|Recent environmental concerns over use of brominated flame retardants (BFRs) have increased interest in halogen-free electronics. However, BFRs are not the only source of halogens in PCBs. Most PCB resins are epoxies and epoxy resins contain measurable levels of chlorine. Additional halogens are added to PCB laminates through glass sizes, wetting agents, curing agents and resin accelerators. It is important to realize that even without use of BFRs, there may be finite levels of halogens present as impurities in your product.
Non-halogenated flame retardants are becoming increasingly popular to replace tetrabromobisphenol A (TBBA) in printed circuit boards (PCBs) due to environmental pressures in the marketplace. While the PCB industry recognizes that one-to-one replacement of TBBA is unlikely, there is growing recognition that mixtures based on non-halogenated alternatives can perform synergistically to provide the cost and performance characteristics necessary for commercial success. By targeting market segments in the mid to high glass transition (Tg), it is now possible to design robust formulations that take into consideration the resin and total flame retardant package to optimize performance and eliminate or reduce TBBA from the system. Non-halogenated blends based on melamine polyphosphate (Ciba® MELAPUR® 200) with inorganic and organic phosphates, metal phosphinates, phosphonates and metal hydrates were evaluated in medium and high Tg epoxy resin systems based on bisphenol A, epichlorohydrin/phenol novolac and benzoxazine type of chemistries. Multi-layer laminates with copper foil were prepared to evaluate flame retardancy, thermal stability, and electrical performance. Synergistic flame retardant effects are shown in several systems herein, to provide UL94 V-0 performance with self-extinguishing properties while maintaining low loading levels of the individual non-halogenated additives (10-15%). Consequently, the lower load levels of the specific additives can lead to improved performance in glass transition temperature and electrical properties (dielectric constant DK, and loss factor Df. The synergistic effects and overall performance is highly influenced by the type of resin. Therefore, PCB designers can adopt a systems approach to achieve the desired performance profile and move to more cost effective combinations of various non-halogenated systems.
Tetrabromobisphenol A (TBBA) is the most popular flame retardant in epoxy printed circuit boards, representing about 95% of the flame retardants consumed in FR4 applications in 2005. TBBA is also under increasing environmental and market pressure because it is the largest volume brominated flame retardant in production today with more than 300 million pounds manufactured annually. While public debate continues over the risk that TBBA poses, market perception is a key driver toward halogen-free printed circuit boards. Many electronic OEMs are therefore aggressively publicizing their intent to become "halogen-free" in the near future. However, there is also a perception that halogen-free systems are inferior to TBBA performance. This perception was the result of attempts to reformulate the epoxy boards in a one-to-one fashion with a halogen-free replacement. Due to the different mechanisms involved, halogen-free materials are often most effective in synergistic combinations. Synergistic blends have the potential to both lower the total additive content and to substantially lower individual additive load levels. This in turn improves the odds that physical and mechanical properties of the board are not negatively impacted by a particular flame retardant. Another consideration is that halogen-free flame retardants often interact with the resin to function, so FR performance might vary somewhat for the same additives in different epoxy systems. This can allow formulators to design optimized boards depending on the total systems approach. The easiest way is to segment the board materials based on glass transition temperature (Tg) of the epoxy resin. Halogen-free systems work best in the medium to high Tg systems. These epoxies are already somewhat inherently flame retardant, and the potential synergies of different FR product combinations can become even more pronounced. The increasing effectiveness of halogen-free FR combinations also coincides with the trend toward higher thermal stability of the boards due to miniaturization.
High growth rates of medium and high Tg resins for thermal stability reasons and the increasing market pressures against TBBA could produce a proliferation of halogen-free circuit boards in the near future. The key is to find the best synergistic combinations of halogen-free flame retardants that meet the overall property requirements of the particular epoxy resin. In this study, we evaluated a melamine polyphosphate as the key building block to form synergistic blends with other nonhalogenated flame retardant components. Performance is discussed in terms of flame retardancy, thermal stability, and electrical properties in the target resin segments (medium to high Tg).
Based on the targeted market segments, it is now possible to design robust formulations that take into consideration both the resin and total flame retardant package and offer a non-halogenated alternative to TBBA. In this approach, MELAPUR 200 can be an efficient and versatile building block to create strong synergies with other non-halogen flame retardants. These synergies are shown to provide excellent UL94 V-0 performance at low levels of individual additives and also maintain the critical properties such as glass transition temperature, dielectric constant and loss factor. The synergistic effects and overall performance are highly influenced by the resin type. Thus, PCB designers will require a systems approach to achieve the desired performance profile.
(Reference: Nikolas Kaprinidis and Sabine Fuchs, Ciba Corporation)