On-Line Measurement of Thermally Induced Warpage of BGAs with High Sensitivity Shadow Moiré

On-Line Measurement of Thermally Induced Warpage of BGAs with High Sensitivity Shadow Moiré On-Line Measurement of Thermally Induced Warpage of BGAs with High Sensitivity Shadow Moiré Yinyan Wang and Patrick Hassell Electronic Packaging Services, Ltd. Co. 430 Tenth Street, Suite S002 Atlanta, Georgia 30318 Phone: 404-881-1114 Fax: 404-881-1614 e-mail: yinyan@warpfinder.com Abstract The thermally induced warpage of electronic substrates is a significant concern in manufacturing processes. The ability to evaluate substrate behavior with respect to temperature is particularly important to manufacturing engineers. Previous measurement of thermally induced warpage on Printed Circuit Boards (PCBs) 1 using general Shadow Moiré cannot provide enough sensitivity for modern electronic substrates and packages. Today s chip-carrier substrates and packages have tighter flatness requirements than the older through-hole connection designs. A successful application of the phase-stepping technique to general Shadow Moiré is reported in this paper. This application has made fringe pattern analysis fully automated and significantly increased the effective sensitivity of the analysis. Key words: Thermal Warpage, Ball Grid Array, Electronic Substrates, Shadow Moiré Method, and Phase-Stepping Technique. 1. Introduction and Background Thermally induced warpage of printed circuit boards and components can cause significant production and reliability problems. Symptoms of adverse warpage include damaged and misregistered components, solder paste bridging and opens, cracked solder joints, and production line jams. Thermally induced warpage is most prevalent during high temperature manufacturing processes, such as reflow soldering, and during cycling operation, such as the powering on and off of a computer. As substrate designs strive to incorporate finer lines, higher component densities, and thinner cross-sections, the relative impact of warped boards and components is increased. Traditional means for measuring warpage have been limited to pre-process and post-process inspections. The ability to evaluate in-process warpage was not possible until recently. In 1989, a group of industrial sponsors initiated research which resulted in a novel approach to substrate warpage measurement with respect to temperature in real-time using general Shadow Moiré technique 2. Originally applied to larger area substrates, primarily PCBs, this technology was used to evaluate thermomechanical behavior on a global (such as whole board) basis. With the introduction of advanced packaging systems (especially the Ball Grid Array (BGA)), to mainstream manufacturing, a second order of warpage concerns for electronic packages and assemblies was created. Component packages and substrates also have design specific warpage behaviors induced by temperature. The ability to evaluate advanced package designs at temperature was important for both reliability and interconnect purposes. The warpage of a BGA is usually on the order of dozens of micrometers. It is necessary to find a suitable measuring method that has adequate sensitivity to detect this magnitude of deformation. The previous on-line thermal warpage system is based on general Shadow Moiré method. Since the technique does not have enough sensitivity, for the deformation of BGAs, the resulting fringes will be too sparse to obtain enough relevant information to analyze. If high sensitivity methods, such as Twyman-Green interferometry and The International Journal of Microcircuits and Electronic Packaging, Volume 21, Number 2, Second Quarter 1998 (ISSN 1063-1674) 191

Intl. Journal of Microcircuits and Electronic Packaging ESPI (Electronic Speckle Pattern Interferometry) are applied to measure the warpage of BGAs, the resulting fringe density is very high and it is often difficult to analyze. For example, if the maximum out-of-plane deformation of a BGA is 25µm, and the size of the sample is 27 mm x 27 mm, there will be approximately 80 fringes in the field. Analyzing this number of fringes can be prone to error since the local surface irregularities will affect the useful global deformation data extraction. In this paper, phase-stepping technique is applied to automatically analyze fringe pattern images of BGAs obtained from Shadow Moiré during a simulated reflow process. The sensitivity of the fringe analysis has been increased significantly compared with the traditional fringe center counting methodology. 2. Shadow Moiré Method Np w = tan α + tan β Where w is the out-of-plane displacement, N represents the fringe order, p is the pitch of the grating, V is the illumination angle, and b is the observation angle. The displacement that each fringe represents is equal to the pitch (p) of the grating if the illumination angle a is 45 and the observation angle b is 0 as it is sketched in Figure 1. In equation (1), w is the out-of-plane displacement and N is the fringe order. Traditionally, the value of the out-of-plane displacement at an interested point in the field is determined by counting the fringe order at that point from a selected reference point. Interpolation is generally necessary since the point does not always fall onto a fringe center. The accuracy of the fringe counting is about 20 percent of a fringe value and the finest displacement that the general Shadow Moiré technique can practically reach is 25 lm. (1) Shadow Moiré method is an optical method that measures the topography of the surface of a solid object, such as its deviation from a planar surface 3,4. Figure 1 illustrates a schematic of Shadow Moiré technique. A reference grating, which is comprised of transparent and opaque equal spaces on a flat glass substrate, is positioned above the surface of the sample. When the light (white light) illuminates the grating, it projects the shadow of the grating onto the surface of the sample. The shadow of the grating is distorted due to the warpage of the sample. Moiré fringes are then generated by the geometric interference of the shadow grating and the real reference grating. Fringes are commonly viewed at an angle normal to the surface of the grating. Fringes are lines composed by the points along the sample having equal distance between the grating and the surface of the sample. 3. Phase-Stepping Technique It has been shown that the fringes produced by Shadow Moiré are equal value lines of the distance between the sample surface and the grating (refer to Figure 1). When the grating is translated (zaxis) closer to or further from the sample surface, a given fringe will move towards a lower order or higher order, respectively. When the grating is translated a distance of p away from the sample, the fringe will shift up one complete fringe. When the grating is translated a distance of fractional p, the fringe will shift a fractional fringe space. Phase-stepping technique uses multiple fringe patterns that are shifted at a certain amount to obtain fine fractional fringe orders. The sensitivity of the fringe pattern image analysis is increased significantly by the technique. One important advantage of the technique is that the direction of the warpage is automatically determined due to the nature of the fringe shifting. The distribution of light intensity of a fringe pattern image obtained from Shadow Moiré can be approximated by a sinusoidal function 5, I = I o + A cos [w (y)] (2) Figure 1. Schematic of Shadow Moiré. When the surface of the sample deviates from a flat plane or is not parallel to the grating, the fringe density is high, otherwise, the fringes are sparse. From the governing equation of Shadow Moiré, where I is the light intensity, I o is the background light, A is the modulation of the fringe, and w is the phase term. The fringe number N is simply equal to w/2o, therefore, finding out the fringe number N is to determine the phase term w in equation (2). The phase term w is determined by taking a number of fringe pattern images, shifting the fringe pattern image a certain amount for each acquisition, and applying a least squares algorithm 6-9 to solve for the unknowns of equation (2). A minimum of three images is necessary since there are three unknowns, the background light I o, the modulation of the fringes A, and the phase w. It is noted that among the three unknowns, only w needs to be solved explicitly. In general, the more images that are taken, the less error that is seen 192

On-Line Measurement of Thermally Induced Warpage of BGAs with High Sensitivity Shadow Moiré due to the shifting system 9. However, in practice, the less images taken, the greater the data acquisition rate and the lower the required storage memory. During on-line measurement of thermally induced warpage of BGAs, the sample is heated according to the designated temperature profile that simulates the manufacturing reflow process. The fringe pattern images need to be recorded as quickly as possible. Three steps of fringe shifting are used in this application. Considering that the three-step technique is sensitive to shifting error, a calibration is performed for the test 5. Equation (3) describes the three phase-shifted fringe patterns. CCD camera records each image after the grating is translated one third of one fringe cycle. I I I 1 2 3 = I o = I = I o o + Acos[ y)] 2π + Acos[ y) ] 3 4π + Acos[ y) ] 3 The phase term w (y) is thereby determined by the following relation, 3( I2 I3) y) = arctan (2I1 I 2 I3) (4) Due to the nature of arc tangent calculation, w (y) can only be determined in the range 0 to 2o. An unwrapping process removes the phase ambiguities. Figure 2(a) shows the fringe pattern image of the wrapped phase of a BGA, Figure 2(b) shows comparison of the contours obtained from the phase-stepping technique and the original fringe pattern image, and Figure 2(c) illustrates a contour plot in which the contour interval is 12.7 lm. From the resulting data matri a contour map can be plotted with a designated contour interval. When the contour interval is set at the level that equals the displacement of one fringe order, the plotted contour lines should fall on the fringe centers if the phase calculation is accurate (see Figure 2(b)). The contour plot reveals the fine features of the sample with fine intervals in Figure 2(c). Such fine features cannot be obtained if traditional fringe counting methodology is used to analyze the fringe pattern image data. (3) Figure 2. Wrapped fringe pattern obtained in phase-stepping technique and the resulting contours. 4. Applications 4.1. System for On-Line Measurement of Thermally Induced Warpage of BGAs Figure 3 shows the system that has been used for the on-line measurement of thermally induced warpage of BGAs. The system was developed in the Advanced Electronic Packaging Laboratory at the Georgia Institute of Technology 1. The heating system is capable of simulating a variety of soldering processes, such as wave soldering and infrared reflow soldering. The optical grating is set on a stage that is driven by a computer controlled stepper-motor. The test sample is set inside of an insulated chamber and its temperature is measured by multiple thermocouples interfaced through signal conditioning circuitry to a desktop computer. The signal of a master thermocouple is compared with a user-entered temperature profile and the heater is driven accordingly. The fringe pattern images are recorded in real-time by a CCD camera and the recorded images are analyzed at a later time. Figure 3. System of on-line measurement of thermally induced warpage of BGAs. The International Journal of Microcircuits and Electronic Packaging, Volume 21, Number 2, Second Quarter 1998 (ISSN 1063-1674) 193

Intl. Journal of Microcircuits and Electronic Packaging 4.2. Normalization of the Sample Position The fringe pattern image obtained from Shadow Moiré technique provides a contour map of the relative distance between the surface of the sample and the fixed reference grating. The rigid body rotation and translation of the sample will not affect the warpage measurement. The fringe pattern image depends upon the relative position of the sample to the grating. Sample warpage/deformation is independent of this positioning and the resulting displacement matrix can be rotated/translated freely in space. Two ways of normalizing the sample position are presented in this work. One is a three-point rotation and the other is a best-fit plane rotation. For the three point rotation, the resulting data matrix from the phase calculation is rotated in free space so that three points (often three corners) of the sample are set to the z = 0 plane. For the best-fit plane rotation, the best-fit plane of the deformed sample is determined and set to z = 0. Figure 4(a) shows the initial fringe pattern image of a BGA, Figure 4(b) shows the 3D surface plot of the deformed BGA without position normalization, Figure 4(c) shows the surface plot with three-corner rotation, and Figure 4(d) is the surface plot with the best fit plane normalization. Figure 4. Normalization of the sample position, (a) initial fringe pattern, (b) surface plot without rotation, (c) surface plot with three-corner rotation, and (d) surface plot with best fit plane rotation. 4.3. On-line Measurement of a PBGA through a Heating-Cooling Cycle Figure 5 shows the results of an on-line measurement of thermally induced warpage of a Plastic Ball Grid Array (PBGA) when experiencing a heating-cooling cycle. The size of the PBGA was 27 mm x 27 mm. The tested PBGA sample was heated and cooled inside of the insulated chamber as shown in Figure 3. A designated temperature profile was followed as shown in Figure 5. The fringe pattern images were recorded at different temperature points. The temperature of the sample was monitored by two thermocouples, one of which was designated as a master. In order to ensure that the sample was heated uniformly, the temperature was held constant for about 100 seconds at each point of data acquisition. Figure 5. Thermally induced warpage of a PBGA through a heating-cooling cycle. The frequency of the grating used in the measurement was 11.8 lines per mm (300 lines per inch). The fringe sensitivity of the setup was 84.7 lm (3.33 mils) per fringe. The measurement was made on the die side (top side of the package). The data matrix of the sample was normalized through a three-point rotation. As seen from Figure 5, the sample starts with a slightly concave shape. When it is first heated, the shape of the deformation remains the same, but the magnitude increases. It is believed that the temperature differential between the substrate (higher) and the encapsulant (lower) during the initial heating is responsible for the increased magnitude of the concave bow. Between 100 C and 200 C heating, the magnitude of the warpage decreases. As temperature increases, a bimetallic strip effect (due in part to the higher CTE of the encapsulant) takes place. The more rapidly expanding encapsulant results in a reduction of concavity up to the packaging temperature (178 C). Continued heating and resulting expansion creates a convex surface on the top of the package. Internal stresses between the die and the encapsulant reach a point during heating where delamination occurs. The top surface of the package deforms to its maximum magnitude at this point. Its warpage magnitude decreases when it is cooled. Subsequent laboratory analysis confirmed that delamination had indeed occurred. There were a total of three samples tested and each behaved similarly. At peak temperature, the maximum deflection of the sample was as large as 161 lm. Potential means to improve this package behavior include performing additional evaluations on different encapsulant materials, altering the processing temperature profile, and evaluating moisture content and its relative effect. 194

On-Line Measurement of Thermally Induced Warpage of BGAs with High Sensitivity Shadow Moiré 4.4. Thermally Induced Warpage Analysis of a BGA and its Seating PWB A BGA (size 38 mm x 38 mm) and its seating PWB were measured independently though a simulated reflow process. For the PWB, the measurement was made on the area where the BGA was to be seated. Each sample was heated inside of the insulated chamber and the fringe pattern images were recorded at different temperature points. The fringe sensitivity was again configured for 84.7 lm (3.33 mils) per fringe. At peak temperature of 225 C, the PWB was bowed downward while the BGA substrate was bowed upward. The maximum gap between the two substrates was found to be as large as 428 lm. Figure 6 plots the deformed BGA and PWB at the peak temperature. Each sample has first been normalized through the best-fit plane rotation, and then the BGA has been moved up along the z-axis so that there is no overlap between the BGA and the PWB. Figure 6(b) shows a 2D plot of the deformed samples along one diagonal (from top-left to bottom-right corners). The maximum gap is the difference between the maximum z-value of the BGA and the minimum z-value of the PWB. Such a large gap between the interconnect points of the BGA and its seating PWB due to their respective thermally induced warpages is very likely to create problems during reflow processing. In this particular example, in-process warpage evaluation revealed that the magnitude of the package deformation in combination with the seating plane deformation was causing a bridging defect. At peak temperature, liquidous solder balls had elongated in the interior regions to maintain contact between the package and the pad on the PWB substrate. The convex shape of the package substrate along with the concave shape of the PWB seating plane caused the majority of the solder connections to pull the component towards the PWB board. This pulling force was being supported by very few solder balls on the corners of the package. The compressive forces on these few solder balls was great enough to cause one or more to increase in diameter to the point where it bridged to an adjacent solder ball. Upon cool-down from peak temperature to 183 C, both the package substrate and the PWB component footprint experienced a significant reduction in warpage magnitude. Good solder ball interconnect was achieved throughout the area array; however, the bridging defect had already occurred and remained. Post processing analysis showed an acceptably flat package and PWB seating plane. Insitu measurement with phase-stepping analysis was responsible for exposing the occurrence of the defect. 5. Conclusion The analysis sensitivity of Shadow Moiré fringe pattern images is increased significantly when phase-stepping technique is applied. Phase-stepping technique makes use of the grayscale information contained in the fringe pattern images so that it resolves the fractional fringe order. Since the approach divides the grayscale of the light intensity of a fringe image into 256 degrees, it can theoretically resolve the fractional fringe order to 1/256. In practice, a resolution of 0.1-0.01 fringe order is widely accepted 9-12. The phase-stepping analysis can be fully automated and the direction of the deformation can be automatically determined. Since the technique increases the sensitivity of the fringe pattern image analysis only, the Shadow Moiré system remains environmentally insensitive. Thermally induced warpage of BGAs and electronic substrates can be successfully measured in real-time through the system described in this work. The application of the phase-stepping technique adds more powerful features to the existing on-line thermal warpage testing system. Acknowledgments Figure 6. Thermally induced warpage of BGA and its seating PWB, (a) 3-D surface plot of the deformed BGA and PWB at peak temperature 225 C, (b) deformation along the diagonal of the BGA and the seating site of the PWB. The authors would like to express their gratitude to EPS s customers who provided samples for testing, and Mr. Sean McCarron for his help with the testing of the BGA and its seating PWB. The International Journal of Microcircuits and Electronic Packaging, Volume 21, Number 2, Second Quarter 1998 (ISSN 1063-1674) References 1. M. Stiteler and C. Ume, System for Real-Time Measurements of Thermally Induced Warpage in a Simulated Infrared Soldering Environment, ASME Journal of Electronic Packaging, Vol. 119, pp. 1-7, 1997. 2. Ifeanyi C. Ume, Method and Apparatus for Measuring Thermally Induced Warpage in Printed Wiring Boards Using Shadow Moiré, United States Patent No. 5601364, February 11, 1997. 3. F. P. Chiang, Manual on Experimental Stress Analysis, J.F. Doyle and J.W. 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Intl. Journal of Microcircuits and Electronic Packaging tion of BGA Using Phase-shifting Shadow Moiré, Electronic/ Numerical Mechanics in Electronic Packaging, Volume II, pp. 32-39, Society for Experimental Mechanics, Inc., 1998. 6. G. Lai and T. Yatagai, Generalized Phase-Shifting Interferometry, Journal of Optical Society of America A, Vol. 8, No. 5, pp. 822-827, 1991. 7. J. E. Greivenkamp, Generalized Data Reduction for Heterodyne Interferometry, Optical Engineering,Vol. 23, No. 4, pp. 350-352, 1984. 8. C. J. Morgan, Least-Squares Estimation in Phase-Measurement Interferometry, Optics Letters, Vol. 7, No. 8, pp. 368-370, 1982. 9. K. Creath, Progress in Optics, Elsevier Science Publishers B.V., New York, Vol. 26, Chapter. 5, pp. 349-393, 1988. 10. J. L. Sullivan, Phase Stepped Fraction Moiré, Experimental Mechanics, Vol. 31, pg. 373, 1991. 11. M. Chang and C. S. Ho, Phase-Measuring Profilometry Using Sinusoidal Grating, Experimental Mechanics, Vol. 33, pg. 117, 1993. 12. M. Chang, C. P. Hu, P. Lam, and J.C. Wyant, High Precision Deformation Measurement by Digital Phase Shifting Holographic Interferometry, Applied Optics, Vol. 24, pg. 3780, 1985. About the authors Yinyan Wang obtained a Master Degree in Engineering Mechanics at Tianjin University (P.R. China) and a Ph.D. Degree in Mechanical Engineering at State University of New York at Stony Brook. She joined the staff of Electronic Packaging Services, Ltd. Co. as Director of Technology in April 1996. Her principal fields of research include advanced optical methods for experimental mechanics and their applications to engineering problems. Patrick Hassell is a co-founder and current Director of Business Development for Electronic Packaging Services, (EPS), Ltd. Co. in Atlanta, Georgia. His three-year tenure at EPS has been focused on effective transfer and commercialization of the Company s core technology developed at Georgia Tech s Advanced Electronic Packaging Laboratory. Mr. Hassell holds a Bachelor of Science in Engineering from the University of Virginia, and an M.B.A with a focus in Management of Technology from the Dupree School of Management at Georgia Tech. 196