each board were tested for detecting presence of cracks using resistance and laser ultrasound testing methods. These measurements were repeated every 10 cycles and stopped after 70 cycles. Laser ultrasound testing results show that starting from 10 cycles, testing values began to increase gradually until 50 cycles when there was a sharp jump (through crack verified at 50 cycles with cross section). Cross section was done only after 70 cycles, and the results show that solder bumps at the corners had through cracks, while solder bumps at the center had partial cracks. Phase II study was a refinement of phase I study. In phase II study, seven boards were thermal cycled starting from 27 cycles to 56 cycles at an increment of approximate five cycles. After every five cycles, only one board was pulled out and tested in order to track crack initialization and propagation in solder bumps. Smaller thermal cycle increments were used in phase II to help determine when cracks initiated and propagated. It was shown that laser ultrasound technique could nondestructively track solder bump crack propagation due to thermal fatigue. The Anand's viscoplastic constitutive model was used in FE simulation to describe the inelastic deformation behavior of solder bumps in thermal cycling. A 3-D FE model was implemented using ANSYS on the same FCP as used in the ATC testing. The stress-strain results were extracted from FE modeling and the inelastic strain energy density (SED) increment per cycle calculated from the critical solder bump was used as a failure parameter. In this paper, it was demonstrated that there is a good correlation between the inelastic SED increment extracted from FE simulation and experimental results from nondestructive laser ultrasound testing.

Laser-generated thermoelastic acoustic sources and acoustic waves in anisotropic plate

The effect of anisotropy on the ultrasound wave generation and propagation in the unidirectional fiber-reinforced composite plate has been investigated. A quantitative numerical model for the laser-generated ultrasound in the thermoelastic regime was presented by using a finite element method. All factors, such as spatial and time distributions of the incident laser beam, optical penetration, thermal diffusivity, and source-receiver distance can be taken into account. Numerical results show that the effect on ultrasound waveform of the size of the laser volume source produces strong bipolar longitudinal waves and improves the amplitude and directivity of the longitudinal waves. A fiber-reinforced composite material exhibits isotropic or homogenous behavior for ultrasonic wave propagation perpendicular to the fiber direction. For ultrasonic propagation along the fiber direction, ultrasonic dispersion resulting from the inhomogeneous nature of the material affects the laser ultrasonic waveforms. As the dimensions of the laser pulse are increased in space and time, the displacement waveform becomes broader and its magnitude decreases.

Detection of Solder Bump Defects in Electronic Packages Using Local Temporal Coherence Analysis of Laser Ultrasonic Signals

Microelectronics packaging technology has evolved from through-hole and bulk configurations to surface-mount and small-profile configurations. Surface mount devices, such as flip chip packages, chip scale packages, and ball grid arrays, use solder bump interconnections between them and substrates/printed wiring boards. Solder bumps, which are hidden between the device and the substrate/board, are difficult to inspect. A solder bump inspection system was developed using laser ultrasound and interferometric techniques. This system has been successfully applied to detect solder joint/bump defects, including missing, misaligned, open, and cracked solder joints/bumps in flip chips, chip scale packages, and multilayer ceramic capacitors. This system uses a pulsed Nd:YAG laser to induce ultrasound in the electronic packages in the thermoelastic regime; it then measures the transient out-of-plane displacement response on the package surface using the interferometric technique. This paper presents a local temporal coherence (LTC) analysis of laser ultrasound signals and compares it to previous signal-processing methods, including error ratio and correlation coefficient methods. The results showed that LTC analysis increased measurement accuracy and sensitivity for inspecting solder bump defects in electronic packages. Laser ultrasound inspection results are also compared with X-ray and C-mode scanning acoustic microscopy results. In particular, this paper discusses defect detection for 6.35×6.35×0.6 mm3 flip chips and flip chips (“SiMAF;” Siemens AG) with lead-free solder bumps.

Ultrasounds Excited by a Laser Line Pulse in a Liquid-Filled Tube

In this paper, a physical model is proposed to describe the propagation of ultrasounds, which are excited in a homogeneous, isotropic, and liquid-filled tube by a laser line source under either the ablation or thermoelastic regime. Since a metal tube is considered, the thermal diffusion and the optical penetration depth are neglected in this modeling. The Fourier series expansion is introduced for a spatial coordinate to provide the transient response of the structure. Theoretical displacements are obtained for aluminum tubes filled with glycerol or air. The corresponding displacements are observed experimentally by the laser ultrasonic technique. Good agreement is found in the arrival time, shape and relative amplitude of various longitudinal and shear bulk waves propagating in the solid and liquid media. These ultrasonic waves are further identified by the ray trajectory analysis.

Acoustic signals from gold nanoparticles irradiated with pulsed lasers

Photoacoustic imaging is a new imaging technique, which enables imaging of living tissue with high resolution. The technique analyses ultrasound pulses generated when absorbing structures in tissue are irradiated with pulsed light. Due to the phenomenon of plasmon resonance gold nanoparticles possess high optical absorption coefficients which makes them potential contrast agents in photoacoustics. Further, the heat generated around these particles when irradiated with pulsed light can lead to nonlinear effects including bubble formation. The acoustic signals produced by gold nanoparticles in the linear thermoelastic and non-linear regimes are expected to have specific acoustic signatures. In this study, we investigate the acoustic signals generated by two sets of gold spheres having 25 nm and 60 nm diameter irradiated by laser pulses with increasing incident fluence rates. We identify and discuss the differences in acoustic signals belonging to the thermoelastic expansion regime and the non-linear regime. Finally, the experimental results are compared with different theoretical models available in literature.

Laser-generated thermoelastic acoustic sources and Lamb waves in anisotropic plates

The effect of anisotropy and temperature on the dispersive Lamb wave generation and propagation in a transversely isotropic thin plate has been investigated. A quantitative numerical model for the laser-generated transient ultrasonic Lamb waves propagating along arbitrary directions is presented by using a finite-element method. All factors, such as spatial and time distributions of the incident laser beam, optical penetration, thermal diffusivity, thickness of the plate, and source–receiver distance, can be taken into account. The effects on the ultrasound waveform of the size of the optoacoustic source are investigated; in the limit of strong optical absorption, a subsurface thermal source gives rise to both vertical and lateral shear tensions. The lateral shear tension is equivalent to applying a shear dipole at the top face; the amplitude of the dipole is a function of material symmetry, contrary to the isotropic case, and the character and strength of the equivalent surface stress are a function of propagation direction. The specific results for the lower anti-symmetric and symmetric mode propagation in all planar directions are presented in the thermoelastic regime; the spatial dispersion (variation of the velocity with the direction of propagation) as well as the frequency dispersion is analyzed.

Local and noncontact measurements of bulk acoustic wave velocities in thin isotropic plates and shells using zero group velocity Lamb modes

An original method for material characterization with acoustic waves is presented. The measurement of the longitudinal and shear wave velocities in thin isotropic plates or shells is performed locally on the same face without any mechanical contact. We exploit the resonance that occurs at the minimum frequency thickness product of the first order symmetric (S1) and of the second order antisymmetric (A2) Lamb modes. At these frequencies the group velocity vanishes, whereas the phase velocity remains finite. Then, the energy, which cannot propagate in the structure, is localized in a zone of diameter half the wavelength. The vibrations are excited in the thermoelastic regime by a laser pulse and detected at the same point by an optical interferometer. For these two Lamb modes we have computed the variations of the frequency thickness product versus Poisson’s ratio. The resonance frequency ratio, which is independent of the plate or shell thickness, provides an absolute and local measurement of Poisson’s ratio. Provided that the plate thickness is known, each resonance frequency allows us to determine in a single shot the bulk acoustic wave velocities VL and VT. Since it is based on frequency measurements, the method, tested on a large number of materials, is very accurate.

Surface breaking crack evaluation with photorefractive quantum wells and laser-generated Rayleigh waves

An adaptive laser ultrasound system using optical two-wave mixing technique with photorefractive quantum wells is developed for crack characterization in noisy environments. AlGaAs∕GaAs photorefractive multiple quantum wells featuring a quick response time of 9.1μs cancel disturbances below 100kHz. A laser-generated Rayleigh wave in a thermoelastic regime is used for crack characterization. The positions and depths of artificial cracks, 3 and 5mm in depth and 0.2mm in width, were evaluated. The crack orientations were also determined using a shear wave that was generated through mode conversion of a Rayleigh wave at the crack tip.

Application of Laser-Generated Ultrasound for Evaluation of Thickness Reduction in Carbon Steel Pipes

In carbon steel pipes of nuclear power plants, local wall thinning may result from erosion-corrosion or flow-accelerated corrosion(FAC) damage. Local wall thinning is one of the major causes for the structural fracture of these pipes. Therefore, assessment of local wall thinning due to corrosion is an important issue in nondestructive evaluation for the integrity of nuclear power plants. In this study, laser-generated ultrasound technique was employed to evaluate local wall thinning due to corrosion. Guided waves were generated in the thermoelastic regime using a Q-switched pulsed Nd:YAG laser with a linear slit array. . In this paper, time-frequency analysis of ultrasonic waveforms using wavelet transform allowed the identification of generated guided wave modes by comparison with the theoretical dispersion curves. Modes conversion and group velocity were employed to detect thickness reduction.