Residual Thermal Strain Distribution Measurement of Underfills in Flip Chip Electronic Packages by an Inverse Approach Based on the Sampling Moiré Method
- 87 Downloads
Residual deformation evaluation of underfill (UF) materials in flip chips is crucial to improve the reliability of electronic packages. In this study, we propose to evaluate the residual thermal strain distributions using an inverse method based on the sampling moiré technique. Even if a grid pattern is fabricated on the specimen at room temperature, the residual strain distributions at an arbitrary temperature relative to the specimen formation temperature can be successfully calculated. The residual strain distributions relative to the free contraction state at an arbitrary temperature can also be measured when the coefficient of thermal expansion is available. A thermal chamber for flip chips was designed under a laser scanning microscope. Using the proposed method, the normal, shear and principal internal strain distributions and deformation characteristics of two kinds of UFs in flip chips were investigated relative to 150 °C. The strains of the UF with low glass transition temperature (UF-A) concentrate near the die material, especially at the die corner, while the strain concentration of the underfill with high glass transition temperature (UF-B) mainly occurs at the die corner and the buffer layer. The maximum principal strain of UF-A is greater than that of UF-B around the die corner. The residual maximum principal strain distributions relative to the free contraction state at 25 °C were compared with the simulation results by the finite element method. The residual strain distribution trends from experiments are consistent with those from simulations.
KeywordsInternal strain Residual stress Flip chip Underfill Moiré Grid Finite element method
With the development of high-density semiconductor packages, to achieve high functions including heat resistance, toughness, heat dissipation, etc., more and more attention has been focused on the underfill (hereinafter referred to as UF) which is a semiconductor encapsulant resin . However, due to the strong nonlinearity of the UF physical properties and the complex micro-nano compositions, optimum design of UF is quite difficult.
UF is a liquid thermosetting resin and injected into a flip chip package (hereinafter referred to as FCPKG) at normal temperature, cured at high temperature, and cooled to room temperature again. Due to the difference of thermal expansion coefficients of UF, die and other materials in FCPKG, there is internal residual stress in UF and other materials. The internal stress is prone to cause a crack, leading to reliability decline of FCPKG. Since stress and strain are directly related, measurement of internal strain in FCPKG is useful for the optimal design of UF and life extension of FCPKG.
Besides residual stress, strain measurement is also essential for evaluating mechanical properties, instability behaviors, and crack initiation and propagation of materials and structures. At present, the full-field, non-contact and non-destructive deformation measurement techniques mainly include the moiré methods [2, 3], the digital image correlation (DIC) method , geometric phase analysis (GPA) [5, 6], the Fourier transform (FT) method, electronic speckle pattern interferometry (ESPI) . Among these techniques, the DIC method is simple but powerless against noise because the deformation carrier is speckle, GPA and FT are weak for complex deformation measurement as multiple frequencies exist, and ESPI is highly sensitive to vibration.
The commonly used moiré methods include the microscope scanning moiré method  such as electron moiré and laser moiré, moiré interferometry , the CCD or CMOS moiré method (abbreviated by the CCD moiré method in the following), the digital/overlapped moiré method, the temporal phase-shifting moiré method and the sampling moiré method [10, 11, 12]. The first four kinds of moiré methods use the fringe-centering technique, and the last two kinds of moiré methods use the phase-shifting technique  to process moiré fringes for calculating deformation distributions.
The microscope scanning moiré method has been reported for residual strain/stress measurement of composite materials [14, 15], and moiré interferometry has been applied to residual strain/stress measurement of electronic packages and composite materials [16, 17, 18, 19, 20].
The present moiré techniques for deformation measurements mainly have three problems. 1) In the moiré methods using the fringe-centering technique, only the information of centerlines of moiré fringes is used, and thus the deformation measurement accuracy is low. Besides, it is difficult to automatically batch deformation measurement, because the centerlines of moiré fringes often need to be manually repaired. 2) Although the temporal phase-shifting technique can improve the deformation measurement accuracy, a phase-shifting device is necessary, and it takes time to record several images making this technique not suitable for dynamic analysis. 3) Since the deformation sensitivity is very high in moiré interferometry, moiré fringes are too dense to be recorded and thus cannot be analyzed in some areas with large deformation, such as the area around a die corner in FCPKG where UF sharply deforms.
Aimed at the above problems, this study concentrates on accurate measurement of internal residual strain distributions including x-direction, y-direction, shear and principal strains from a single-shot periodic pattern even in the case of large deformation. For nondestructive measurement of residual thermal strain distributions, an inverse approach based on the sampling moiré technique is proposed by combining the spatial phase-shifting sampling moiré method and an inverse idea. The residual deformation behaviors of two kinds of UFs with different glass transition temperatures in FCPKGs were investigated, and compared with the simulation results of the finite element method (FEM).
Principle of Residual Thermal Strain Measurement
Sampling Moiré Generation and Phase Extraction
Similarly, sampling moiré fringes can also be generated from grating X, and the moiré phase difference in the x direction at Tt relative to Tr can also be calculated using the above equations by replacing the direction symbol ‘y’ with ‘x’.
Thermal Strain Measurement Relative to Room Temperature
Residual Thermal Strain Measurement
Consequently, the residual normal strains in the x and y directions at an arbitrary temperature relative to the specimen formation temperature are measurable using Eqs. (7), (8) and (10), where the thermal strains at Tt and Tf, i.e., εx(Tt), εy(Tt), γxy(Tt), εx(Tf), εy(Tf) and γxy(Tf) can be calculated from Eq. (5).
Note that Eqs. (7), (8) and (10) mean the residual strains relative to the specimen formation temperature ignoring the coefficient of thermal expansion (CTE). The residual strain relative to the free contraction state at any temperature can also be calculated from linear transformation of Eqs. (7), (8) and (10) by considering CTE. In defining the residual strain, the stress-free state was used as the reference state in , and the free contraction state refers to the stress-free state in this manuscript. In some cases, the residual strain distribution relative to the free contraction state is difficult to be plotted as the CTEs of different materials in a composite structure are different. For example, the CTEs of UF, die and the buffer layer are different, and the CTEs of resin and filler are also different inside the UF.
In the case that CTE is available, by considering the influence of CTE, Eqs. (7) and (8) can be revised for residual strain measurement in the x and y directions. To simplify the symbols of variables and directions, we simply use P to express the grid pitch at the specimen formation temperature Tf in the x direction '(Fig. 4). When the temperature becomes an arbitrary temperature Tt, the grid pitch changes to P + ΔPR = (1 + εx(Tt)_R)P in the actual state with residual stress due to constrains, where εx(Tt)_R can be obtained from Eqs. (7) and (8) in different directions.
Principal Residual Strain Measurement
When the thermal strains εx, εy and γxy relative to the room temperature in Eq. (5) are substituted into Eq. (13), the principal thermal strains are obtainable. When the residual strains εx(Tt)_R_rev,εy(Tt)_R_rev and γxy(Tt)_R in Eqs. (11), (12) and (10) are substituted into Eq. (13), the principal residual strains relative to the specimen formation temperature are measurable.
Besides, the principal residual stresses can be further measured using Hooke’s law for plane-stress problems when Young’s modulus and the Poisson’s ratio are known.
Simulation Verification and Method Discussion
Simulation Verification of Residual Strain Measurement
The grating at state T1 was deformed to the grating at state T2 by exerting normal and shear strains. The grating at state T2 could be transformed to the grating at state T1 by applying residual normal strains in the x and y directions and residual shear strain. The theoretical residual strains were known from the strain Mohr’s circle and the residual strain Mohr’s circle, a little bit different with the opposite values of the theoretical strains. A series of theoretical residual strains were applied to the grating at state T2.
Discussion of this Method
Full-field residual strain measurements: The precise distributions of 2D residual strains including normal, shear and principal strains are simultaneously measurable;
Fast and accurate: High accuracy, automatic batch processing and fast image storage for displacement, strain, residual strain and residual stress measurements;
Dynamic deformation measurement: The proposed moiré technique is suitable for dynamic deformation measurement and able to display results in real time;
Wide strain measurement range: Both large deformation and small deformation can be accurately measured even if the periodic pattern is inclined at a large angle.
The proposed residual deformation measurement technique can be used in industrial fields such as aerospace, automobiles, electronic packages, biomedicine, military, material manufacturing, etc. It is mainly useful for failure analysis, residual stress measurement, strengthening and toughening of materials ranging from nanoscale to meter-scale, optimal design of interfaces, production quality control, structural health monitoring, etc.
Besides, the inverse approach can also be combined with other grid-based methods for residual strain measurement, including geometric phase analysis (GPA) using Fourier transform (FT) or windowed Fourier transform (WFT). Compared with GPA using FT, the sampling moiré method uses local phase information, and is more suitable for objects with complex boundaries. Compared with GPA using WFT, the sampling moiré method has almost the same measurement accuracy but has much higher calculation speed .
Experiments to Flip Chip Packages
Specimen Preparation and Grid Fabrication
Two FCPKGs containing two different kinds of UFs formed at 150 °Cwere used as the objects of study, where the UF having low glass transition temperature (Tg) was called as UF-A, and the other one having high Tg was referred to as UF-B. These two square FCPKGs with a size of 25 × 25 mm2 were cut along their diagonal lines, and half of both were used as two triangular specimens containing UF-A and UF-B, respectively. The cross sections of the specimens were polished with sandpapers and polishing solution successively.
On the cross section of the specimens, a kind of UV nanoimprint resist PAK01 was dropped and spin-coated. Since the thermally stable temperature for PAK01 is approximately 270 °C , it can be used for deformation measurement of specimens lower than 150 °C. Then, a grid with a pitch of 3 μm was fabricated on the cross section using UV nanoimprint lithography at 25 °C (room temperature). The grid fabrication process of UV nanoimprint lithography was drawn in Fig. 7(b). The pressure was 0.2 MPa, the used UV wavelength was 375 nm, and the UV illumination time was 30s. The grid mold was from SCIVAX Corporation. Next, the grid on either FCPKG specimen was coated with a layer of platinum with a thickness of about 3 nm to increase the contrast of the grid image.
Compared with commercially available grid plate for moiré interferometry, the cost of the UV nanoimprint mold is not much different. However, the UV nanoimprint polymer mold is easy to cut and can be used for several times, and the grid can be quickly fabricated by UV nanoimprint lithography .
Heating Experiment under a Laser Microscope Observation
The thermal chamber was placed on a 5-axis stage, where the back-forth and left-right tilt angles are controllable by a tilt adjustment stage installed at the bottom side of the thermal chamber, and the in-plane rotation and translational motions in the x and y directions are adjustable by the affiliated sample stage of the laser microscope. Besides, the movement in the z direction can also be implemented by the focus adjustment unit of the laser microscope. Thus, the heating system made up by the thermal chamber and the laser microscope can carry out 6-axis or 6-dimensional adjustment including 3-dimensional translation and 3-dimensional rotation.
During the heating experiment, the temperature of either specimen was increased from 25 °C to 150 °C, i.e., from the room temperature to the specimen formation temperature. The grid images on the cross section were recorded at 25 °C, 75 °C, 125 °C and 150 °C by the laser microscope. As the aim is to evaluate the residual strain of UF, we mainly focus on the area of UF around one corner of the die (Figs. 9(b) and 9(c)).
Note that some chemical changes in the FCPKG will occur and some gas will be released during the heating experiment. When the gas meets the glass, some misty liquid will be generated on the inner surface of the glass. To avoid the misty liquid area, during the image observation under the microscope, the glass whose plane view size is much greater than the observation window of the thermal chamber should be slightly moved. Unnoticed misty liquid area will affect the quality of the grid image. Fortunately, the moiré method has strong noise-resistance ability and the strain measurement is almost unaffected.
Residual Strain Distributions of Underfills
For reference, the thermal displacement distributions of two types of underfills at 150 °C relative to 25 °C are depicted in Appendix 1. Compared with the nano-pattern recognition and correlation technique , the proposed technique can measure not only the thermal deformation, but also the residual thermal deformation of materials. The residual strain distributions of both underfills relative to 150 °C are measured using the proposed technique.
Residual Strain Measurement of Underfill A
This section shows the 2D residual thermal strain measurement results of the low-Tg (345 K) underfill (UF-A) in an FCPKG using the proposed technique. Figures 9(b)-9(d) presents the laser microscope images of the FCPKG surface before and after grid fabrication and the enlarged images of the fabricated 3-μm-pitch grid. The laser wavelength was 405 nm in the used laser microscope.
Using Eq. (13), the maximum and minimum residual principal strains (Figs. 11(d) and 11(e)) were also determined at 125 °C, 75 °C and 25 °C, respectively. The maximum principal residual strain is highest at the bottom area below the die, and the absolute value of the minimum principal strain is highest at the corner around the die.
Residual Strain Measurement of Underfill B
The calculated residual principal strains at 125 °C, 75 °C and 25 °C are depicted in Figs. 13(d) and 13(e). Both the maximum and minimum residual principal strains tend to concentrate at the buffer layer besides the corner.
Comparation with FEM Simulation and Discussion
To compare with the simulation results, the residual strain distributions relative to the free contraction state at 25 °C of both underfills were measured using Eqs. (10)–(12), where the used CTE curves of underfills were plotted in Appendix 2. Then the residual principal strain distributions relative to the free contraction state at 25 °C were calculated using Eq. (13). Since the CTEs of underfills, the buffer layer and the substrate are different and the emphasis is on underfills, only the residual principal strain distributions in the underfill area are compared with the simulation results.
Although the strain trends are the same, the strain values from experiments and simulations are a little bit different. The maximum principal strain of UF-A around the die has a maximum value of 0.06 from the experiment, whereas the maximum value is 0.04 from the simulation. The maximum value of the maximum principal strain of UF-B around the die from the experiment is 0.03, also higher than the maximum value of 0.02 obtained by the simulation. It is presumed that the actual UF has low elasticity on the lower surface caused by filler sedimentation, but the UF with high elasticity is used in FEM. The other possible reason lies in the influence of the non-uniformity of the actual filler dispersion in the UF, whereas the UF is uniform in the FEM simulation.
A future work is to compare the proposed image processing technique with other methods such as the hole-drilling method and X-ray analysis in terms of residual strain measurement. Another future work is to improve the FEM model in numerical analysis in the light of experimental results.
The inverse approach based on the sampling moiré method was proposed for measuring the internal thermal strain distributions nondestructively. The measurement principle of the residual strain at different temperatures relative to the specimen formation temperature or the free contraction state was developed. The strain measurement accuracy was verified from the numerical simulation. To investigate the residual thermal strain distributions measurement of UFs in FCPKGs which are most concerned, the clamping jig for nanoimprint lithography and the thermal chamber under the laser scanning microscope were designed. The normal, shear and principal internal strain distributions of two kinds of UFs at 125 °C, 75 °C and 25 °C were measured relative to 150 °C. The residual strains of UF-A with low Tg had greater absolute values around the die, especially at the die corner, whereas the strain localization of UF-B with high Tg mainly emerged at the buffer layer. The residual strain distributions trends from experiments are the same as that from FEM simulations. The proposed method is expected to be used for measuring the internal thermal strains in thermal cycle tests for predicting the life of UFs, finding the cause of crack formation, and optimizing the UF resin design.
- 3.Patorski K (1993) Handbook of the moiré fringe technique. Elsevier Science,Google Scholar
- 4.Kehoe L, Guenebaut V, Kelly P Measurement of deformation and strain in flip chip on BGA (FC-BGA). In: Electronic Components and Technology Conference, 2004. Proceedings. 54th, 2004. IEEE, pp 120–127Google Scholar
- 7.Jang W, Lee B-W, Kim D-W, Nah J-W, Paik K-W, Kwon D Evaluation of thermal shear strains in flip-chip package by electronic speckle pattern interferometry (ESPI). In: Electronic Materials and Packaging, 2001. EMAP 2001. Advances in, 2001. IEEE, pp 310–314Google Scholar
- 11.Lei ZK, Wang ZL Vibration testing parameters measured by sampling moire method. In: Applied Mechanics and Materials, 2012. Trans Tech Publ, pp 1975–1980Google Scholar
- 12.Zhang Q, Xie H, Liu Z, Shi WJO, Engineering Li (2018) Sampling moiré method and its application to determine modulus of thermal barrier coatings under scanning electron microscope 107:315–324Google Scholar
- 17.Cho S, Han B (2001) Effect of Underfill on Flip-Chip solder bumps: an experimental study by microscopic moiréinterferometry. Int J Microcirc Electron Packag 24(3):217–239Google Scholar
- 20.Post D, Han B (2008) Moiré interferometry. In: Springer Handbook of Experimental Solid Mechanics. Springer, pp 627–654Google Scholar
- 23.Omens JH, Fung Y CJCr (1990) Residual strain in rat left ventricle. 66 (1):37–45Google Scholar
- 25.Hirasawa T, Taniguchi J, Ohtaguchi M, Sakai N (2007) Photo-curable resins and the evaluation methods for UV-Nanoimprint lithography. IEEJ Trans Electron Inform Syst 127:174–178Google Scholar
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.