IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 7, NO. 3, MARCH

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1 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 7, NO. 3, MARCH Moisture Effects on NCF Adhesion and Solder Joint Reliability of Chip-on-Board Assembly Using Cu Pillar/Sn Ag Microbump Youngsoon Kim, Taeshik Yoon, Tae-Wan Kim, Taek-Soo Kim, and Kyung-Wook Paik Abstract Electronic devices as well as electronic packaging technology have required higher speed, I/O capability, and density. To meet these requirements, flip-chip solder bumps interconnection combined with reflow assembly process has been a widely used. In spite of the many advantages of flip-chip solder bump joints, there is a limitation for less than 100 µm fine pitch interconnection due to the solder bump bridging during the assembly process. Therefore, nonconductive films (NCFs) and Cu pillar/sn Ag microbump interconnection become a promising interconnection solution for fine pitch assembly. However, NCFs technology has some problems for flip-chip assembly. Epoxy-based NCF can easily absorb moisture, causing delamination reliability problem at moisture environment. In order to solve the interconnection, the NCF adhesion should be enhanced. Silane coupling agent (SCA) was added to NCFs to secure the microbump joint reliability for chip-on-board assembly by increasing the adhesion strength in the pressure cooker test. After the humidity test, the NCFs modulus and Tg were increased by adding SCA content. Moreover, the measured adhesion strength and energy showed similar results after the humidity test that higher SCA content showed high adhesion strength and energy than lower SCA content and unmodified NCF at the interface between NCF and solder resist of the printed circuit board substrate. The bump joint lifetime of 5wt% SCA NCF was longer than 1wt% SCA and unmodified NCF after the humidity test. In this paper, we report results of our investigations on effects of employing SCA in NCF composition for improved moisture resistance. Index Terms Adhesion, moisture, nonconductive films (NCFs), solder joint reliability. I. INTRODUCTION To ensure the reliability of the flip-chip packaging, underfills are necessary to protect solder bump joint from mechanical and chemical attacks and to improve the reliability [1] [3]. Thus, the underfill material should have higher modulus, higher Tg, lower moisture absorption, and good adhesion. Due to the trends of integrated circuit devices such as high I/O capabilities, high speed, and high density, the bump pitch of the flip-chip packaging becomes narrower. The fine pitch of flip-chip interconnection can cause severe reliability Manuscript received August 24, 2015; revised August 31, 2016; accepted November 6, Date of publication February 14, 2017; date of current version March 14, Recommended for publication by Associate Editor R. N. Das upon evaluation of reviewers comments. Y. Kim, T.-W. Kim, and K.-W. Paik are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon , South Korea ( kwpaik@kaist.ac.kr). T. Yoon and T.-S. Kim are with the Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon , South Korea. Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TCPMT problems due to the underfill voids and flux residue after the capillary underfill process [4]. Therefore, B-stage preapplied nonconductive films (NCFs) can be a promising technology to solve the reliability problems of capillary underfill and make stable microsolder joints without underfill voids. Moreover, B-stage preapplied NCFs can reduce packaging process steps such as flux treatment and flux cleaning. To achieve stable solder joints and void suppression at NCFs, NCFs should have proper material properties to secure interconnection reliability. First, the resin material of the NCF should flow easily during the bonding process to avoid the resin trapping to occur between the metal bumps and substrate electrode. Second, the NCF resin material should have a flux function to remove the solder oxide at the bump in order to form stable solder joints. Finally, there should be no voids when the resin is completely cured after final bonding at high temperature. It was reported that many NCFs used epoxy resin to secure the reliability of interconnection [5] [8]. Epoxy resins are widely used in electronic packaging due to its advantages such as excellent mechanical and chemical properties. However, epoxy resin has a critical problem of moisture absorption. Epoxy resin can easily absorb moisture that can produce a severe reliability issue of delamination since the absorbed moisture is expanded when exposed to high temperature during packaging process [9] [14]. The absorbed moisture within the epoxy resin can be classified into two states. The first state of the absorbed moisture is the bound water where hydrogen bonding occurs between the epoxy resin chains and the interfaces. The second state is called the unbound water, which sits at voids and free volume within an epoxy resin. Normally, more than 90% of absorbed moisture in the epoxy resin is known as the bound water. Moreover, the bound water is mainly involved in hygroscopic swelling of epoxy resin, whereas unbound water is not involved in the epoxy swelling [15]. As the moisture absorption increases, the vapor pressure in epoxy resin is also increased, while interfacial adhesion is decreased. Although the vapor pressure saturates during the absorption process, the interfacial adhesion continually decreases. Thus, resin delamination can occur when the effects of interfacial adhesion are below the effects of the vapor pressure [16]. Preventing the moisture absorption is the main challenge at the solder flipchip interconnection, since this will improve adhesion, which is the most important factor to prevent delamination at the interface. Thus, preventing moisture absorption will make sure that NCFs have sufficient interfacial adhesion. In this paper, the effects of silane coupling agent (SCA) on NCF adhesion between the Si chips and the printed circuit IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See for more information.

2 372 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 7, NO. 3, MARCH 2017 Fig. 1. Molecular structure of glycidoxypropyltrimethoxysilane. Fig. 3. (a) Schematic of the DCB specimen. (b) Photograph of the fabricated DCB specimen. Fig. 2. (a) Image and (b) schematic of the die shear test. board (PCB) substrates were evaluated. Here, NCFs were applied on the microbumps on a Si chip. The die shear and DCB test were performed to evaluate the NCF adhesion at the interface between the NCFs and the solder-resist (SR) laminated PCB substrates depending on the SCA contents in the NCFs. The pressure cooker test [121 C, 2 atm, and 100% relative humidity (RH)] was performed up to 264 h to evaluate the solder joint reliability depending on the SCA content in NCFs. The contact resistance was measured using the fourpoint probe system. II. EXPERIMENTS A. NCF Material Preparation and Mechanical Property The NCFs used in this experiment are composed of three main parts: 1) resin; 2) anhydride; and 3) additives. Epoxy resin was mainly used for the resin material, and multifunctional resin and phenoxy resin were mixed for enhanced film properties and had the characteristic of film formation, respectively. The anhydride material was used as a latent epoxy curing agent. In addition, some additives such as the silica filler and the curing accelerator were used as a supporter. In this experiment, three different types of NCFs were used: 1) the unmodified NCF; 2) the NCF with 1wt% SCA; and 3) the NCF with 5wt% SCA, respectively. Fig. 1 shows the molecular structure of the glycidoxypropyltrimethoxysilane. The hydrolyzable group is made of hydrogen bonding with OH on the substrate surface, and the organofunctional group is involved in cure of ether in NCFs. The mechanical properties such as storage modulus and loss tangent (tan δ) were measured using the dynamic mechanical analyzer (DMA) using a sinusoidal force of 100 ± 20 mn at 0.02 Hz, and the temperature ranged from 30 C to 180 C with a heating rate of 5 C/min. The moisture uptake ratio was measured by weight changes after a humidity test using mm 2 size and 500-μm-thickness NCFs. B. Test Vehicle 1) Die Shear Test: For the top dummy test chip, a blank Si wafer was diced into mm 2 size. For the bottom Fig. 4. Image of a (a) Si test chip and a (b) PCB substrate for COB assembly. Fig. 5. Top and cross-sectional images of the Cu pillar/sn Ag microbump on a Si test chip. substrate, copper clad lamination (CCL) with a thickness of 60 μm was used as the core layer. The top and bottom of the CCL was first electroplated with a 15-μm Cu layer, and a 20-μm-thick solder resist was coated on top of the Cu layer. Fig. 2 shows the top view and the schematic of the die shear test specimen. 2) Double Cantilever Beam Test: PCBs of size mm 2 that consisted of CCL, electroplated Cu, and solder resist were used as beam materials in the DCB specimen. As seen in Fig. 3, 25-μm-thick NCFs were laminated between two symmetric PCB beams and were cured at 210 C under the 50-mbar pressure. NCFs were partially applied on PCB beams to make a precrack region of 10 mm, which ensures the stable crack initiation during the fracture test. After the bonding of PCB beams with NCF, the aluminum loading tabs were attached on both edges of PCB beams to apply tensile forces on the DCB specimen. 3) Reliability Evaluation: For the top chip, mm 2 - size Si chips with a bump pitch of 130 μm were used. The microbump structure on a Si chip was Cu pillar/sn Ag where the Cu pillar and Sn Ag heights were 50 and 20 μm, respectively. When observing the top view of the microbump, the diameter was 70 μm. For the bottom PCB, mm 2 -size PCB was used. For the PCB substrate, the core thickness of PCB was 150 μm, and a 15-μm thickness

3 KIM et al.: MOISTURE EFFECTS ON NCF ADHESION AND SOLDER JOINT RELIABILITY OF COB ASSEMBLY 373 Fig. 6. Top and cross-sectional images of coined solder bumps on a PCB substrate after the coining process. Fig. 7. Photograph of the DCB fracture test. of Cu layer was electroplated followed by a 20-μm solder resist covering. A 75-μm-diameter size (top view) coined SAC solder bump with a height of 35 μm was formed on Cu bump pads with electroless nickel electroless palladium immersion gold (ENEPIG) surface finish (Ni: 7 μm, Pd: 0.06 μm, and Au: 0.1 μm). The coined SAC solder bump was 15 μm higher than the solder-resist surface. Fig. 4 shows the top images of a Si chip and a PCB substrate used for the chip-on-board (COB) assembly. Figs. 5 and 6 show the top and cross-sectional images of the Cu pillar/sn Ag microbump on a Si test chip and coined solder bumps on a PCB substrate after the coining process, respectively. C. Evaluation of Adhesion Strength of NCF The adhesion strength of the NCFs with various SCA content (0wt%, 1wt%, and 5wt%) were evaluated using the die shear test before and after the pressure cooker test at 121 C, 2 atm, and 100% RH. For the die sheer test, COB samples were prepared as follows. First, a silicon wafer was diced into mm 2 size and the PCB substrate was diced into mm 2 size. Then, the 20-μm-thickness NCF was laminated on the diced silicon wafer surface using a roll laminator. After the NCF was laminated, thermal compression bonding was performed at 250 C and 1 Mpa for 60 s to attach the silicon chip on a PCB substrate. Finally, vacuum lamination was performed at 210 C for 10 min to fully cure the NCF resin. The die sheer test was performed at room temperature, and the shear speed and shear height were 700 μm/s and 100 μm, respectively. D. Evaluation of Adhesion Energy of NCF/Solder-Resist Interface Using DCB Test The adhesion energy between the NCF and SR was measured using the DCB fracture mechanics test [17]. The contents of SCA were 0wt%, 1wt%, and 5wt%. The as-bonded Fig. 8. Modulus and tan δ of NCFs before and after the humidity test depending on various amounts of SCA. (a) Unmodified NCF. (b) 1wt% SCA added NCF. (c) 5wt% SCA added NCF. and pressure-cooker-tested (48 and 120 h) samples were tested to characterize the humidity reliability. The PCB was used as a beam material, and the flexural modulus of a single PCB beam was measured by a three-point bending test. The DCB specimens were tested by a high-precision micromechanical tester, which controls the displacement rate and monitors the applied load as shown in Fig. 7. The DCB specimens were loaded and unloaded under a constant displacement

4 374 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 7, NO. 3, MARCH 2017 TABLE I SUMMARY OF THE DMA RESULTS OF NCFs Fig. 9. Moisture absorption ratios of various NCFs. Fig. 11. test. Adhesion strength of various NCFs on PCB substrates after a PCT 5wt%) were cured at 210 C for 10 min using a vacuum laminator and baked at 150 C for 24 h for full cure before measuring the moisture weight gain. The moisture weight gain of the NCFs was calculated using the electronic balance equipment. After all the weights of NCFs before the PCT were measured, they were placed in a PCT chamber to perform the PCT test at 121 C, 100% RH, and 2 atm. The weights of the NCFs were measured after 0, 2, 4, 8, 12, 24, 48, 72, and 120 h to observe the amount of moisture absorbed depending on the SCA content. Fig. 10. Adhesion strength depending on the added amount of SCA. rate of 50 μm/s, and then a load displacement curve can be obtained. By considering the dimensions and flexural modulus of the PCB beam, the adhesion energy can be calculated as a function of crack length and critical load, based on the fracture mechanics model [18], [19]. The multiple values of the adhesion energy can be probed in a specimen by repeating loading, crack growth, and unloading cycles. E. Moisture Absorption The moisture uptake rate is the key parameter when considering NCFs adhesion strength since the adhesion strength of the NCFs decreased when vapor pressure increased. Three types of NCFs with various SCA content (0wt%, 1wt%, and F. Reliability Evaluation Three NCFs with various SCA content (0wt%, 1wt%, and 5wt%) were used to investigate the NCF adhesion strength effects on the microbump joint reliability of the COB assembly. A Si chip having dimension mm 2 size was bonded on mm 2 PCB using thermal compression bonding after laminating the B-stage preapplied NCFs on a Si chip surface. The thermal compression bonding was performed at 250 C for 30 s with a heating rate of 6 C/s. After the thermal compression bonding, the PCT test was performed at 121 C, 100% RH, and 2 atm up to 216 h, and the contact resistances of the microbump joints were measured using a four-point probe to verify microbump reliability.

5 KIM et al.: MOISTURE EFFECTS ON NCF ADHESION AND SOLDER JOINT RELIABILITY OF COB ASSEMBLY 375 Fig. 12. (c) NCF. Type of fracture mode after a die shear test of a Si chip bonded on a solder-resist applied PCB using NCFs. (a) Si, NCF, and SR. (b) Si and NCF. III. RESULTS AND DISCUSSION A. Evaluation of Mechanical Properties of NCFs The mechanical properties of the cured NCFs were determined by measuring the changes of modulus and loss tangent (tan δ) of the three NCFs depending on the SCA content before and after 72 h of the PCT reliability test at 121 C, 100% RH, and 2 atm. Fig. 8 shows the modulus and tan δ results of three NCFs. The modulus of all NCFs decreased compared to as-cured NCFs. Especially at high temperature, the degradation of the mechanical properties of NCFs was presumably due to moisture absorption, which acts as a plasticizer [20]. The modulus slightly increased by adding SCA into NCFs. The glass temperature (T g ) was defined at the maximum peak of tan δ. T g also increased by adding SCA. This is presumably because the organofunctional groups of SCA participated in the NCF resin curing process. As a result, the cross-linking density increased by the added SCA. The modulus and T g data results were measured by the DMA and summarized in the Table I. B. Moisture Uptake Ratio The moisture uptake ratio of epoxy resin is a critical factor because the moisture can be easily absorbed in NCFs and diffuse at the interface between the NCFs and the PCB substrates. Absorbed moisture can increase the vapor pressure at the interface between the NCFs and the PCB substrates resulting in weakening of interfacial adhesion. Thus, the moisture uptake ratio of NCFs and the diffusivity at the interface between the NCFs and the PCB substrates are important material properties [12], [21], [22]. The moisture uptake ratio is defined as the difference in weight before and after the humidity test. All NCFs were exposed at 121 C, 100% RH, and 2 atm in the PCT chamber for 120 h. The moisture absorption of NCFs decreased by the added SCA, as shown in Fig. 9. Although the moisture uptake ratio of the three types of NCFs was different, all NCFs reached 90% absorption ratio of moisture at an early stage. This corresponds to the general moisture absorption mechanism [23]. The moisture uptake ratio was different depending on the amount of SCA added in NCFs because the organofunctional group of SCA was involved in the epoxy resin curing system. The moisture uptake ratio was 4.83% for the unmodified NCF, 4.15% for the NCF with 1wt% SCA, and 3.53% for the NCF with 5wt% SCA, respectively. By increasing the SCA content, the cross-linking density of the NCFs increases because the moisture absorption site such as nanovoid/microvoid or free volume decreases. Eventually, Fig. 13. Failure modes of a die shear test after a humidity test. (a) Fracture mode of the unmodified NCF. (b) Fracture mode of the 1wt% SCA added NCF. (c) Fracture mode of the 5wt% SCA added NCF. the mechanical property of NCFs can be improved by adding SCA. C. Evaluation of the Adhesion Strength of NCFs Fig. 10 shows the adhesion strength results of the NCFs measured by a die shear test. As the SCA contents in NCFs

6 376 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 7, NO. 3, MARCH 2017 Fig. 14. DCB specimens before and after a humidity test. (a) After as-bonded. (b) After 48 h of humidity test. (c) After 120 h of humidity test. to the adhesion strength before the humidity test, as shown in Fig. 11. However, the loss of adhesion strength was less significant for 1wt% SCA and 5wt% SCA NCFs compared to the same for the unmodified NCF. The failure sites after the die shear test were classified into five categories depending on the remaining component on the failure surface, as shown in Fig. 12. The initial fracture surface mode after the die shear test showed the remaining three layers of Si chip, NCF, and solder resist, as shown in Fig. 13(a). The fracture mode of the SCA-added NCFs shows that the NCF portion decreases at the failure site, as the SCA content increases, because the adhesion strength between the NCFs and the solder resists increased. The adhesion strength after die shear test showed a dramatic change after a humidity test. In the case of unmodified NCF, the adhesion strength between the SR and the NCF disappears after 12 h of PCT, as shown in Fig. 13(a). Here, the fracture site was the interface between the NCF and the SR. However, as the added SCA in NCFs increased, the adhesion between the SR and the NCF was somewhat maintained even after the humidity test, as shown in Fig. 13(b) and (c). The fracture surface of the NCF with 1wt% SCA was the mixed mode of Fig. 12(a) and (d) up to 24 h of humidity test. After 24 h of humidity test, the fracture surface was the mode of Fig. 12(d). On the other hand, the NCF with 5wt% SCA showed the modes of Fig. 12(a) and (d) for the fracture surface up to 120 h of humidity test. The remaining Si chip at the fracture surface implies that the adhesion strength was significantly enhanced at the interface between the NCFs and the SRs by adding SCA. The decrease in adhesion strength at the interface was presumably due to the moisture absorption effect in the NCFs. The absorbed moisture was squeezed between the OH sites at interface between the NCF and the SR during a humidity test. Therefore, the absorbed moisture prevents the chemical bonding between the NCFs and SRs through the hydrolysis reaction. By adding epoxy SCA into NCFs, this allows the hydrozable group in the SCA to form a hydrogen bonding with the substrate surface. Eventually, this strengthens the chemical bonding at the interface between NCFs and SR as well as reduces the OH sites on the solder-resist surface that can absorb moisture during the humidity test. Fig. 15. Measured adhesion energy of NCFs on the SR during the PCT test using the DCB test. increase, the adhesion strength increases slightly. This is presumably because the hydrolyzable group in the SCA reacts with the OH sites on the organic substrate surface. Also when observing the die shear test results of NCFs in the humidity test at 121 C, 100% RH, and 2 atm, all the three NCFs showed a decrease in adhesion strength value compared D. Evaluation of the Adhesion Energy of NCFs The adhesion energy between NCFs and SRs was measured using the DCB test while the adhesion strength was measured using the die shear test. The adhesion energy represents the required energy to fracture the interface; therefore, the characterization of the adhesion energy is also critical, particularly at the impact loading condition. Fig. 14 shows the interface of DCB specimens after they are as-bonded in the DCB test and after 48 and 120 h of humidity test. As shown in Fig. 15, the SCA successfully enhanced the NCF adhesion energy, and the energy was decreased as the PCT time increased. The trends are similar to the adhesion strength data using the die shear test. However, the adhesion energy values of NCF with 1wt% SCA were slightly higher than those of NCF with 5wt% SCA, presumably due to the

7 KIM et al.: MOISTURE EFFECTS ON NCF ADHESION AND SOLDER JOINT RELIABILITY OF COB ASSEMBLY Fig Microbump joint images of (a) unmodified NCF, (b) 1wt% SCA added NCF, and (c) 5wt% SCA added NCF after bonding. excellent bump joint stability. The contact resistance results of three different NCFs obviously showed a significant SCA effect on the bump joint reliability after the PCT test. Fig. 18 shows the microbump joint failure mode of the COB assemblies after the PCT test. Three different types of failure modes were observed: 1) microsolder bump crack; 2) delamination at the NCFs and the SRs; and 3) opened bump joints. At the early stage of PCT test, microsolder crack occurs as shown in Fig. 18(a). As the adhesion strength between the NCF and SR decreased further due to the moisture absorption at NCFs, delamination occurs at the NCF and SR interface as shown in Fig. 18(b). Microsolder bump joints were completely failed as shown in Fig. 18(c). In the case of the unmodified NCF and the NCF with 1wt% SCA, completely failed solder joints were observed after 264 h of PCT test. On the other hand, the NCF with 5wt% SCA showed no bump joint failure even after 264 h of PCT, which demonstrates the excellent NCF adhesion property causing the stable solder bump joint reliability. Fig. 17. Contact resistances of microbump joints of (a) unmodified, (b) 1wt% SCA, and (c) 5wt% SCA added NCFs after PCT. high storage modulus of the NCF with 5wt% SCA as described in Table I. In general, polymers become brittle at higher modulus. Therefore, too much SCA reduced the ductility of NCFs resulting in the reduced fracture absorbing ability. It is good that silane-contained NCFs have a sufficiently high adhesion energy compared to the unmodified NCF. E. Verification of Silane Coupling Agent on the Microbump Joint Reliability of COB Assembly Fig. 16 shows the microbump joint images bonded by a thermocompression bonding using preapplied NCFs. Three different SCA contents were added in the NCFs for comparison. All microbump joints showed well-defined solder joints. The contact resistance results of microbump joints after the PCT using various silane contents (0wt%, 1wt%, and 5wt%) are shown in Fig. 17. For the unmodified NCF, the contact resistance increased by 20% after 12 h. Some joint regions showed open failure after 120 h and complete open failure after 264 h during a PCT test. For the NCF with 1wt% SCA, the contact resistance increased by 20% after 48 h, and some joint regions show a complete open failure after 264 h during a PCT test. However, for the NCF with 5wt% SCA, the contact resistances increased less than 20% after 72 h, and no bump joints were failed even after 264 h of the PCT test, showing IV. C ONCLUSION In this paper, the mechanical properties, moisture absorption, and adhesion strength of NCFs modified with various amounts of SCA were investigated. In addition, we evaluated the lifetimes of the modified NCFs upon exposure to temperature and humidity. The added SCA significantly affects the NCF mechanical properties such as the adhesion strength of the NCFs and the solder resist interface of PCB substrates. When observing the NCF properties, higher SCA-added NCFs showed higher modulus and Tg as well as less moisture absorption rate during high temperature and high humidity aging presumably due to the higher cross-linking density of SCA-added NCFs. The adhesion strength was significantly improved by adding SCA to the NCFs. SCA allows the OH bond to chemically bond with the solder resist, resulting in an increase of the adhesion strength between NCF and SR. Furthermore, SCA probably reduced the moisture absorption sites on the solder resist surface. The SCA effectively improved the adhesion energy of NCFs. The NCF with 1wt% SCA had higher adhesion energy than the NCT with 5wt% SCA presumably because of the increased storage modulus of the NCF with 5wt% SCA.

8 378 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 7, NO. 3, MARCH 2017 Fig. 18. Failure modes of the microbump joint. (a) Microsolder bump crack. (b) Delamination at NCFs and SR. (c) Opened bump joint failure. The microbump joint reliability evaluation of the unmodified NCF and the NCF modified with two different loadings of SCA shows that NCFs with a higher silane content offer longer lifetime upon exposure to humidity. This confirms excellent improvement in the adhesion property of the NCFs upon the modification of SCA. REFERENCES [1] Z. Zhang and C. P. Wong, Recent advances in flip-chip underfill: Materialsprocess, and reliability, IEEE Trans. Adv. Packag., vol. 27, no. 3, pp , Mar [2] B. Han and Y. Guo, Thermal deformation analysis of various electronic packaging products by Moiré and microscopic Moiré interferometry, J. Electron. Packag., vol. 117, no. 3, pp , [3] D. Suryanrayana, R. Hsiao, T. P. Gall, and J. M. 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Packag., vol. 18, no. 2, pp , Youngsoon Kim, photograph and biography not available at the time of Taeshik Yoon, photograph and biography not available at the time of Tae-Wan Kim, photograph and biography not available at the time of Taek-Soo Kim, photograph and biography not available at the time of Kyung-Wook Paik, photograph and biography not available at the time of