Compression molding encapsulants for wafer-level embedded active devices

Transcription

1 2017 IEEE 67th Electronic Components and Technology Conference Compression molding encapsulants for wafer-level embedded active devices Wafer warpage control by epoxy molding compounds Kihyeok Kwon, Yoonman Lee, Junghwa Kim, Joo Young Chung, Kyunghag Jung, Yong-Yeop Park, Donghwan Lee, Sang Kyun Kim Development Group, Semiconductor Material Business Team, Samsung SDI Co., Ltd 130, Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-Do, Korea, Abstract Fan-out wafer level packaging technology (FOWLP) is one of the advanced packages in semiconductor industry. Recently, its interest has been raised due to the benefits such as thin package, board fan-out capability, high I/O, substrate-less process, integration of passives into structure, good thermal resistance, and electrical performance. Despite these many advantages, FOWLP is reluctant to be applied packaging industries due to the difficulty of controlling wafer warpage. Its broad molding area causes warpage control of FOWLP to be more difficult than other packages. The relationship between molding area and warpage have trade-offs, which have to be optimized beforehand in order to ensure successful subsequent processing. In order to control both factors, it is necessary to study the properties of Young s modulus, coefficient of thermal expansion (CTE), glass transition temperature (T g ), and flow-ability for wafer level mold compounds of interest. In this paper, our group introduces an evaluation tool based on all above parameters and explains how to optimize warpage of FOWLP based on this tool. Challenges that FOWLP packaging technology is confronted with include wafer warpage [4-6], die shift/protrusion [7], and board level reliability [8]. Among these challenges, wafer warpage is a crucial one to be resolved for successful subsequent wafer processing. There are many efforts to resolve this warpage issue in FOWLP, but few effective tools have been reported to date [4-5]. There are two types of wafer level molding materials in FOWLP, liquid and powder (Fig. 1). The properties of each material must be understood in order to optimize FOWLP. Liquid materials have been successfully used in manufacturing since 2016, but their disadvantages such as low storage stability, high material cost, and complicated dispensing patterns have led to a new demand for powder type materials. These powder type materials show relatively better storage stability, lower cost, and simple dispensing patterns, which are more desirable for FLOWP [9]. Keywords - component; fan-out wafer level package, compression molding encapsulant, wafer warpage, young s modulus, coefficient of thermal expansion, glass transition temperature I. INTRODUCTION Semiconductor packaging technologies have been advanced toward a variety of applications such as entertainment, sensor, communication, authentication, security, and environment system for mobile, portable devices, and automotive. These highly functional electronic devices have been leading the packaging industry toward more specific and condensed methods such as 3D integrated circuit (IC) packaging with through silicon-via (TSV), 3D and 2.5D system in package (SiP), 2.5D with TSV interposer (TSI), package on package (PoP), wafer level chip scale packaging (WLCSP), fan-out wafer level packaging (FOWLP), and so on [1-2]. Among the many advanced packaging technologies, FOWLP technology draws attention because of its advantages, such as high integration capability, small form factor, high I/O density, high performance, and cost effectiveness over a wide range of applications [3]. Additionally, successful mass production of an application processor (AP) in 2016 showed a huge market potential. However, there are certain challenges to be resolved. Figure 1. Epoxy molding compound types for FOWLP In this study, we will discuss wafer warpage control using wafer level compression molding materials, whereby a finite element technique is used to simulate the molded wafer and experimental technique based on the properties of the compression encapsulants. II. RESULTS AND DISSCUSION A. Simulation results A statistic structural simulation tool was constructed based on a 12 inch diameter disc ( m), which consisted of a bilayer that included a top mold with clearance of 320 μm and silicon die of 700 μm. The aspect ratio (R) of the thickness of die over the total was Seven different epoxy molding compounds which are commercially produced by Samsung SDI were prepared according to the following properties: Young s modulus, CTE1, CTE2, and T g as shown in Table 1. Standardized samples were fabricated with each compound, and their /17 $ IEEE DOI /ECTC

2 wafer warpages were measured. These results are summarized in Fig. 2. Then, a statistic simulation tool was derived upon those properties by finite element modeling analysis (FEA) using ABAQUS The results show a smile shape warpage, which indicates higher shrinkage of a compression molding material over that of the silicon wafer. TABLE I. MATERIAL PROPERTIES USED IN FEA MODELING Material Young s modulus (GPa) CTE1 (ppm/ CTE2 (ppm/ T g ( EMC A EMC B EMC C EMC D EMC E EMC F EMC G B. Screening epoxy resins and epoxy resin systems After screening the standard molding compound formulations to determine our model, we then screened a series of alternative epoxy resins with successively decreasing Young s modulus. In addition, T g was also varied. The Young s module and T g values for each epoxy formulation are shown in Fig. 3. They were measured using samples molded at 135 C for 600 sec. Relatively, the epoxy resins had low Young s modulus with a suitable T g (Resin7, 8, 9, and 10). Additionally, we did further screenings by combining different resins to further modify their final properties. The combination of and resulted in the lowest Young s modulus (Fig. 4). Especially, gave the biggest drop in Young s modulus and CTE, as well as the biggest elevation of T g. The material property changes from are expected to prevent incomplete fills during a molding process by increasing their flow-ability. The incomplete fills is another major component to be controlled in addition to the wafer warpage in order to commercialize FOWLP. As can be seen in Fig. 5 and Fig. 6, most epoxy resin systems meet their minimum flow-ability except the Rein1 + Resin3. With the consideration of modulus, CTE, T g, as well as flow-ability, the combinations of and were the best performing formulations. Figure 2. Simulated wafer warpage at different EMC materials Various parameters influencing wafer warpage were screened by the simulated calculation. Among all these parameters, Young s modulus, CTE, and T g have a significant effect on the controlling warpage. Generally, wafer warpage is reduced by lowering the Young s modulus and CTE, and increasing the T g. The low CTE of EMC C was the main contributing factor that lowered wafer warpage by minimizing the CTE mismatch between the molding compound and silicon wafer. The low modulus of EMC D was found to reduce wafer warpage by lowering stress relaxation. In general, wafer warpage can be decreased by increasing the T g of the molding compound because the CTE of epoxy molding compound decreases with increasing T g. Based on these results, in order to reduce wafer warpage after the molding process, the molding compound materials should have low Young s modulus, low CTE, and high T g. Figure 3. Screening epoxy resin s properties. Figure 4. Screening epoxy resin system s properties 320

3 lengths. Results of each experimental group are shown in Fig. 7. Note the labeling nomenclature: A, B, and C represent polymer functional group. 1, 2, and 3 represent different chain length. (1) Long Chain, (2) Medium Chain, (3) Short Chain. Figure 5. EMC s spiral flow using epoxy resin systems As shown in Fig. 7, additive A with medium and short chain length (A2 and A3) and additive B with medium and short chain (B2 and B3) were found to be the most highly efficient additives to control Young s modulus. Further material property investigation also revealed that these additive and chain length combinations also provided the most optimum CTE 1, as shown in Fig. 8. In addition, these two functional additives with medium and short chain lengths showed a very good balance of reducing both modulus and CTE while maintaining suitable flow-ability. This behavior is captured in the spiral flow measurement results shown in Fig. 9. From these results, it was envisioned that different functional groups and chain lengths play an important role in the epoxy molding compound s properties after the epoxy resin-phenol hardener curing reaction, and these additives should be the key raw materials modulated to tune final warpage of compression molding compounds. Figure 7. Screening additives with different functional group that have different chain length Figure 6. a) Optical image of incomplete filling molded wafer, b) Optical image of complete filling molded wafer C. Screening low modulus/low CTE additives Next, we investigated the effective additives, which can control Young s modulus and CTE for compression molding compounds. A total of 9 different additives were screened while varying polymer functional groups and chain Figure 8. Young s modulus and CTE1 variation depending on additive 321

4 Figure 7. Spiral flow results of EMC with additives D. Evaluation of wafer warpage performance with newly developed EMC The current trend of wafer molding is that wafer size and thickness is becoming larger and thinner. The size of WLP now varies from 6 inch to 12 inch and now even larger than 12 inch for WLP is available (i.e. large panels). Here, controlling wafer warpage is one of the most significant subjects to develop large size of FOWLP [10]. Since package design is changed frequently based on market requests, in the real world, evaluating each of these packages individually is very difficult and expensive. Therefore, a simple evaluation method is needed to predict the wafer warpage conveniently. We propose to use a bilayer test structure with silicon wafer and epoxy molding compound as a standardized evaluation vehicle. Each layer is set at 300 µm thickness. To further standardize, the molding conditions can be fixed at 135 x 600 sec with a post mold cure (PMC) of 150 x 2 hrs. By standardizing the test vehicle and processing conditions, we can then be certain that the warpage behavior between mold compounds can be directly compared, and any differences are solely caused by the EMC. Based on this tool, we prepared several specimens using different Young s modulus, CTE, and T g properties, which were easily manipulated by epoxy resin systems and additives. As shown by the data in Table 2, low Young s modulus, low CTE, and high T g resulted in the lowest wafer warpage. We set the experiment by using two different resin systems ( resin10 and Resin 10) and four different additives (A2, A3, B2, and B3) to screen the EMC for FOWLP, with warpage being the primary metric. The EMC behaviors after PMC were characterized by shadow moire, and the results are summarized in Table 2 and Fig. 11. Among all the options, the best result in terms of wafer warpage was the condition which used and A2 additive, which has the lowest Young s modulus, CTE, and highest T g values. Experimental results were closely matched with the simulation data. TABLE II. PROPERTIES OF EPOXY MOLDING COMPOUNDS WITH LOW MODULUS/CTE MATERIALS Ref. EMC1 EMC2 EMC3 EMC4 EMC5 EMC6 EMC7 EMC8 Epoxy resin system Resin4 Additive - A2 A3 B2 B3 A2 A3 B2 B3 Young s modulus (GPa) CTE1 (ppm/ T g ( ) Warpage (μm) Figure 10. Optical image of wafer warpage for reference and EMC1 322

5 Figure 11. Wafer warpage for each EMC detected by Shadow moire III. CONCLUSION Fan-out wafer level packaging (FOWLP) is a relatively new packaging method that has great advantages and potential for success in mass production. However, its use is limited due to the difficulty of warpage control, which is necessary to achieve success in subsequent process steps. Therefore, we developed a tool that provides a standardized way to express and predict the warpage of different mold compounds. In this study, we developed novel compression epoxy molding compounds by varying resin and additive compositions, which can easily control the wafer warpage with suitable flow-ability. Although concurrent optimization of Young s modulus, CTE, and T g of a mold compound s properties is very difficult because of tradeoffs for modifying each component, we developed new compression molding compounds with both low Young s modulus and CTE, with relatively high T g for use in specialized FOWLP. We anticipate that the application of these novel compression molding compounds can be extended to a wide range of applications for successful fan-out wafer level package implementation. ACKNOWLEGNTMENTS The authors would like to thank Joongkeun Kwag, Hankyu Park, and Hoyun Kim for analysis of wafer warpage, sample preparation, and experiment set-up. REFERENCES 1. C. F. Tseng, C. S. Liu, C. H. Wu, D. Yu, InFO (Wafer Level Integrated Fan-Out) Technology, Proc. IEEE Electronic Components and Technology Conference (ECTC 16), IEEE press, Aug. 2016, pp. 1-6, doi: /ECTC C. T. Wang, D. Yu, Signal and Power Integrity Analysis on Integrated Fan-Out PoP (InFO_PoP) Technology for Next Generation Mobile Application, Proc. IEEE Electronic Components and Technology Conference (ECTC 16), IEEE press, Aug. 2016, pp , doi: /ECTC M. Brunnbauer, T. Meyer, G. Ofner, K. Mueller, R. Hagen, Embedded Wafer Level Ball Grid Array (ewlb), Proc. International Electronics Manufacturing Technology Conference (EPTC 08), IEEE press, Jan. 2009, pp , doi: /EPTC H. W. Liu, Y. W. Liu, J. Ji, J. Liao, A. Chen, Y. H. Chen, N. Kao, Y. C. Lai, Warpage Characterization of Panel Fan-out (P-FO) Package, Proc. IEEE Electronic Components and Technology Conference (ECTC 14), IEEE press, Sept., 2014, pp , doi: /ECTC

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