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20 DESIGN007 MAGAZINE I JANUARY 2024 cloth impregnated with a resin bonding agent furnished to the circuit board fabricator as a partially cured dielectric sheet (referred to as B-stage or bonding ply) material. In prepara- tion for joining the circuit board layers, the B-stage prepreg material is placed between the rigid laminate materials or as shown in Figure 1, between the copper-clad laminate base and the succeeding copper foil layers. e process for joining the multi-layer circuit uses special- ized lamination systems employing both heat and pressure. FR-4 Glass/Epoxy Laminate ($) is family of glass-reinforced material developed for printed circuit fabrication has been (and will continue to be) the workhorse of interconnection technology. e materi- als are widely available, very versatile, and tailored to accommodate different product functions or applications: hand-held, portable electronic products to large multilayer system- level boards. e dielectric is defined as a high- pressure thermoset plastic laminate that exhib- its, in a panel form, a robust strength-to-weight ratio and performs well in most environmental conditions. • Glass transition temp (Tg): 150–200°C • Surface resistivity: 10 4 MΩ minimum • Dielectric breakdown: 40kV minimum ese attributes, along with good fabrica- tion characteristics, make it a secure choice for a variety of electrical and mechanical applica- tions, retaining its high mechanical values and electrical insulating qualities in both dry and humid conditions. Although all epoxy/glass laminates have similar physical attributes, the products that adopt these materials oen have very different manufacturing focus and perfor- mance requirements. Note: e acronym "FR" stands for flame retardant, denoting that the material com- prises woven fiberglass cloth with an epoxy resin binder. e material complies with the Underwriters Laboratory Standard, UL94V- 0, defined as glass fiber/epoxy composite with copper foil laminated on one or both sides. Bismaleimide/Triazine (BT)/ Epoxy Laminate ($$) BT laminate is one of the preferred materials for products developed to operate in the more physically challenging environments and has proven ideal for semiconductor package sub- strate applications. e company that devel- oped the material reports that, when compared to standard epoxy resin systems, the blending of bismaleimide/triazine with epoxy provides: • Enhanced thermal and mechanical stability • Improved electrical performance • Furnishes a higher Tg (180°C) • Lower coefficient of thermal expansion • Improved electrical insulation in high humidity and temperature. Polyimide (PI)/Glass Laminate ($$$) Polyimide-based laminates are comprised of a high-strength and high-temperature polymer material system engineered with an all-poly- imide resin chemistry suitable for any elec- tronic package application requiring uncom- promised performance. ermal stability of the polyimide composition and glass fiber Figure 1: Four-layer printed circuit board stack-up.