Laser Spallation Technique Research Articles

In-situ measurement of solder joint strength in board-mounted chip-scale packages using a quantitative laser spallation technique

A previously developed laser spallation (LS) technique for measuring the interface strengths of planar films is modified to determine the interfacial tensile strengths of solder joints, in situ, in board-mounted chip-scale packages (CSPs). The new technique is demonstrated on packages with bare Cu pads and Pb-free solders. The solder/pad interface strength is determined by first using a laser-generated stress wave to separate the geometrically heterogeneous interface, in situ, and then quantifying the failure stress by a combination of interferometry and wave mechanics simulation. The solder to pad interfacial strengths were found to be 705 ± 102, 510 ± 71, and 369 ± 55 MPa for packages stressed by baking at 150 °C for 0, 48, and 100 h, respectively. The interface strengths were found to be roughly proportional to the critical laser energies for separation of solder joints. Thus, it may suffice to use the critical laser energy as a parameter for the purposes of material selection and solder joint quality inspection during manufacturing. The quantitative capability of the LS test for testing board-level packages now provides an opportunity to enhance the board-level drop test that is widely conducted in the industry today.

Measurement of solder joint strength in freestanding chip-scale packages using a quantitative laser spallation technique

A previously developed laser spallation technique is adapted to measure in situ the tensile strengths of geometrically heterogeneous interfaces, in multilayer freestanding chip-scale packages that were baked for specific temperatures and times. The test procedure involved quantification of the stress waves inside the packages using interferometry, and subsequent stress field quantification, including that at the failed interface, using a wave mechanics simulation. The technique is generally applicable and can be used to test any type of freestanding package. In this work, it is demonstrated on freestanding 0.5 mm-pitch MicroStar BGATM packages. The ball grid array joint strengths for Pb-free solders on bare Cu pads were measured to be 942 ± 91, 703 ± 69, 666 ± 70, 441 ± 42, and 392 ± 45 MPa for samples that were thermally aged for 3, 10, 20, 40, and 80 days, respectively. An important result of our study is that the critical laser energy for causing joint failure was found to be approximately proportional to the peak tensile stress at the joint. This validates the discussion earlier where no stress quantification was carried out, but the interfaces were characterized only in terms of the critical laser energies and the deteriorations in the material microstructure in the solder joint region caused by thermal aging were related to the reductions in the measured critical laser energies. This powerful result shows that in the future it may suffice to use the critical laser energy for material selection and quality control during manufacturing.

Dynamic tensile strength of polyurea-bonded steel/E-glass composite joints

The dynamic tensile strengths of E-glass composite/polyurea and polyurea/steel interfaces within the E-glass composite/polyurea/AL-6XN stainless steel joint were measured using a laser spallation technique. Values of 370 ± 20 MPa were obtained for the polyurea/composite interface while a much higher value of 486 ± 20 MPa was obtained for the steel/polyurea interface. Because of the transient nature of the stress pulse, the strain rate changes continuously as the interface stress builds up. A peak strain rate of 5 × 105 s−1 was estimated. The effect of moisture on the tensile strength of the E-glass/polyurea interface was also examined. The effect was found to be minimal, with the tensile strength stabilizing at 320 ± 25 MPa after 30 days of exposure to a 90%RH, 50 °C environment. When comparing the strengths of corresponding interfaces in an epoxy-bonded joint from a previous study, it was concluded that polyurea results in a much stronger and durable joint.

Evaluation of bonding strength between yttria coating and vanadium alloys for development of self-cooled blanket

A laser spallation technique was utilized for measurement of the bonding strength between yttria coating and vanadium alloys. The bonding strength between the alloys containing small amounts of yttrium made by levitation melting method and the yttria coating prepared by vacuum plasma-spray was evaluated to be about 950MPa. It was not clearly observed the effects of alloying elements on the bonding strength. The strength varied about 100MPa by specimens and by alloy compositions. Detailed observation of the failure type at the interface indicated that crack formation in the coating reduced the stress at the interface, so that the evaluation might be overestimated. It was demonstrated that application of the laser spallation technique to measure the bonding strength between ceramics coating and base material was useful for the evaluation of mechanical integrity of the coating.

A hybrid experimental/numerical approach to characterize interfacial adhesion in multilayer low-κ thin film specimens

The strength of multilayer low dielectric (low-κ) constant organo–silicate glass (OSG) and silicon–carbon–nitride (SiCN) thin film interfaces is characterized by a laser spallation technique. Two specimen sets with different OSG/SiCN stacking sequences are evaluated. The effect of helium (He) pretreatment is also investigated. The stress at the low-κ interface is enhanced through the use of a fused silica backing layer to shape the incident pulse and the addition of a thin gold (Au) top layer to increase inertial force during the dynamic failure event. The weakest interface in the multilayer stack (first to fail) is identified through optical images, profilometry and scanning electron microscope images of the spallation zone. The strength of the failed interface is inferred from the incident stress history in combination with a one dimensional dynamic wave propagation analysis. The adhesion strength of the OSG/SiCN interface (372 MPa) is 32% larger than that of Si/OSG (282 MPa) for specimens with no He pretreatement. The interfacial strength of both interfaces is significantly increased by a He pretreatment, making failure by spallation difficult.

An Understanding of the Mechanism That Promotes Adhesion Between Roughened Titanium Implants and Mineralized Tissue

A previously developed laser spallation technique to determine the tensile strength of thin film interfaces was successfully adopted to study the effect of microsurface roughness of titanium disks on the adhesion strength of mineralized bone tissue. The study demonstrated that mineralized tissue has about 25% higher interfacial strength when it is cultured on the acid-etched titanium surface than on its machined counterpart. Specifically, interfacial tensile strength of 179+/-4.4 MPa and 224+/-2.6 MPa were measured when the mineralized tissue was processed on the machined titanium and acid-etched titanium surfaces, respectively. Since in the laser spallation experiment, the mineralized tissue is pulled normal to the interface, this increase is attributed to the stronger interfacial bonding on account of higher surface energy associated with the acid-etched surface. This enhanced local chemical bonding further enhances the roughness-related mechanical interlocking effect. These two effects at very different length scales--atomic (enhanced bonding) versus continuum (roughness-related interlocking)-act synergistically and explain the widely observed clinical success of roughened dental implants.

Interface strength measurement of tungsten coatings on F82H substrates

In the current work, hot isostatic pressing is adopted to deposit tungsten coatings on F82H substrates. The interface strength of the W/F82H samples is measured using the Laser Spallation technique and the microstructure is analyzed to determine the strength of the coating. Finally, the failure mechanisms of the hot isostatic pressing versus vacuum plasma spraying tungsten coatings and their different failure strengths are compared. It is concluded that the hot isostatic pressing process ensures a good adhesion for the W/F82H interface while the vacuum plasma spraying process results in relatively lower failure strength for the W-coating itself due to the high porosity in the coating.

Measurement of the tensile strength of cell–biomaterial interface using the laser spallation technique

A previously developed laser spallation technique to determine the tensile strength of thin film interfaces was successfully adopted to determine the tensile strength of interfaces between three different live mammalian cells (osteoblast, chondrocyte and fibroblast) and polystyrene (untreated and fibronectin coated) and titanium surfaces. No noticeable differences in the interfacial tensile strength values were found across the three cell types on the same substrate although osteoblasts showed slightly lower adhesion strength when cultured on untreated polystyrene surfaces. Significant differences were, however, measured for cells treated on different surfaces. Use of fibronectin increased the interfacial tensile strength for all cell types, and cells bonded much better to titanium than to untreated polystyrene surfaces. Cell interfacial strength was higher when cultured with serum than in a serum-free environment. The results demonstrate the remarkable sensitivity of the laser spallation experiment in determining the effects of local interfacial microstructure and chemistry on cell adhesion.

Failure Strength Measurements of VPS Tungsten Coatings for HAPL First Wall Armor

The High Average Power Laser (HAPL) project is pursuing development of an IFE power reactor using a solid first wall chamber. Tungsten has been chosen as the primary candidate armor material protecting the low activation ferritic steel chamber wall structure. The tungsten armor is less than 1-mm thick and is applied by vacuum plasma spraying (VPS). The failure strength of the tungsten-armor is critical, which is measured using a state-of-the-art spallation technology developed at UCLA. A nano-second laser is used to propagate a compression/tension stress wave through the composite layered structure. The tensile strength in the coating is then related to the displacement velocity of the free surface of the tungsten coating. VPS tungsten coated steel samples were tested using the laser spallation technique and coating strengths were evaluated and are reported.

Evaluation of laser spallation as a technique for measurement of cell adhesion strength

Cell adhesion to material surfaces is one of the fundamental phenomena of cellular response to implanted devices. Controlling the strength, dynamics, and mechanics of cell adhesion offer opportunities for designing novel biomaterials for tissue engineering and biotechnology. Many techniques have been developed for the purpose of quantifying various types of cell-biomaterial interaction. One method to evaluate cell affinity for a biomaterial is to measure the stress required to remove adherent cells from the material. This study investigates the possibility of using laser spallation, a technique previously developed for measuring the tensile strength of thin film interfaces, for evaluation of initial cell attachment strength. MC3T3-E1 preosteoblasts were cultured on fibronectin-coated polystyrene, a surface known to engage cells in receptor mediated adhesion, and untreated polystyrene, which elicit nonspecific adhesion mechanisms during early stages of cell attachment. The laser spallation technique effectively detached cells from polymer substrates and also distinguished relative cell adhesion strengths to surfaces with known differences in cell binding affinities. Scanning electron micrographs determined that cell detachment resulting from laser spallation left a cleaner surface than jet impingement, possibly suggesting a more complete detachment mechanism. Absolute values of adhesion strengths determined by laser spallation were significantly higher than those found using jet impingement, a previously reported hydrodynamic technique.

Glycosaminoglycan degradation reduces mineralized tissue–titanium interfacial strength

Although the localization of the proteoglycan/glycosaminoglycan (GAG) complex at the bone-titanium implant interface has been implied, the role of proteoglycans on the establishment of bone-titanium integration is unknown. The hypothesis to be tested was that proteoglycans play an important role in establishing bone-titanium interfacial adhesion. The objective of this study is to investigate the effect of proteoglycan knockdown by GAG enzymatic degradation on the interfacial strength between mineralized tissue and titanium having different surface topographies. Rat bone marrow-derived osteoblastic cells were cultured on either a machined titanium disk or an acid-etched titanium disk. At day 21 of culture, one of the three following GAG degradation enzymes was added into the culture; chondroitinase AC, chondroitinase B, or keratanase. After 3 days of incubation (at day 24 of culture), the laser spallation technique was applied to the samples in order to assess the tissue-titanium interfacial strength. In this technique, a laser-generated stress wave is used to separate the tissue-titanium interface, and the interfacial strength is determined interferometrically by recording the transient free surface velocity of the tissue. Mineralized tissue cultured on the acid-etched titanium showed 20-30% higher tissue interfacial strength than that cultured on the machined titanium (p < 0.0001). For both the machined and acid-etched surface cultures, administration of the enzyme reduced the interfacial strength by 25-30% compared with the untreated control cultures (p < 0.0001). There were no differences in the effect among the three different enzymes tested. A nanoindentation study revealed that the enzyme treatment did not affect the elastic modulus of the mineralized tissue. Scanning electron microscopic and energy dispersive spectroscopic analyses revealed less post-spallation tissue remnant on the titanium substrates when treated with the enzymes. The tissue remnant was greater in amount on the acid-etched surface than on the machined surface. The results suggest that there exists not only mechanical interlocking but also biological interfacial adhesion between the mineralized tissue and titanium, in which the proteoglycan/GAG complex is involved.

Interfacial adhesion of nanoporous zeolite thin films

Nanoporous silica zeolite thin films are promising candidates for future generation low-dielectric constant (low-k) materials. During the integration with metal interconnects, residual stresses resulting from the packaging processes may cause the low-k thin films to fracture or delaminate from the substrates. To achieve high-quality low-k zeolite thin films, it is important to carefully evaluate their adhesion performance. In this paper, a previously reported laser spallation technique is modified to investigate the interfacial adhesion of zeolite thin film-Si substrate interfaces fabricated using three different methods: spin-on, seeded growth, and in situ growth. The experimental results reported here show that seeded growth generates films with the highest measured adhesion strength (801 ± 68 MPa), followed by the in situ growth (324 ± 17 MPa), then by the spin-on (111 ± 29 MPa). The influence of the deposition method on film–substrate adhesion is discussed. This is the first time that the interfacial strength of zeolite thin films-Si substrates has been quantitatively evaluated. This paper is of great significance for the future applications of low-k zeolite thin film materials.

An advanced method for measuring the residual stress of deposited film utilizing laser spallation technique

Residual stress of thin diamond films deposited by chemical vapor deposition (CVD) method was measured using a new method for residual stress measurement we developed. Compressive residual stresses of CVD diamond films were determined from the height and diameter of protuberance of film delaminated by pulse laser spallation technique. We produced the protuberance with different diameter and height by changing the laser energy. Compressive stresses were calculated from the momentum balance of atmospheric pressure and internal compressive force of the film. The residual stress of the well-faceted diamond film was measured as −338±22 MPa and agreed well with the stress (−348±17 MPa) measured by the X-ray diffraction method. In three other polycrystalline diamond films with different grain structure, the compressive stresses decreased from 300 to 147 MPa with decreasing the grain size from a few microns to tens of nanometers. Small compressive stresses of fine grain films are correlated with the grain boundary structure according to visible Raman spectra study conducted.

A quantitative study of moisture adsorption in polyimide and its effect on the strength of the polyimide/silicon nitride interface

Polyimide films on silicon nitride substrates were exposed to moisture under varying conditions of relative humidity, time and temperature. The moisture content of the films was measured by FTIR spectroscopy, and the polyimide/silicon nitride interface strength was measured at room temperature by a laser spallation technique. The moisture adsorption by polyimide films was analyzed using a diffusion model. Under the experimental conditions of this study, it was found that the rate of moisture adsorption was controlled the surface exchange reaction. For samples exposed at 38 °C, the interface strength was found to decrease linearly with increasing interface moisture concentration. A critical interface moisture concentration was identified, where the strength is expected to go to zero. The interface strengths of all the measured samples were combined into one empirical equation that can be used as a basis to construct strength charts as a function of exposure conditions. Such strength charts should help in the development of more rational standards for handling packaged ICs during manufacturing and integration from the viewpoint of avoiding moisture-related failures.

Interfacial Adhesion of Pure-Silica-Zeolite Low-k Thin Films

AbstractNanoporous zeolite thin films are promising candidates as future low dielectric constant (low-k) materials. During the integration process with other semiconductor materials, the residual stresses resulting from the synthesis processes may cause fracture or delamination of the thin films. In order to achieve high quality low-k zeolite thin films, the evaluation of the adhesion performance is important. In this paper, laser spallation technique is utilized to investigate the interfacial adhesion of zeolite thin film-Si substrate interfaces prepared using three different processes. The experimental results demonstrate that the nature of the deposition method has a great effect on the resulted interfacial adhesion of the film-substrate interfaces. This is the first time that the interfacial strength of zeolite thin films-Si substrates is quantitatively evaluated. The results have great significance in the future applications of low-k zeolite thin film materials.

Study on the interface strength of zirconia coatings by a laser spallation technique

Tensile strength of interfaces between zirconia coatings and stainless steel substrates were measured using a laser spallation technique. In this technique, a laser-generated compression stress wave reflects into a tensile wave from the free surface of the coating and pries off its interface at a critical amplitude. Zirconia coatings were deposited using the gas tunnel-type plasma spraying process which was previously shown to result in denser and harder deposits compared with those produced by conventional spray methods. The interface strength was found to increase with a decrease in the spray distance and traverse speed, and with an increase in the traverse number, and coating and substrate thickness. The variation in the measured strength with these variables could be explained through their effect on controlling the temperature of the particles and that in the interfacial region. Higher temperature in the interfacial region during coating deposition is expected to increase adhesion by enhancing chemical and physical bonding mechanisms. Depending upon the processing conditions used, the tensile strength for the zirconia/steel interface system was found to vary anywhere between 206 and 501 MPa.

Finite element simulation of the film spallation process induced by the pulsed laser peening

The laser spallation technique for measuring the interface strength between a coating and a substrate is similar to laser shock peening, in which the stress wave induced by laser shock cause debond on the interface between a hard coating with micron thickness and a metal substrate. According to the modified experiment setup of the laser spallation technique, finite element analysis simulated the process of the film spallation by taking the laser loading as a direct input. We presented a numerical model of finite element that the laser spallation process includes two related, but uncoupled procedures. One was transient heat transfer in a two-layer medium. The other was the related transient elastic wave propagation in the same two-layer media, which was the result of the thermal misfit by transient heating. Based on the threshold of film spallation, we analyzed the process of laser shocking to study the propagation of stress wave and evaluate the spall resistance of sputtered films. The analysis result showed the dynamic adhesive strength of the interface between the TiN coating and the 304 stainless steel substrate was 193.0 MPa.

The effect of structure and chemistry on the strength of FeCrAl(Y)/sapphire interfaces: II. Strength of interfaces

The interfacial strengths of the FeCrAl/Al2O3 and FeCrAlY/Al2O3 interfaces in both as-deposited and annealed state were measured quantitatively by the laser spallation technique. The results show that in the as-deposited state, the presence of Y significantly improved the interfacial strength from 330±31 to 686±36 MPa. However, after annealing the films at 850°C for 16 h, the interfacial strength of the Y-free samples increased to 545±68 MPa, while the interfacial strength of Y-containing sample decreased to 599±22 MPa. The increase in the interfacial strength of the Y-free films was attributed to an improvement in the crystalline quality of the interface. The decrease in interfacial strength of the annealed Y-containing film in spite of the improvement in the crystalline quality of the interface was attributed to the depletion of Y at the interface due to formation of yttrium oxide precipitates. This is further proved that the presence of Y improves the strength of FeCrAl/Al2O3 interfaces significantly.

In-Situ Intrinsic Interface Strength Measurement at Elevated Temperatures and its Relationship to Interfacial Structure

A previously developed laser spallation technique has been modified to measure the tensile strength of thin film interfaces in-situ at temperatures up to 1100°C. Tensile strengths of Nb/A-plane sapphire, FeCrAl/A-plane sapphire and FeCrAlY/A-plane sapphire were measured up to 950°C. The measured strengths at high temperatures were substantially lower compared with their corresponding strengths at ambient temperature. For example, at 850°C, the interface tensile strength for the Nb/sapphire (151 ± 17 MPa), FeCrAl/sapphire (62 ± 8 MPa) and FeCrAlY/sapphire (82 ± 11 MPa) interface systems were lower by factors of approximately, 3, 5, and 8, respectively, over their corresponding ambient values. These results underscore the importance of using such in-situ measured values under operating conditions as the failure criterion in any life prediction or reliability models of such coated systems where local interface temperature excursions are expected. The results on alloy film interfaces also demonstrate that the presence of Y increases the strength of FeCrAl/Al2O3 interfaces.

Adhesion measurement of thin films by a modified laser spallation technique: theoretical analysis and experimental investigation

Laser driven shocks can lead to a dynamic failure, called film spallation. Here, we use a modified laser spallation set-up to measure the dynamic adhesion of thin films and we propose a novel diagnostic technology. Based on correlation theory, new spallation criteria for characterizing the progressive damage at the interface between the film and the substrate are established, such as interface delamination, film spallation and film expulsion. With the help of the theory, the degree of damage and the dimension of damage (i.e. fracture), such as the minimum width of delamination radius, the thickness of the film etc., are estimated. Experiments are carried out on epoxy/stainless steel and epoxy/Al, and the experimental results show that their dynamic bonding strengths are about 25 MPa and 20 MPa, respectively. The detailed results, analyses and discussions are presented in this paper.