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Status of the Polymeric Dielectric Materials for Wafer Level Packaging
Keywords: Dielectric distribution RDL, Polymide (PI), Polybenzoxazoles (PBO), Benzocyclobutenes (BCB), Packaging
A wide range of polymeric material is nowadays available on the market and is widely used in the Advanced Packaging market. These materials are used as stress buffer, passivation layer for bumping,dielectric material for redistribution layer (RDL) such as Polyimides (PI), Polybenzoxozoles (PBO), WPR as well as benzocyclobutene (BCB). Dielectric materials applied for RDL in Wafer Level Packaging (WLP) designs require in general high technical specifications and need to exhibit the following properties: low dielectric constant, low cure temperature, high reliability, high mechanical properties andchemical stability. Today, PI and BCB are the main dielectric materials applied in High Volume Manufacturing for the redistribution layers to all WLP applications. Indeed, in 2014, PI and BCB still accounted for 70% of the total dielectric material market. On one hand, PI is the key polymer material used for WLP and flip chip wafer bumping applications since its exhibits high thermal properties, high chemical resistance and process window is much higher than PBO which make this material suitable for bumping processes. Due to its advantages, there has been significant consolidation in the PI industry in the past 15 years. For instance, Hitachi and Dupont merged to form HD Microsystems. In addition, Fujifilm acquired Arch Chem, which has acquired the Olin/Ciba Geigy PI business. Toray also provides PI materials for the packaging market. On the other side, BCB was popular 5 – 10 years ago and is mainly supported by WLCSP today. The dielectric materials market for Advanced Packaging is dominated by HD Microsystems with more than 45% of the total dielectric passivation polymers business following by Fujifilm and DOW Chemical . However, advanced RDL dielectric require new and aggressive specifications by the development of alternative or next generation dielectric material that cure at lower temperature and at faster cure time. This will allow respectively to reduce wafer warpage issues especially for wafer thinning down to 50 µm and enhance the process throughput. Indeed, standard PI has a temperature limitation due to its high curing temperature (usually ~350°C) which is not suitable for 12 inch wafer where warpage can be created (especially for Fan Out WLP). Moreover, despite WPR offers low cure temperature down to 200 °C, it is being replaced for FO WLP because of its better chemical resistance and thermal stability issues. The industry is looking for an alternative passivation material that exhibits lower stress properties than WPR. Some of the main OSATs have decided to switch from WPR to PI as passivation layer as dielectric redistribution for FO WLP due to the challenges that need to be addressed in terms of cure temperature and reliability. Moreover, due to the higher cost of BCB, the industry might look for alternative dielectric solutions especially for high volume such as mobile applications. Although PI and BCB are the most mature dielectric material available on the WLP applications, next generation of PBO material will also contribute to market growth in coming years with clear advantages such as a lower cure temperature (250°C), cost and wafer warpage. In addition, improved resolution with PBO compared to PI has been reported which allows to reduce bump size and increase bump density. Some companies such as HD Microsystems are currently working on PBO in low-temperature cure applications dedicated to FO WLP and TSV packaging with low thermal budget We also expect that main fab-less companies will certainly adopt either PI or PBO for WLCSP applications. There is a trade-off for selecting the right dielectric passivation between warpage, good chemical resistance and good adhesion. Key technical trends, requirements and challenges regarding the dielectric material for all WLP platforms applications will be analysed. In addition, more insights on the current and emerging dielectric materials for Advanced Packaging will be included, as well as market forecast, competitive landscape of the major materials suppliers addressing this field.
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