Novel Silicone Hotmelt Solutions for Electronic Components

Abstract

Silicone materials are well recognized for their excellent photo/thermal stability, owing to which they are often used as adhesives or encapsulants in electronics applications. However, silicone materials can also present a challenge due to their high Coefficient of Thermal Expansion (CTE) which can generate thermal stress because of CTE mismatching with the substrate. While a “soft” silicone compensates for CTE mismatch through deformation during thermal stress in a device setting, this mismatch limits the use of “hard” silicone products in the market (this limitation was the first market need we addressed in our study). On the other hand, readily available silicone adhesives or encapsulants are mostly curable liquid or paste forms. Organic counterparts such as epoxy or acrylic materials offer “hotmelt” products to cover specific market needs in transfer molding (cylindrical tablet) or large area encapsulation (film/sheet). While the market demand for curable silicone hotmelt products is emerging, only a few such products have been realized thus far (this unmet demand is the second market need we investigated). Our recent study on silicone hotmelt has shown that thermal stress management is feasible even with silicone compositions producing relatively “hard” cured monolith; silicone hotmelt enables an extreme ratio of the raw materials, which is impossible by typical curable liquid compositions. In this presentation, we will introduce novel silicone hotmelt solutions to meet emerging urgent technological requirements such as ease of handling, thermal stability or reliability against thermal stress. The first solution is heat curable silicone hotmelt cylindrical tablet for transfer molding. This aims to achieve similar handling/property to epoxy molding compound (EMC) tablet with superior thermal stability. Silicone hotmelt technology combined with novel compounding technology enabled extreme hardness and CTE of the cured piece; it provides a tensile modulus of 8 GPa and a CTE of as low as 11 ppm/°C, which are comparable to typical EMC. It has been confirmed that molded piece with a PCB board did not show any warpage, indicating matched CTEs in a molded body. Furthermore, the prototype showed no significant degeneration in mechanical and adhesive properties up to 1000-hour exposure to 250°C. In conjunction with optimized melt/flow performance of the prototype, this can be a novel solution for electronics encapsulant applications requiring extreme thermal stability. The second solution is heat curable silicone hotmelt film for large area encapsulation or adhesion. Large area molding is an emerging trend in electronics to achieve higher production output. Utilizing curable liquid products in this application is cumbersome because it requires dam material for precise control of the layer thickness. Furthermore, CTE mismatch between silicone and the substrate becomes a serious issue because of the relatively large thermal stress coming from the large area size. On the other hand, material providing a modulus in GPa range (such as the aforementioned cylindrical tablet material) cannot be applied to flexible devices. To meet these needs, novel silicone hotmelt film prototypes have been developed. Cured monolith from this prototype provides a tensile modulus of 10–100 MPa and a CTE of 220 ppm/°C. While the CTE is high, the designed rheological property enabled excellent stress relaxation capability to minimize the thermal stress in a molded body. No warpage has been observed when molded onto an 8-inch wafer. Controlled modulus of the cured piece bestowed flexibility and low surface tackiness at the same time. The melt/flow performance was optimized for vacuum lamination with excellent gap-filling capability. Furthermore, this prototype exhibited excellent adhesion capability to various substrates including fluoro-based materials and precious metals such as gold. Unique features realized by this silicone hotmelt film are expected to meet emerging market needs in materials for large area encapsulation or adhesion. General trends in electronics include miniaturization of devices and simplification of the production process. As devices become smaller and thinner, encapsulant or adhesive is likely to be exposed to harsher conditions, i.e. higher temperature or stronger light. In light of these trends, silicone is becoming more and more suitable, but current usage of conventional silicone products is limited because of its low modulus and product form (liquid or paste). The novel technologies described in this presentation will break some of these barriers and open a door to utilize silicone in applications where it has never been used before.