- Email: [email protected]
Micro- and nanomaterials characterization by image correlation methods
Sensors and Actuators A 99 (2002) 165–171
Micro- and nanomaterials characterization by image correlation methods D. Vogel*, A. Gollhardt, B. Michel Department of Mechanical Reliability and Micromaterials, Fraunhofer Institute for Reliability and Microintegration (IZM), Gustav-Meyer-Allee 25, D-13355 Berlin, Germany
Abstract The authors present a new approach to deformation analysis based on localized correlation analysis on load state images. Displacement and strain fields are extracted from images, originating from different kinds of highest resolution equipment, e.g., from scanning electron and scanning force microscopes. As a result, object load response can be recorded within microscopic or nanoscopic material areas. This unique technique is utilized for deformation mapping as well as for direct determination of thermo-mechanical material properties. The method has been established as microDAC/nanoDAC for strain field mapping and as mTest for the measurement of material properties, currently for coefficients of thermal expansion (CTE) and Poisson ratios. This paper includes an introduction in the measurement technique, a view on the method’s capability and the developed hardware for mechanical and thermal testing. The application of the tools is illustrated by examples from electronics and microsystem packaging. Deformation measurements on chip scale packages and measurement of CTE are discussed more in detail. Front-end application of correlation techniques using atomic force microscope imaging is demonstrated. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Mechanical strain; Correlation; Electronics packaging; Material properties
1. Introduction Improvement of thermo-mechanical reliability of advanced electronic packages and microsystems to a large extend depends on the understanding of possible failure mechanisms and factors influencing their lifetime. Finite element simulation, used as a standard tool to analyze thermo-mechanical behavior of devices, can suffer from insufficient input data. Advanced reliability assessment based on finite element analysis (FEA) demands widespread input data for mechanical modeling. The real knowledge of time- and temperature-dependent constitutive material behavior, failure mechanisms and failure occurrence quite often is insufficient for microelectronic components and devices under investigation. Consequently, a strong need exists to accompany finite element simulations by experimental methods of strain and stress analyses. This approach allows to verify selected results from FEA with measurements on real components to ensure appropriate modeling. Because of the small scale of the considered objects and the required field data, only a few methods are available for *
Corresponding author. Tel.: þ49-30-4640-3214; fax: þ49-30-4640-3211. E-mail address: [email protected] (D. Vogel).
that purpose. At present, test chips with silicon piezoresistive stress sensors [1], Raman spectroscopy [2] and optical methods for strain measurement [3–5] are used. Among the optical methods, the Moire´ interferometry is one of the most applied tools. Disadvantages of the Moire´ technique are the cumbersome specimen preparation and the resolution limit set by the optical wavelengths. An alternative approach is image correlation on digital micrographs. It allows to determine strain fields at the surface of loaded objects from the mutual displacement of local image structures. Its main advantage is based on the downscaling capability connected with the use of high magnification equipment, which makes possible to measure deformation inside microscopic objects not accessible by other means. This paper reports the development and application of the corresponding microDAC/nanoDAC (micro/nano-deformation analysis by correlation) tools. With regard to electronic packaging, the paper highlights the capability to study thermal deformation behavior inside such tiny structures as area array interconnects used in flip chip and chip scale packaging technology. Finite element model verification and refinement have been obtained for the thermo-mechanical analysis (TMA) of flip chips assembled to organic laminate substrates. Recent advances in microelectronics and microsystem packaging has put on the agenda the development and
0924-4247/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 9 0 8 - 6
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D. Vogel et al. / Sensors and Actuators A 99 (2002) 165–171
Fig. 1. Principle of displacement field measurement by the comparison of load state images. Black arrows: displacement vectors in a regular grid of measurement point.
application of a multitude of new materials. Their thermomechanical characterization cannot only be achieved with conventional measurement technique and equipment like macroscopic testing machines, TMA, dynamic mechanical analysis (DMA), etc. Several reasons exist to look for new approaches. For example, many test specimens cannot be provided with macroscopic size necessary for classical equipment, or material properties strongly depend on manufacturing and aging conditions of real electronic devices. Sometimes, thermo-mechanical constitutive behavior of micromaterials, as for example in thin polymer layers, is completely different from that of the same material in a bulk specimen. So material properties more often have to be determined on microscopic-scale specimens or even on place, where they are applied [6–9]. microDAC techniques [10,11] originally developed for the measurements of nonhomogeneous strain fields can be modified for this purpose. With regard to the measurements of material properties, this paper focuses on the determination of coefficients of thermal expansion (CTE). Providing of CTE data, e.g., for materials like underfills, molding compounds, or polymers used in passivation and stress compensation layers, can be a crucial issue for design optimization with FEA. Traditional equipment, e.g., for TMA, runs out of availability, if small-sized, soft and thin material specimens have to be analyzed. A microDAC-based method allows access to these materials.
2. Strain determination from local cross-correlation on micrographs Images captured in different kinds of microscopes often exhibit local natural pattern, which can be used as unique markers for deformation measurements. Commonly, these patterns maintain even during severe thermal or mechanical loading of objects. Consequently, incremental displacement fields can be determined from the comparison of subsequent load state micrographs, applying cross-correlation algorithms to a set of small image subwindows. Fig. 1 illustrates the measurement principle by two optical micrographs regarding to two thermal load states from a homogeneous piece of plastics. The mechanically unrestricted thermal expansion of the material is obviously, following the movement of pronounced pattern with regard to the fixed measurement grid. The black arrows are the respective displacement vectors derived by the help of the correlation analysis. Computing derivatives of measured displacement fields or evaluating displacement values in a particular vicinity of a measurement point, it is possible to provide strain data. In the past few years, this approach has been established for strain field determination on micro-objects [11]. In most of the cases, it allows to measure spatially resolved displacement and strain values with an accuracy of approximately 0.1 image pixel and 1 103 , respectively. So, besides the
D. Vogel et al. / Sensors and Actuators A 99 (2002) 165–171
167
Fig. 2. Deformation measurement at a solder bump utilizing an ABAQUS finite element mesh. Gray mesh: part of the complete undistorted FE mesh; white mesh: deformed FE mesh obtained from a microDAC measurement (displacement values three times enhanced). Approximate micrograph field of view: 100 mm.
two in-plane displacement fields ux and uy the following components are available: @ux @uy ; eyy ¼ ; @x @y 1 @ux @uy þ exy ¼ ðin-plane strainsÞ 2 @y @x 1 @ux @uy rxy ¼ ðlocal in-plane rotation angleÞ 2 @y @x exx ¼
(1)
mesh. The later approach implies that the mesh has been prepared for the object under investigation by the help of a finite element preprocessor. The measurement at node points of an FE mesh aims at the direct comparison between results of FEA and of results of corresponding measurements on real components. Fig. 2 shows a respective measurement example.
(2)
Correlation analysis can be performed on whatever micrographs, if they exhibit sufficient natural object pattern and if the imaging process itself is stable enough. Especially for scanning types of image acquisition, like in scanning electron microscope (SEM) and atomic force microscope (AFM) equipment, different drifts in the scanning process itself and between the object and the scanner can introduce distortions to micrographs. They must be avoided, otherwise pseudostrains will be obtained from micrograph comparison. Once the mentioned conditions are fulfilled, thermo-mechanical strains can be extracted from micrographs, including imaging tools with very high magnification. The microDAC software allows two alternative ways to extract displacement and strain data from load state images. In the first case, deformation data is determined for a grid of equidistant measurement points. In the second case, deformation values are found for node points of a finite element
3. Deformation measurement on chip scale packages Besides flip chip technology, chip scale and wafer level packaging were the most pushed electronic packaging approaches within the last years. Namely, size and weight considerations are driving forces for these developments. Because individual chip scale package (CSP) and wafer level package (WLP) underfilling on board level is undesired from cost considerations, one of the main reliability concern is the integrity of the free-standing package-to-board interconnects. microDAC deformation measurements on crosssectioned packages are being used to analyze the impact of thermal mismatch stresses on board level interconnects. They allow to compare packages and to judge upon the suitability of different mechanisms of stress suppression and compensation used by particular CSP or WLP. With regard to thermo-mechanical solder reliability, it is essential in which way the stiff and low expansion silicon die
168
D. Vogel et al. / Sensors and Actuators A 99 (2002) 165–171
Fig. 3. Thermal deformation of solder interconnects on a mBGA, assembly heating from room temperature to 145 8C, strains perpendicular to the board 4 the thermal expansion indicate package bending.
forces the solder interconnects to deform. Materials and components in between the die and the solder interconnect and also board properties influence the solder deformation. Consequently, CSP with rigid or flex interposer as well as WLP with only thin redistribution layers between the die and the solder exhibit quite different mechanical behavior. In order to decouple the die from the PCB one of the first CSP, Tessera’s mBGA, has introduced a compliant elastomer layer between the flex with solder balls and the die. microDAC strain measurements on assembled mBGAs have confirmed the intended suppression of shear strain in the solder material. Anyway, a solder strain perpendicular to the board direction four times higher than the unrestricted material expansion was detected (see Fig. 3). Although this strain level is not effecting severe thermal solder fatigue, it indicates that the solder interconnects are rigid enough to force
the mBGA to bend similarly as underfilled flip chips on organics. Common bending of mounted CSPs with PCBs seems to be also a general issue if rigid type interposers are applied. Stress relaxation takes place as the whole structure bends. Too high solder strain is avoided in this way. On the other hand, as a design rule, package reliability can be sensitive to a possible backside assembly. Prevented board bending underneath the package can redistribute severely stresses and strains and lead to unacceptable solder strains. As an example, Fig. 4 shows a displacement contour line map pointing out the common board and package bending. Measurement of thermal solder deformation on an S3Diepack WLP developed at the IZM revealed a solder ball rotation, which accommodates the remaining CTE mismatch between WLP and board. Aiming at lower solder shear for
Fig. 4. Rigid carrier CSP warpage due to sample heating from room temperature to 90 8C. Left side: cross-section of the investigated CSP, right side: optical microDAC measurement, uy displacement contour lines (displacement component perpendicular to board direction, displacement values in mm units).
D. Vogel et al. / Sensors and Actuators A 99 (2002) 165–171
169
Fig. 5. Solder ball deformation on an S3-Diepack for heating the mounted WLP from room temperature to approximately 125 8C. (a) Package design, (b) deformed virtual object mesh (20) after heating the specimen, determined by microDAC.
outermost balls the stand-off height of the S3-Diepack has been increased by a pair of stacked solder balls. The die-side ball has an envelope of epoxy (S3 solder support structure) to prevent the solder height from collapsing during reflow. Fig. 5 illustrates the typical solder deformation caused by heating from room temperature to approximately 125 8C. The deformation of the originally rectangular and regular spaced mesh clearly shows the rotation of an outer boardside ball. This motion allows the die to slide against the board, similarly to a ball bearing. The deformed mesh of Fig. 5 also demonstrates that the upper ball does not transmit the die-toboard shear to the same extent as the lower one, i.e. a softer S3-material could improve the effect of increased stand-off height.
4. Displacement measurements on cracks by AFM The application of micrographs acquired by AFM to deformation measurement holds a twofold advantage. The higher resolution of AFMs yields, respectively, higher measurement resolution for displacement values but also better resolution of object details. Moreover, using AFM topography maps, a real 3D displacement measurement is possible with one tool, only. Taking AFM micrographs as a base for deformation analysis requires careful choice of appropriate equipment. Mainly reproducibility of the cantilever scans and drift suppression between the object and the scanner is necessary. Fig. 6 shows a measurement carried out on a CT crack test
Fig. 6. Displacement measurement from AFM images nearby a crack tip on an epoxy specimen. (a) AFM topography scan (30 mm 30 mm image size, height scale: 0.22 mm/div) after crack opening, with line scan, (b) displacement field ux (component perpendicular to the crack boundaries).
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D. Vogel et al. / Sensors and Actuators A 99 (2002) 165–171
Fig. 7. CTE measurement from mTest: (a) typical strain vs. temperature for a filled epoxy material (two perpendicular strain components), (b) dependency of CTE values on specimen thickness.
specimen nearby the crack tip location. AFM non-contact topography scans have been picked up for two crack opening displacements. The mutual crack boundary displacement between the load states at the micrograph location is about 200 nm. The AFM images have been taken with a 256 256 image array, i.e. with a lateral resolution of approx. 130 nm/ pixel. Consequently, the crack cannot be identified from the AFM images, because the crack opening is in the order of 1 image pixel. This fact is illustrated by the line scan from the topography plot, where scratches and the crack are not clearly distinguished from each other.
5. Measurement of material properties by a modified microDAC approach The microDAC displacement accuracy on real specimens achieved until today is approximately 0.1 image pixel. Considering, e.g., a desired measurement of CTE values for a medium thermal expansion (Cu, CTE ¼ 1617 ppm K1) and a temperature interval of 100 K, only 1.7 pixel mutual displacement is caused between the image edges of a 1024 1024 pixel detector. That is, thermal expansion for CTE measurements cannot be determined accurately from two single structures in one micrograph image. The same situation takes place for the measurement of strains on a sample within a testing machine, if moderate final strains of less than 1% are considered. Consequently, either the mutual displacement of two images has to be determined, where the image distance is significantly larger than the image size, or statistical properties of multiple measurements on one micrograph have to be taken into account. The second solution seems to be advantageous in cases where two far away detectors are impossible (e.g., in non-optical microscopes for image capture), and has been accomplished. Under the assumption of homogeneous material expansion over the micrograph area and a dominating random error without essential systematic parts, the specimen expansion
can be determined by averaging a whole set of local strain measurements (within one micrograph). However, this approach requires extensive computation, because of the amount of images (i.e. temperature steps) to be treated. Efficient numerical correlation has to be performed, including a reliable discrimination of individual wrong data points. In order to test this approach, hardware and software have been developed together with Image Instruments Chemnitz, which allows automatic CTE measurements on small-scale specimens. Equipment calibration on well-defined specimens with known CTE values, like silver or aluminum, resulted in a measurement accuracy for CTE values of 0.5 ppm K1. CTE values are determined from linear strain curve-fitting. Measurements are possible between room temperature and approximately 200 8C. Fig. 7a and b illustrates how CTE values are evaluated from strains determined over a particular temperature interval. Strains are measured independently in two perpendicular directions, i.e. measurement on anisotropic materials can be carried out without additional runs. The example shown above regards to a flip chip underfill material, which is 70 wt.% filled with silica. Obviously, material inhomogeneities either in the epoxy constitution or in the filler distribution have led to a slight anisotropy of the CTE behavior. Fig. 7b gives the CTE values for the same material but determined on foils of different thickness. As can be seen, no essential CTE variation takes place for thickness values in between 100 mm and 1 mm. Nevertheless, some spread of the coefficient exists with regard to the two perpendicular measurement directions.
6. Conclusions In this paper, an advanced strain measurement technique based on cross-correlation algorithms applied to micrographs has been described. It has been shown that micrographs from scanning electron microscopes as well as from atomic force microscopes can be used to determine experimentally
D. Vogel et al. / Sensors and Actuators A 99 (2002) 165–171
displacements and strains inside very tiny structures. The application of the method was demonstrated mainly for examples from electronics packaging.
[5] [6]
References [1] R.C. Jaeger, J.C. Suhling, Proceedings of the 1997 ASME International Mechanical Engineering Congress and Exposition, EEP-Vol. 22, AMD-Vol. 226, Dallas, 1997, pp. 1–5. [2] I. de Wolf, et al., Proceedings of the Micromaterial’97, Berlin, 1997, pp. 261–269. [3] B. Han, Y. Gou, J. Electron. Packaging 117 (1995) 185. [4] X. Yan, R.K. Agarwal, Proceedings of the SEM Spring Conference on Experimental and Applied Mechanics and Experimental/
[7] [8] [9] [10] [11]
171
Numerical Mechanics in Electronic Packaging II, Houston, 1998, pp. 73–83. D. Post, B. Han, P. Ifju, High Sensitivity Moire´ , Springer, New York, 1994, pp. 331–347. J. Alison (Ed.), Materials Handbook for Hybrid Microelectronics, Artech House, Inc., Norwood, 1988. G. Elsner, J. Appl. Polym. Sci. 34 (1987) 815. H. Ogawa, Proceedings of the Micromaterials’97, Berlin, 1997, pp. 716–719. R. Willecke, P.S. Ho, Proceedings of the Micromaterials’97, Berlin, 1997, pp. 721–724. D. Vogel, A. Schubert, W. Faust, R. Dudek, B. Michel, Microelectron. Reliab. 36 (1996) 1339. D. Vogel, J. Auersperg, A. Schubert, B. Michel, H. Reichl, Proceedings of the Reliability of Solders and Solder Joints Symposium at 126th TMS Annual Meeting and Exhibition, TMS Publication Cat. No. 96-80433, Orlando, 1997, pp. 429–438.
Micro- and nanomaterials characterization by image correlation methods D. Vogel*, A. Gollhardt, B. Michel Department of Mechanical Reliability and Micromaterials, Fraunhofer Institute for Reliability and Microintegration (IZM), Gustav-Meyer-Allee 25, D-13355 Berlin, Germany
Abstract The authors present a new approach to deformation analysis based on localized correlation analysis on load state images. Displacement and strain fields are extracted from images, originating from different kinds of highest resolution equipment, e.g., from scanning electron and scanning force microscopes. As a result, object load response can be recorded within microscopic or nanoscopic material areas. This unique technique is utilized for deformation mapping as well as for direct determination of thermo-mechanical material properties. The method has been established as microDAC/nanoDAC for strain field mapping and as mTest for the measurement of material properties, currently for coefficients of thermal expansion (CTE) and Poisson ratios. This paper includes an introduction in the measurement technique, a view on the method’s capability and the developed hardware for mechanical and thermal testing. The application of the tools is illustrated by examples from electronics and microsystem packaging. Deformation measurements on chip scale packages and measurement of CTE are discussed more in detail. Front-end application of correlation techniques using atomic force microscope imaging is demonstrated. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Mechanical strain; Correlation; Electronics packaging; Material properties
1. Introduction Improvement of thermo-mechanical reliability of advanced electronic packages and microsystems to a large extend depends on the understanding of possible failure mechanisms and factors influencing their lifetime. Finite element simulation, used as a standard tool to analyze thermo-mechanical behavior of devices, can suffer from insufficient input data. Advanced reliability assessment based on finite element analysis (FEA) demands widespread input data for mechanical modeling. The real knowledge of time- and temperature-dependent constitutive material behavior, failure mechanisms and failure occurrence quite often is insufficient for microelectronic components and devices under investigation. Consequently, a strong need exists to accompany finite element simulations by experimental methods of strain and stress analyses. This approach allows to verify selected results from FEA with measurements on real components to ensure appropriate modeling. Because of the small scale of the considered objects and the required field data, only a few methods are available for *
Corresponding author. Tel.: þ49-30-4640-3214; fax: þ49-30-4640-3211. E-mail address: [email protected] (D. Vogel).
that purpose. At present, test chips with silicon piezoresistive stress sensors [1], Raman spectroscopy [2] and optical methods for strain measurement [3–5] are used. Among the optical methods, the Moire´ interferometry is one of the most applied tools. Disadvantages of the Moire´ technique are the cumbersome specimen preparation and the resolution limit set by the optical wavelengths. An alternative approach is image correlation on digital micrographs. It allows to determine strain fields at the surface of loaded objects from the mutual displacement of local image structures. Its main advantage is based on the downscaling capability connected with the use of high magnification equipment, which makes possible to measure deformation inside microscopic objects not accessible by other means. This paper reports the development and application of the corresponding microDAC/nanoDAC (micro/nano-deformation analysis by correlation) tools. With regard to electronic packaging, the paper highlights the capability to study thermal deformation behavior inside such tiny structures as area array interconnects used in flip chip and chip scale packaging technology. Finite element model verification and refinement have been obtained for the thermo-mechanical analysis (TMA) of flip chips assembled to organic laminate substrates. Recent advances in microelectronics and microsystem packaging has put on the agenda the development and
0924-4247/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 9 0 8 - 6
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D. Vogel et al. / Sensors and Actuators A 99 (2002) 165–171
Fig. 1. Principle of displacement field measurement by the comparison of load state images. Black arrows: displacement vectors in a regular grid of measurement point.
application of a multitude of new materials. Their thermomechanical characterization cannot only be achieved with conventional measurement technique and equipment like macroscopic testing machines, TMA, dynamic mechanical analysis (DMA), etc. Several reasons exist to look for new approaches. For example, many test specimens cannot be provided with macroscopic size necessary for classical equipment, or material properties strongly depend on manufacturing and aging conditions of real electronic devices. Sometimes, thermo-mechanical constitutive behavior of micromaterials, as for example in thin polymer layers, is completely different from that of the same material in a bulk specimen. So material properties more often have to be determined on microscopic-scale specimens or even on place, where they are applied [6–9]. microDAC techniques [10,11] originally developed for the measurements of nonhomogeneous strain fields can be modified for this purpose. With regard to the measurements of material properties, this paper focuses on the determination of coefficients of thermal expansion (CTE). Providing of CTE data, e.g., for materials like underfills, molding compounds, or polymers used in passivation and stress compensation layers, can be a crucial issue for design optimization with FEA. Traditional equipment, e.g., for TMA, runs out of availability, if small-sized, soft and thin material specimens have to be analyzed. A microDAC-based method allows access to these materials.
2. Strain determination from local cross-correlation on micrographs Images captured in different kinds of microscopes often exhibit local natural pattern, which can be used as unique markers for deformation measurements. Commonly, these patterns maintain even during severe thermal or mechanical loading of objects. Consequently, incremental displacement fields can be determined from the comparison of subsequent load state micrographs, applying cross-correlation algorithms to a set of small image subwindows. Fig. 1 illustrates the measurement principle by two optical micrographs regarding to two thermal load states from a homogeneous piece of plastics. The mechanically unrestricted thermal expansion of the material is obviously, following the movement of pronounced pattern with regard to the fixed measurement grid. The black arrows are the respective displacement vectors derived by the help of the correlation analysis. Computing derivatives of measured displacement fields or evaluating displacement values in a particular vicinity of a measurement point, it is possible to provide strain data. In the past few years, this approach has been established for strain field determination on micro-objects [11]. In most of the cases, it allows to measure spatially resolved displacement and strain values with an accuracy of approximately 0.1 image pixel and 1 103 , respectively. So, besides the
D. Vogel et al. / Sensors and Actuators A 99 (2002) 165–171
167
Fig. 2. Deformation measurement at a solder bump utilizing an ABAQUS finite element mesh. Gray mesh: part of the complete undistorted FE mesh; white mesh: deformed FE mesh obtained from a microDAC measurement (displacement values three times enhanced). Approximate micrograph field of view: 100 mm.
two in-plane displacement fields ux and uy the following components are available: @ux @uy ; eyy ¼ ; @x @y 1 @ux @uy þ exy ¼ ðin-plane strainsÞ 2 @y @x 1 @ux @uy rxy ¼ ðlocal in-plane rotation angleÞ 2 @y @x exx ¼
(1)
mesh. The later approach implies that the mesh has been prepared for the object under investigation by the help of a finite element preprocessor. The measurement at node points of an FE mesh aims at the direct comparison between results of FEA and of results of corresponding measurements on real components. Fig. 2 shows a respective measurement example.
(2)
Correlation analysis can be performed on whatever micrographs, if they exhibit sufficient natural object pattern and if the imaging process itself is stable enough. Especially for scanning types of image acquisition, like in scanning electron microscope (SEM) and atomic force microscope (AFM) equipment, different drifts in the scanning process itself and between the object and the scanner can introduce distortions to micrographs. They must be avoided, otherwise pseudostrains will be obtained from micrograph comparison. Once the mentioned conditions are fulfilled, thermo-mechanical strains can be extracted from micrographs, including imaging tools with very high magnification. The microDAC software allows two alternative ways to extract displacement and strain data from load state images. In the first case, deformation data is determined for a grid of equidistant measurement points. In the second case, deformation values are found for node points of a finite element
3. Deformation measurement on chip scale packages Besides flip chip technology, chip scale and wafer level packaging were the most pushed electronic packaging approaches within the last years. Namely, size and weight considerations are driving forces for these developments. Because individual chip scale package (CSP) and wafer level package (WLP) underfilling on board level is undesired from cost considerations, one of the main reliability concern is the integrity of the free-standing package-to-board interconnects. microDAC deformation measurements on crosssectioned packages are being used to analyze the impact of thermal mismatch stresses on board level interconnects. They allow to compare packages and to judge upon the suitability of different mechanisms of stress suppression and compensation used by particular CSP or WLP. With regard to thermo-mechanical solder reliability, it is essential in which way the stiff and low expansion silicon die
168
D. Vogel et al. / Sensors and Actuators A 99 (2002) 165–171
Fig. 3. Thermal deformation of solder interconnects on a mBGA, assembly heating from room temperature to 145 8C, strains perpendicular to the board 4 the thermal expansion indicate package bending.
forces the solder interconnects to deform. Materials and components in between the die and the solder interconnect and also board properties influence the solder deformation. Consequently, CSP with rigid or flex interposer as well as WLP with only thin redistribution layers between the die and the solder exhibit quite different mechanical behavior. In order to decouple the die from the PCB one of the first CSP, Tessera’s mBGA, has introduced a compliant elastomer layer between the flex with solder balls and the die. microDAC strain measurements on assembled mBGAs have confirmed the intended suppression of shear strain in the solder material. Anyway, a solder strain perpendicular to the board direction four times higher than the unrestricted material expansion was detected (see Fig. 3). Although this strain level is not effecting severe thermal solder fatigue, it indicates that the solder interconnects are rigid enough to force
the mBGA to bend similarly as underfilled flip chips on organics. Common bending of mounted CSPs with PCBs seems to be also a general issue if rigid type interposers are applied. Stress relaxation takes place as the whole structure bends. Too high solder strain is avoided in this way. On the other hand, as a design rule, package reliability can be sensitive to a possible backside assembly. Prevented board bending underneath the package can redistribute severely stresses and strains and lead to unacceptable solder strains. As an example, Fig. 4 shows a displacement contour line map pointing out the common board and package bending. Measurement of thermal solder deformation on an S3Diepack WLP developed at the IZM revealed a solder ball rotation, which accommodates the remaining CTE mismatch between WLP and board. Aiming at lower solder shear for
Fig. 4. Rigid carrier CSP warpage due to sample heating from room temperature to 90 8C. Left side: cross-section of the investigated CSP, right side: optical microDAC measurement, uy displacement contour lines (displacement component perpendicular to board direction, displacement values in mm units).
D. Vogel et al. / Sensors and Actuators A 99 (2002) 165–171
169
Fig. 5. Solder ball deformation on an S3-Diepack for heating the mounted WLP from room temperature to approximately 125 8C. (a) Package design, (b) deformed virtual object mesh (20) after heating the specimen, determined by microDAC.
outermost balls the stand-off height of the S3-Diepack has been increased by a pair of stacked solder balls. The die-side ball has an envelope of epoxy (S3 solder support structure) to prevent the solder height from collapsing during reflow. Fig. 5 illustrates the typical solder deformation caused by heating from room temperature to approximately 125 8C. The deformation of the originally rectangular and regular spaced mesh clearly shows the rotation of an outer boardside ball. This motion allows the die to slide against the board, similarly to a ball bearing. The deformed mesh of Fig. 5 also demonstrates that the upper ball does not transmit the die-toboard shear to the same extent as the lower one, i.e. a softer S3-material could improve the effect of increased stand-off height.
4. Displacement measurements on cracks by AFM The application of micrographs acquired by AFM to deformation measurement holds a twofold advantage. The higher resolution of AFMs yields, respectively, higher measurement resolution for displacement values but also better resolution of object details. Moreover, using AFM topography maps, a real 3D displacement measurement is possible with one tool, only. Taking AFM micrographs as a base for deformation analysis requires careful choice of appropriate equipment. Mainly reproducibility of the cantilever scans and drift suppression between the object and the scanner is necessary. Fig. 6 shows a measurement carried out on a CT crack test
Fig. 6. Displacement measurement from AFM images nearby a crack tip on an epoxy specimen. (a) AFM topography scan (30 mm 30 mm image size, height scale: 0.22 mm/div) after crack opening, with line scan, (b) displacement field ux (component perpendicular to the crack boundaries).
170
D. Vogel et al. / Sensors and Actuators A 99 (2002) 165–171
Fig. 7. CTE measurement from mTest: (a) typical strain vs. temperature for a filled epoxy material (two perpendicular strain components), (b) dependency of CTE values on specimen thickness.
specimen nearby the crack tip location. AFM non-contact topography scans have been picked up for two crack opening displacements. The mutual crack boundary displacement between the load states at the micrograph location is about 200 nm. The AFM images have been taken with a 256 256 image array, i.e. with a lateral resolution of approx. 130 nm/ pixel. Consequently, the crack cannot be identified from the AFM images, because the crack opening is in the order of 1 image pixel. This fact is illustrated by the line scan from the topography plot, where scratches and the crack are not clearly distinguished from each other.
5. Measurement of material properties by a modified microDAC approach The microDAC displacement accuracy on real specimens achieved until today is approximately 0.1 image pixel. Considering, e.g., a desired measurement of CTE values for a medium thermal expansion (Cu, CTE ¼ 1617 ppm K1) and a temperature interval of 100 K, only 1.7 pixel mutual displacement is caused between the image edges of a 1024 1024 pixel detector. That is, thermal expansion for CTE measurements cannot be determined accurately from two single structures in one micrograph image. The same situation takes place for the measurement of strains on a sample within a testing machine, if moderate final strains of less than 1% are considered. Consequently, either the mutual displacement of two images has to be determined, where the image distance is significantly larger than the image size, or statistical properties of multiple measurements on one micrograph have to be taken into account. The second solution seems to be advantageous in cases where two far away detectors are impossible (e.g., in non-optical microscopes for image capture), and has been accomplished. Under the assumption of homogeneous material expansion over the micrograph area and a dominating random error without essential systematic parts, the specimen expansion
can be determined by averaging a whole set of local strain measurements (within one micrograph). However, this approach requires extensive computation, because of the amount of images (i.e. temperature steps) to be treated. Efficient numerical correlation has to be performed, including a reliable discrimination of individual wrong data points. In order to test this approach, hardware and software have been developed together with Image Instruments Chemnitz, which allows automatic CTE measurements on small-scale specimens. Equipment calibration on well-defined specimens with known CTE values, like silver or aluminum, resulted in a measurement accuracy for CTE values of 0.5 ppm K1. CTE values are determined from linear strain curve-fitting. Measurements are possible between room temperature and approximately 200 8C. Fig. 7a and b illustrates how CTE values are evaluated from strains determined over a particular temperature interval. Strains are measured independently in two perpendicular directions, i.e. measurement on anisotropic materials can be carried out without additional runs. The example shown above regards to a flip chip underfill material, which is 70 wt.% filled with silica. Obviously, material inhomogeneities either in the epoxy constitution or in the filler distribution have led to a slight anisotropy of the CTE behavior. Fig. 7b gives the CTE values for the same material but determined on foils of different thickness. As can be seen, no essential CTE variation takes place for thickness values in between 100 mm and 1 mm. Nevertheless, some spread of the coefficient exists with regard to the two perpendicular measurement directions.
6. Conclusions In this paper, an advanced strain measurement technique based on cross-correlation algorithms applied to micrographs has been described. It has been shown that micrographs from scanning electron microscopes as well as from atomic force microscopes can be used to determine experimentally
D. Vogel et al. / Sensors and Actuators A 99 (2002) 165–171
displacements and strains inside very tiny structures. The application of the method was demonstrated mainly for examples from electronics packaging.
[5] [6]
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