Comparative Analysis of BGA Deformations and Strains Using Digital Image Correlation and Moiré Interferometry

Transcription

1 Comparative Analysis of BGA Deformations and Strains Using Digital Image Correlation and Moiré Interferometry Seungbae Park, Ramji Dhakal and Rahul Joshi Mechanical Engineering Department Binghamton University Binghamton, NY 1392 Abstract The thermo-mechanical behavior of BGA solder joints under thermal cycling is characterized by using full field optical measurement techniques, namely Moiré Interferometry (MI) and Digital Image Correlation (DIC), and each technique s advantages and disadvantages are discussed. Moiré Interferometry has been widely used in the electronics packaging for development and reliability assessment. Its high signal to noise ratio and in-situ measurement capabilities made the technique popular in the last two decades. Its application was further disseminated by the development of fringe shifting algorithms and fringe analysis software that made its virtual sensitivity as high as tens of nanometer per fringe order. However, the technique requires application of high frequency diffraction gratings and flatness of the inspected surface. Increases in computing capabilities, efficient algorithms, and affordable higher resolution CCD cameras have made Digital Image Correlation (DIC) a practical and quantitative deformation measurement technique. It does not require surface treatment in most of the applications, and this relaxed surface condition requirement appeals positively to the technical community. For these comparative analyses, a flip chip plastic ball grid array package (FC-PBGA) was cross sectioned to reveal a row of BGA solder interconnects and was subjected to a thermal cycle in an environmental chamber for in-situ measurement. The deformations and strains for the solder balls and the global deformations of the substrate and PCB have been compared for a T of -8 C. 1. Introduction The CTE mismatch between the chip, substrate and printed circuit board (PCB) produces thermally induced mechanical stresses within the electronic package assembly. Repeated thermal and/or power cycling would build up stresses in the interconnects and eventually leads to failure of the device during operation. Full field optical Measurement techniques have been widely used to quantify the deformations and associated strains and to locate the risk sites of electronic packages. Optical measurement techniques can provide actual package behavior and therefore, can be used as an experimental verification of the reliability assessment of different electronic and MEMS packages predicted by numerical simulation tools. These methods have a wide spectrum of application in material characterization and strain and deformation analyses. Digital Image Correlation (DIC), a form of Photogrammetry, is a non-contact optical deformation measurement technique in which the surface features of the object are traced in digital images. Initially used to obtain the measurements of natural and man made features on earth, it is getting increasingly popular in measuring the deformations of complex parts in small scale with remarkable sensitivity. For 2-D (in-plane) measurements, DIC consists of only one camera while, as an enhanced capability, 3-D DIC uses stereovision with two cameras at a certain angle. Simplicity in measurement procedure and relaxed surface condition requirement are the key features of DIC, which make it applicable in diverse fields like aerospace, biomechanics, remote sensing in addition to electronics packaging [1]. The ability of 3D DIC to simultaneously measure inplane as well as out-of-plane deformations and 3D shape of the component can be particularly useful. Moiré Interferometry [7] is another full field optical method to measure deformation. It has a high measurement sensitivity of.417 µm per fringe order [2]. High spatial resolution, high signal-to-noise ratio and wide dynamic range make it ideal for measuring deformations of multiple layers like those encountered in electronic packages. Moiré Interferometry has been used effectively to measure thermal deformations of electronic packages [9, 1]. Several experiments have been performed to characterize the residual strains in different packages subjected to isothermal loading. Recently, several studies using Moiré interferometry have been conducted to measure inelastic strains accumulated during thermal cycles or time-dependent creep behaviors [4]. In this work, DIC and MI are compared for in-plane analyses of electronic packages and their advantages and disadvantages are discussed. For the comparison, a flip chip plastic ball grid array (FC-PBGA) assembly was cooled from 1 C to room temperature (2 C) in an environmental chamber. The global deformations are quantified for the PCB and substrate by the in-

2 situ measurement scheme and the local deformations and strains of some of the solder balls have been studied and the results have been discussed. 2. Digital Image Correlation (DIC) 2.1 Principle Chip Substrate Output CCD camera Specimen PCB (b) (a) Camera Bar (c) Fig.1 (a) A schematic of the 3-D Digital Image Correlation set up (b) schematic of the test sample with the assignment of solder ball number (c) cross section of the test sample. DIC is a full field optical measurement technique in which both the in-plane and out-of-plane deformations can be computed by comparing the pictures of the target object at initial and deformed stages. Thousands of unique correlation areas (known as facets) are defined across the entire imaging area. The center of each facet is a measurement point that can be thought of as an extensometer or strain rosette. These facet centers are tracked, in each successive pair of images, with accuracy of up to one hundredth of a pixel. Then, using the principles of photogrammetry, the coordinates of each facet are determined for each set of images. The results are the shape of the component, the displacements, and the strains. Rigid body motion can first be quantified and then removed to reveal local deformations. For a 2-D measurement, only one camera is used and the images before and after deformation are used to quantify in-plane deformations. Unlike for a 3-D measurement, 2-D measurements do not require calibration, the only challenge being the gray scale variation on the sample and the field of view possible with the optics available. 3-D DIC can also be used to understand the initial shape of the specimen. It compares the two images from two cameras placed at an angle of about 3 and extracts both the in-plane and the out-of-plane deformations from the 3-D coordinates. For out-of-plane measurement, the camera and the lenses have to be calibrated for a specific field of view of interest. The sample to be tested needs to have a random distribution of gray scale (fig.1c). In certain cases, the inherent gray scale variation in a package is enough for the software to detect the points, thus requiring no sample preparation at all. 2.2 Sample preparation In this work, a flip-chip PBGA package of 14mm x 22mm body size with 119 interconnections in a 7 x 17 eutectic tin-lead solder ball array with 1.27mm pitch was cross sectioned to the middle of the first row of solder balls and polished flat. Sample preparation consists of applying paint to create a random variation of gray scale on the specimen. For the measurement of global deformations, the cross section was sprayed with a white paint (fig.1c). But, for the measurement of solder-balldeformations, the scratches and minute surface defects inherent to the polishing process give a good variation of gray scale on the picture (fig.2b). Because the software traces the gray scale on the facets, which comprise a certain number of pixels in the picture, spraying paint on the sample surface does not give a good variation of gray scale when the measurement field of view is only a solder ball area. In one of the studies to characterize the inter-grain deformation and failure mode of Pb-free BGA, the intermetallics present in the Sn matrix were used as the irregular pattern in the picture [6]. 2.3 Procedure The 3-D DIC set up is calibrated using calibration panels for the required field of view. A sequence of pictures of the panel at different distances from the cameras and with different orientations is captured. Then a photogrammetry process known as bundle adjustment is used to establish the precise relationship between the two cameras. Calibration also educates the system about the associated optics. The calibration was done through the window of the environmental chamber that was used for in-situ measurement of the deformations of the package.

3 (a) (b) Fig.2 (a) Picture showing the calibration of a 3D DIC set up, (b) surface of the die shadow solder ball showing the gray scale variations because of minute surface defects 3 Moiré Interferometry (MI) 3.1 Principle Moiré Interferometry is based on the principle of interference of light, whereby a diffraction grating replicated on the specimen interacts with the reference grating of the interferometer to produce the interference fringe pattern. The fringe pattern holds information about the displacement of the specimen. Depending on the wavelength of light used, the grating frequency and the accompanying optics, the sensitivity of the interferometry system varies. 3.2 Sample Preparation After the specimen is polished flat, it is preheated to 1 C and held there for about an hour until it is uniformly heated. A high frequency (12 lines/mm) cross-line grating from an ultra low expansion (ULE) grating mold is then replicated at 1 C using a high temperature curing epoxy and is held there until the epoxy cures. This temperature can be treated as the reference temperature or undeformed state. When the specimen is brought back to room temperature, it possesses the deformation signature due to the isothermal load ( T = -8 C) via the deformation of the grating. 3.3 Procedure The high frequency crossed-line diffraction grating applied to the specimen surface imitates the deformation of the specimen with high fidelity. In the interferometer, the ULE grating is used to tune a null field setting at room temperature, corresponding to the uniform original grating frequency. Then, the interaction of the deformed specimen grating with the virtual reference grating of the interferometer creates the moiré fringe pattern, which is viewed by the imaging system [7]. The contour interval between fringes is.417 µm per fringe order. Deformation and strain information is extracted from these fringe patterns by using mathematical relations [5]. However, sometimes even such a high resolution is inadequate in providing accurate deformation information in small areas/regions such as the solder joint. The measured lengths are so small that deformations have to be interpolated in these regions. This inadequacy can be overcome by the fringe shifting method [11], which uses multiple fringe patterns that are shifted by a certain amount to obtain fractional fringe orders. The practical sensitivity of the fringe analysis is increased significantly. From the resulting data matrix of the calculation, a contour map can be plotted with a designated contour interval. 4. Results and Discussion The assembly was heated up to 1 C in an environmental chamber and dwelled for an hour. The subsequent measurements, having 1 C as the reference temperature, were taken at the room temperature (2 C). Accordingly, the thermal loading in this study was 8 C for both Moiré and DIC. 4.1 Global Deformation The deformations of the package in the x and y directions (u and v fields, respectively) are presented only for the right half of the assembly, due to symmetry. Figure 3 shows a contour plot of u and v field deformations where each band (in DIC) and fringe (in MI) shows 2µm per band and.417µm per fringe, respectively. The fringes are the loci of different points with the same deformation. As shown in the v-field images (fig. 3b), because the CTE of PCB is higher than effective CTE of the module, the assembly warps down globally (vertical deformation).

4 2µm/fringe 2µm/fringe Digital Image Correlation.417µm/fringe.417µm/fringe (a) U-field Moiré Interferometry (b) V-field Fig. 3: Contour plots of (a) u and (b) v deformation fields of the die-shadow solder joint (ball 6) obtained from DIC and Moiré Interferometry. Fig. 4a shows the plot of global deformations at the bottom of substrate and top of PCB for the isothermal load, T of -8 C. The deformations for the substrate and PCB are nearly the same and show a change in curvature near the 6 th solder ball, which is attributed to CTE mismatches between the chip region and the bare substrate region of the package [3]. The U and V field deformation mismatches between the PCB and substrate are presented in fig. 4b. Again, the highest deformation mismatch is seen near the 6 th solder ball, which is located at the end of the chip. It is a well known fact that the 6 th solder ball, which is located at the end of the chip, fails earlier for this type of package and it is called the die shadow effect. In fig.4a, the v-deformation field of the PCB is at a certain offset from the substrate field. It can be explained by the free thermal contraction of solder. In Moiré, the boundaries of the substrate and PCB are very well defined in the image (fig. 3). Hence the v-field deformation mismatch in fig. 4b provides accurate information on the vertical deformation of the solder joints. However, in DIC, the v-field deformations were plotted along the lines slightly offset from the solder pad since the boundaries are hard to trace and have a higher noise floor. For this reason, the offset at the center solder ball (ball no.1) was calculated using the known distances and CTE (4ppm/ C) of substrate and PCB. In general, the two techniques provide similar results, which are reasonable physically. 2 Digital Image Correlation Moiré Interferometry Deformation, microns u-pcb u-sub v-pcb v-sub Deformation,microns u-pcb u-sub v-pcb v-sub -12 solder ball no -12 solder ball no (a) u and v deformation of PCB and Substrate for a T of 8 C as cooled from 1 C to Room temperature (2 C)

5 Deformation mismatch, micron Digital Image Correlation u, pcb-sub v, sub-pcb solder ball no. Deformation mismatch, micron Moiré Interferometry u, pcb-sub v,sub-pcb solder ball no (b) Deformation mismatch of PCB and Substrate for a T of 8 C as cooled from 1 C to Room temperature (2 C) Fig. 4 Plots showing the global deformation and deformation mismatch of substrate and PCB in u and v directions 4.2 Solder Joint strains The strains in the solder balls were computed for the central (first), die shadow (sixth) and the corner (ninth) joints. Normal strains, ε x and ε y, were evaluated along the horizontal and vertical centerlines of the solder balls, respectively. The shear strain, γ xy, was computed along the vertical centerline. In the analysis using MI, the solder joint strain was computed using the fringe shifting technique described above. Because of the symmetry of the package, theoretically, the central solder ball is expected to have uniform compressive thermal contraction in ε x, in the amount of 192 µstrain for solders. The measured value of ε x should be less than 192 µstrain considering the constraint of the solder by the pads on the top and bottom of the joint. Both techniques provide a value of about 12 µstrain in average. The shear strain at the center ball should be minimal also due to the symmetry. The results from both the techniques, DIC and MI, confirm the fact (fig.5a and b). Small offset in the values can be attributed to the mechanical strains induced due to warpage (global v-deformation). The value of ε y is affected by compressive mechanical strain, which is associated with vertical deformation of the assembly. Strains on the interconnects tend to increase with the DNP. However, the sixth solder ball located in the die-shadow is the point of interest in the reliability assessment. Effective CTEs of the chip region and the bare substrate region with the constant PCB CTE generates a change in curvature. The sixth ball, therefore, is expected to experience the largest shear strains and it is most likely to fail earlier when the assembly is subjected to thermal cycling. The γ xy plot of Moiré shows a high and uniform shear strain on the solder ball whereas DIC shows a relatively lower value. In case of DIC, the strains are sensitive to the averaging scheme used in the software. The discrepancy in the obtained values is attributed to the averaging scheme used. For the corner solder ball, the largest effect of CTE mismatch between the module and PCB is reflected in the strain plots. A bump that appears on the γ xy plot (fig. 5e) can be attributed to a noise usually appearing at the edges of a sample. Free thermal contraction and some mechanical strain at the highest DNP of this joint are well reflected on the strain distribution. The strain values given by DIC can be misleading if we include the noise at the edges (fig.5e). A close investigation of the result is warranted to eliminate the misleading results. In case of MI, on the other hand, the deformation values are extracted by counting the fringes manually and calculated strains have minimal noise at the edges (fig.5f). Strain calculation in MI can be sometimes tedious due to the manual fringe analysis. In general, results are similar for both DIC and MI. Figure 6 shows the contour plots of x- and y-deformations of the die shadow ball (no.6) and the corresponding fringe patterns from Moiré Interferometry. The deformation pattern matches very well for both techniques.

6 Strain results using Digital Image Correlation Strain results using Moire Interferometry (a) Central solder joint ε x ball1 ε y ball1 γ xy ball (b) Central solder joint ε x ball1 ε y ball1 γ xy ball ε x ball6 ε y ball6 γ xy ball ε x ball6 ε y ball6 γ xy ball6 (c) Die shadow joint (no.6) (d) Die shadow joint (no.6) ε x -ball9 ε y -ball9 γ xy -ball ε x -ball9 ε y -ball9 γ xy -ball (e) Corner joint (no.9) (f) Corner joint (no.9) Fig.5 Strains along center lines of solder ball for (a) Central ball, (b) Die shadow ball and (c) corner ball obtained by Digital Image correlation and Moire Interferometry

7 u-field u-field.42µm/band.428µm/fringe v-field v-field.3µm/band.35µm/fringe (a) (b) Fig. 6 Contour plots showing the u- and v- deformations of the die shadow solder ball (no.6) as obtained from (a) DIC and (b) Moire Interferometry It is important to note that for 2-D DIC, although the camera and associated optics need not be calibrated, it is required that the images remain focused to minimize out-of-plane movements. In this study, the camera was mounted on an x-y stage to compensate for the movement of the sample towards the camera during in-situ experiments. Certain features of the sample surface were focused to ensure a consistent focus. If the image of the sample at the deformed stage is not properly focused, an error will be imposed in the form of uniform normal strains. In case of Moiré, however, focusing does not cause erroneous results. In the case of a 3-D analysis by using DIC, a measurement depth is specified in the calibration process. Hence, the out-of plane motion of up to a certain value does not deteriorate the focus quality of the image. However, the requirement of stereo-vision imposes some limitations in terms of minimum field of view ( 5mm) and lens size in the application of DIC for very small parts like BGA interconnects. In case of Moiré Interferometry, the presence of defects or poor transfer of the grating may generate unnecessary fringes, which interferes proper measurement. Also, the thickness of epoxy used for grating replication should be uniform and minimal to minimize shear lag and get clear information on the edges. A visual judgment has to be made in calculating the deformations. In case of DIC, natural surface features or defects can be used as the gray scale variation. However, too deep scratches or defects may mislead the deformation. For both measurement techniques, extreme caution is required when using the software for the interpretation of data, the errors stemming out primarily from the grating defects in Moiré and the averaging scheme in DIC. Minimal sample preparation, relaxed surface requirement and simplicity of the whole procedure are the advantages of DIC whereas it demands a caution to eliminate the noise on the edges. Moiré technique is well known for high displacement sensitivity and a good resolution on the edges too but it demands a compromise with a cumbersome sample preparation and comparatively tedious data processing. Summary Thermo-mechanical behavior of an FC-PBGA under an isothermal load of T=-8 C was investigated using two optical full field deformation measurement techniques; digital image correlation (DIC) and moiré interferometry (MI). The advantages and disadvantages of each method are discussed.

8 DIC is an emerging technique that can be used in measuring both in-plane and out-of-plane deformations for a broad range of component sizes. Compared with other full field measurement techniques shown in Table 1, DIC offers a simple procedure and repeatable results with a relaxed surface requirement. It requires significantly less sample preparation than MI. Convenient in-situ measurement capability and ability to measure complex 3D geometry are the major advantage of DIC. Digital images taken from other general high magnification imaging instruments like SEM and optical microscope can be analyzed and this feature make it very useful for the deformation measurement of extremely small scale objects like MEMS. Maintaining the focal distance during the entire testing is critical to assure the accuracy of the results and careful evaluation of the results are required since the results may vary depending on the averaging scheme at the boundaries of dissimilar materials. DIC has variable field of view and its measurement sensitivity depends on the field of view (FOV) unlike fixed sensitivity in MI. On the other hand, moiré interferometry has been used for last two decades and proved its versatility and effectiveness in high sensitivity deformation measurement. High spatial resolution, high signal-to-noise ratio and wide dynamic range make it ideal for measuring deformations of multiple layers like those encountered in electronic packages. Fringe shifting and micro-moiré methods enhanced its sensitivity further. One drawback of this technique is tedious sample preparation and requirement of costly high frequency diffraction gratings. Reliable and repeatable automated fringe analysis software is most warranted for the dissemination of the technique. Moiré Interferometry Table 1: Capabilities of current optical tools Shadow Moiré Interferometry [8] Twyman-Green Interferometry Type In-plane Out-of-plane Out-of-plane Basic Resolution (µm) Range of Deformation (µm) Digital Image Correlation In-plane, Out-of-plane Variable (1/3, FOV) 1~5 25~25 1~5 Variable Surface treatment Required - Grating Required - Diffusive Required - Specula May be required, Tedious Simple Tedious Simple Measurable Surface Flat Single Plateau Multiple Plateau Multiple Plateau Field of view < 5 mm variable Variable > 5mm Acknowledgement The authors wish to thank Mr. Tim Schmidt at Trilion Technology Inc. for technical support on GOM s Aramis system used in this work. References 1. Schmidt, Tyson and Galanulis, Full-Field Dynamic Displacement and Strain Measurement Using Advanced 3D Image Correlation Photogrammetry, Experimental Techniques, Part I: May/June 23, Vol. 27 #3, p.47-5, Part II: July/Aug, Vol. 27 #4, p S.B. Park, B. Han, W. Ackerman and K. Verma, On the Design Parameters of Flip Chip Package Assembly for Optimum Solder Ball Reliability, IEEE, Components and Packaging Technologies, vol.24, No.2, pp3-37, S. B. Park and I. Ahmed, Shorter Field Life in Power Cycling for Organic Packages, ASME J. of Electronic Packages, submitted for publication, S.B. Park, Rahul Joshi, Lewis Goldmann, Reliability of Lead-Free Cu Columns in Comparison with Sn-Pb Solder Column Interconnects, Proc. of 54th Electronic Components and Technology Conference, Las Vegas, 24, pp S.J. Ham, S.B. Lee, Measurement of Creep and Relaxation Behaviors of Wafer-level CSP Assembly Using Moiré Interferometry, ASME Journal of Electronic Packaging, Vol. 125, 23, pp S B Park, Ramji Dhakal, Eric Cotts, LP Lehman, Grain boundary deformation and intergrain stresses in Sn3.5Ag.8Cu BGA solder balls, accepted for InterPACK, D. Post et al, High Sensitivity Moiré, Springer-Verlag (New York, 1994), pp , Y. Wang, and P. Hassell, Measurement of Thermally Induced Warpage of BGA Packages/Substrates Using Phase Stepping Shadow Moiré, 9. B. Han and Y. Guo, Photomechanics Tools as applied to Electronic Packaging Product Development, Experimental/Numerical Mechanics in Electronic Packaging, Vol. 1, pp , Society for Experimental Mechanics, Bethel, CT, A. F. Bastawros and A. S. Voloshin, Transient Thermal strain Measurements in Electronic packages, IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. 3, No. 4, pp , K. Andersen, Displacement and strain calculation by the phase shift method, Appl. Optics, 26, (1987)