13.1 Introduction

Semiconductor packaging technologies are state of the art for various industries and applications including automotive, machinery, entertainment, communication, sensor, security, authentication, medical and environment systems, and many more. Today, many variations of electronics are available, and indeed the entire world and environment could not function without these in the manner that people have become used to. Recently, fan‐out wafer‐level packaging (FO‐WLP) has attracted attention as a high performance and very cost‐efficient packaging solution [1]. This packaging is highly suitable for wireless devices, most common in mobile and automotive sensor applications [2]. Nagase, as a diversified epoxy material supplier, began active development for these applications in the mid‐2000s, and LMC was successfully implemented into FO‐WLP at the end of 2007.

Epoxy resin has two specific features: excellent adhesion to various surfaces and superb electrical insulation. Therefore, epoxy compounds became a very popular encapsulant for semiconductor packaging some time ago. During its development history, epoxy compound encapsulant has been used in many different forms of packaging in various applications with very specific requirements. Generally, epoxy molding compound (EMC) is a solid‐type resin, which consists of epoxy and phenol chemicals and was originally developed for transfer molding systems. In contrast, liquid‐type epoxy molding compound was developed for several specific applications that inevitably require a liquid state such as unique flowability and/or a dust‐free requirement.

There are various liquid epoxy compound products including LMC for compression molding, capillary underfill (CUF), pre‐applied underfill called NCP (nonconductive paste), and a glob top material that facilitates the dam and fill process for encapsulation and environmental protection of various electronic modules, devices, and other components. Liquid‐type epoxy molding compound (LMC) was developed for the latest applications and process adoption of FO‐WLP where it has become indispensable.

13.2 The Necessity of Liquid Molding Compound for FO‐WLP

The original chip‐first FO‐WLP manufacturing process developed by Infineon Technologies AG using LMC is called extended wafer ball grid array (eWLB), and will be described as shown in Figure 13.1 [1].

1. Temporary adhesive tape is laminated on a metal carrier.
2. Die are mounted with active side face‐down on the temporary adhesive tape.
3. LMC is dispensed over the die on the carrier.
4. LMC is molded using a compression molding system, creating a die embedded resin wafer.
5. The molded die embedded resin wafer is then post mold cured.
6. The molded die embedded resin wafer is separated from the temporary adhesive tape using thermal treatment process.
7. A redistribution layer (RDL) process is created, providing insulation between the copper (Cu) metal routing layers.
8. Solder balls are then mounted on die embedded resin wafer.
9. Finally, package singulation or dicing of die embedded resin wafer into final FO‐WLP is completed.

There are three major advantages of using LMC in this process. Firstly, FO‐WLP manufacturing takes place in a class 1000 clean room environment, which is much stricter than traditional assembly, and since LMC is in a liquid state, it is free of any dust. This kind of dust‐free material can be used in the entire semiconductor manufacturing processes including sensitive clean room environments. In comparison, solid‐state EMC and granule type resin used for traditional flip‐chip ball grid array (fc‐BGA) and wire‐bond ball grid array (wb‐BGA) packages are not dust‐free and thus cannot be used in such clean dedicated areas. Recently, EMC suppliers have been controlling fine particles to minimize the dust in the EMC manufacturing process itself, but it cannot be guaranteed that fine particle dust powder is completely eliminated because during transportation and handling an unavoidable collision of epoxy parts generates fine powder. To minimize this, molding equipment manufacturers developed specific dust preventing measures for their equipment, but it remains a major concern in large‐scale mass production. Once a machine stops or requires adjustment, a technician has to fix it. Thus, it is generally impossible to prevent contamination by fine particles of dust while under repair. This pollution would generate major problems in further processes, contaminating the treated wafers and their devices, thus reducing the production yield dramatically. So, dust and contamination control and dust and contamination‐free materials are indispensable in the semiconductor assembly manufacturing world today.

Secondly, LMC has, due to its specific properties, the ability to cure at lower temperatures in the range of 110–125 °C in comparison with transfer molding systems and solid‐state EMC that typically require a curing temperature of 175 °C. The lower temperature is in general a major advantage for many process‐related issues. It minimizes the thermal stress between the curing temperature and room temperature, which results in lower warpage and better warpage controllability. Another advantage of LMC is the prevention of so‐called flying die. The temporary adhesive tapes’ bonding declines at increased temperatures, and especially smaller die are easily detached from the adhesive, becoming so‐called “flying die.” By keeping the temperature moderate, the die do not move, enhancing the processability for smaller die, which is one of the main merits of FO‐WLP and fan‐out process efficiency.

Thirdly, LMC has the excellent characteristic of high inorganic filler loading in the final compound. High filler loading is one of the key characteristics of LMC performance and very important for warpage control due to minimization of thermal strain on silicon. LMC can have higher filler loading in comparison with solid resins due to its lower viscosity, which provides very good flowability while molding, and this phenomenon effectively prevents flying die. Finally, those LMC‐specific performance characteristics provide larger manufacturing process design flexibility for FO‐WLP and also for the resin formulation capability. Therefore LMC is the preferred choice for FO‐WLP worldwide manufacturing today.

13.3 The Required Parameters of Liquid Molding Compound for FO‐WLP

FO‐WLP packaging demands several specific characteristics from an LMC encapsulation material. One of these is a low coefficient of thermal expansion (CTE), which minimizes the thermal stress difference between the silicon die and the LMC, resulting in less thermomechanical strain on the die, and leads to less warpage. The current target value of CTE below T_g (glass transition temperature) is less than 10 ppm °C⁻¹. A second requirement is to have a well‐balanced viscosity level, which enables excellent handling and flowability during the molding process. LMC has significantly lower viscosity than solid epoxies in the molding process. However, LMC has a high viscosity value for dispensing due to typical characteristics and especially the filler loading (filler content). This has led to the development of a new dispenser technology that enables fluids with high viscosity to be integrated in a fully automatic compression molding machine where the material should not exceed the viscosity value of 1000 Pa s. Filler content is one of the key factors for the two parameters shown in Figure 13.2. On the one hand CTE decreases when the filler content is increased, while on the other hand viscosity increases. Therefore a well‐balanced material in the molding process is a must to enable dispensing with viscosities below the critical value of 1000 Pa s.

Another very important factor is LMC’s excellent flowability during the molding process. The aim to be ever smaller, thinner, and lighter in the semiconductor industry has always been a standard requirement. In some cases, when the thickness of LMC over the die becomes very thin, failure can occur. This failure, known as flow marks, is shown in Figure 13.3. Flow marks are caused in LMC by filler clogging during compression molding as shown in Figure 13.4. In a standard compression molding system using LMC, the LMC will be dispensed in the center of the wafer, and during compression it flows radially toward the wafer edge. When there is a narrow space for LMC between the die and the machine tool, normally one can observe at the edge of the wafer larger filler particles clogging, and a flow mark can be observed.

This flow mark is strongly related to the filler size of the LMC as shown in Figure 13.5. Three kinds of LMCs with different filler size averages have been evaluated regarding their flowability performance to verify the flow mark dependency of the filler size. This exercise shows that 25 μm average filler size can be used if the gap between the die top and mold machine tool, the so‐called mold thickness, is at least 500 μm. If this dimension declines, the flow mark will appear. In the case of a smaller filler average size, a smaller mold thickness can be achieved.

In addition, redistribution layer (RDL) processes require low die‐shift performance of LMC. Die shift is the difference of the die position before and after molding and curing. One main factor generated by LMC material for the die‐shift phenomenon is physical (thermal) and chemical shrinkage shown in Figure 13.6. Generally, the FO‐WLP process preferably uses LMCs with lower thermal and chemical shrinkage related to the filler content and the described CTE parameters. However, die shift is also a result of the configuration of the whole arrangement of the die on the wafer, their positions, the die pitch (distance between the die), and the final resin‐to‐die ratio in the whole wafer (occupation) after molding. The influence of the resin declines with the smaller resin‐to‐die ratio in the whole wafer. In consequence, requirements of LMCs vary depending on the different wafer configurations; thus, lower thermal shrinkage is more advantageous for die‐shift performance than low CTE.

RDL compatibility presents a further design challenge for LMC. Here LMC is preferred because it does not include any release agents like solid EMC in its formulation. Since the LMC molding process uses releasing tape to prevent sticking to the machine parts, while a solid material uses wax to deform from the molding machine die (from the cavities), LMC does not require a release agent. Should the surface quality of the molding material be poor due to the internal release agents, the RDL chemicals during the coating process will shed, and the quality of the process decline drastically.

13.4 Design of LMC Resin Formulation

Generally, a liquid molding compound consists of epoxy resin, hardener, filler, and some additives such as coloring agent, accelerator, adhesion promoter, and further special chemicals as required. All these components and chemicals have their own functions and influence on the performance of the final LMC. FO‐WLP requirements demand the best possible balance of materials and substances so the final LMC fulfills all the requirements shown in Figure 13.7. Acid anhydride derivatives that are suitable as hardeners for LMCs provide low viscosity and low temperature fast curing as well as maintaining thermal performance such as T_g. Recently, some derivatives of acid anhydride have already been listed by the European Chemicals Agency (ECHA) as substances of very high concern (SVHC). As a result of this situation, an LMC was proactively developed with non‐acid anhydride to comply with European regulations. Meanwhile, the Anhydride Joint Industry Taskforce (AJIT) was established by acid anhydride producers, importers, formulators, and end users to re‐enhance the safety precautions concerning the usage of acid anhydride [3]. The FO‐WLP market continues to observe AIJT activities and the future possibility to continue using acid anhydride in the European Union.

Silica is suitable as a filler for LMCs. It influences the CTE and thermal shrinkage properties. As explained above, high filler loading leads to lower CTE but can also generate higher viscosity of LMCs, and a balance of those is very important for FO‐WLP application. Therefore selection of fillers is a key factor to help provide high filler loading with appropriate viscosity values. The main mechanism of viscosity changes is the friction between filler and the liquid component of the LMC [4]. High filler loading has a large friction area and generates higher viscosity. The method to minimize this influence is to use low friction fillers. The suitable filler has a spherical shape, smooth surface, specific size distribution, and surface compatibility to liquid components. Furthermore, the control of filler size and the filler loading in LMC is very important for FO‐WLP technology. The fine filler technology in LMC is suitable for thinner molding, fine‐pitch die‐to‐die distance where LMC is required to fill in‐between die, and better accuracy of through‐mold via (TMV). TMV is a laser‐drilled channel in the molding compound to enable further connection in multidimensional scale, e.g. 3D packaging. In contrast, fine filler loading causes higher viscosity because it is increasing surface area in LMC. Therefore, next LMC generation has to overcome this obstacle between filler size and filler loading for future FO‐WLP.

Recently, functional fillers such as thermal conductors and electromagnetic shielding are coming into focus as well. Beside these components, LMC includes more additives such as carbon black that is important for laser marking on the final package, adhesion promoters to enhance adhesion, stress release agents to improve warpage behavior, and others depending on the requested functionality modification.

13.5 Development of LMC in Connection with Latest Requirements

The current packaging trends are mainly large molding areas, fine‐pitch lines and space (L/S), and complex packages like system in package (SiP). To be able to follow the technology, these trends require very challenging LMC development. There is fine filler loading LMC development to fulfill the thinner molding and fine TMV performance to fill into narrow spaces between the die, but at the same time, it has the obstacle of increased viscosity due to the friction increase caused by the decreased filler size, resulting in a dramatic surface area increase. Therefore, developing new fillers that have special spherical shapes, smooth surfaces, and a specific size distribution to minimize the roll resistance and provide dedicated surface compatibility to liquid components is imperative [4].

13.6 Current LMC Representative Proprieties

Three types of standard LMC’s with different filler cuts (maximal filler size) are shown in Table 13.1. These are produced by Nagase ChemteX Corporation for FO‐WLP. All these materials have high filler loading above 85% of volume and maintain the dispensability requirement of a viscosity below 1000 Pa s. Nagase ChemteX has developed, launched, and released several kinds of LMCs into the semiconductor market for more than 10 years where the main application is the FO‐WLP. Using this experience and knowledge, Nagase ChemteX continually works on further development and new LMC materials to support the semiconductor market with its fast‐moving trends and technologies and challenging environmental requirements.

13.7 Conclusions

FO‐WLP requires LMC and has been using it now for a decade. At the beginning there were many different issues that required time to be solved [1, 2, 4]. Today, the LMC is well adjusted for current standard FO‐WLP, but new applications and functions arise, and the requirements for the LMC are changing with these new developments. For further progress in the semiconductor electronic world, there is a need for design and development cooperation between LMC formulators, equipment, and packaging designers.

Acknowledgment

Authors would like to thank Nagase ChemteX Semiconductor Packaging team, Nagase High Performance Material Section, and our partners for their excellent support.

References

1. 1 Brunnbauer, M., Fürgut, E., Beer, G., and Meyer, T. (2006). Embedded wafer level ball grid array (eWLB). Proceedings of 8th Electronic Packaging Technology Conference (2006).
2. 2 Obori, T., Kan, K., Nishikawa, Y. (2009). Development of liquid molding compound for wafer level package. Microelectronics Symposium, Japan (2009).
3. 3 Anhydride Joint Industry Taskforce (AJIT). http://anhydrides.eu (accessed 6 August 2018).
4. 4 Kan, K. (2016). The novel liquid molding compound for FAN‐OUT wafer level package. IWLPC (2016).