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30 DESIGN007 MAGAZINE I FEBRUARY 2019 The transition (melting) temperature of these films must be considered to assure the right material match to the application. Poly- ethylene has the lowest melt temperature from approximately 190–250°F (88–121°C), depending on the density of the resin and degree of molecular cross-linking. CTFE is about 380°F (193°C), which is prohibitive for PCBs that will see manufacturing processes with higher temperatures such as hot air sol - der leveling (HASL). FEP has a transition point of 500°F (260°C) and is capable of handling HASL temperatures. PTFE has the highest transition temperature above 630°F (332°C), so it will survive subsequent high-temperature processes. The primary advantage of using thermo- plastic films is their low electrical loss factor. PTFE multilayer boards are well known for their excellent electrical properties, but using a hybrid construction of a high-loss epoxy-based prepreg would defeat the purpose of the PTFE. While a particular bonding film may not exactly match the dielectric constants of the laminate, the effect of any difference is typically negli- gible, or if not, the board can be designed to allow for the difference. The downside is that thermoplastic film bonding is typically limited to low layer PCBs and not suited for sequential lamination. Thermoset Prepreg Bonding Thermoset prepregs harden and cure as a result of a thermochemical reaction such as the reaction that hardens the two components of epoxy when mixed together that you can buy at the hardware store. Thermoset prepregs can also be reinforced with fillers to improve the stability of the final product. Once hardened or cured, thermoset materials are typically harder than their thermoplastic counterparts. Unlike thermoplastic materials, thermoset materials go through the thermochemical reaction only once, and cannot be re-melted like a thermo- plastic. Before the cure of thermoset prepregs, they have a limited shelf life compared to ther- moplastic films. The primary advantages of thermoset pre- pregs are the ability to manufacture sequen- tially laminated PCBs and produce higher layer count stackups, and a closer match to tradi- tional laminate properties in hybrid construc- tions. The primary disadvantage is a high elec- trical loss factor. Fusion Bonding The first two methods require additional films or prepreg materials, which function like glue to keep the multiple layers in one piece. The third approach for forming RF and micro- wave PCBs, fusion bonding, uses heat and pressure to direct bond the material substrate layers into one piece. The layers are joined together through extremely high temperature and precisely controlled pressure without the addition of any bonding materials. This method has its challenges such as the additional control over the lamination fixture, pressure, and elevated temperature. But the increased performance over thermoplastic and thermoset bonding are significant in the right application. The fusion bonding results yield a fully homogeneous dielectric constant structure with no mismatch of properties from different films or prepregs. Fusion bonding produces a sin - gle, uniform dielectric constant value through- out the PCB package, which can instrumental for high-frequency applications that must meet critical performance requirements. Comparison of Pros and Cons Thermoplastic Films Pros • Good to excellent loss characteristics • Lower dielectric constant (E r ) than most thermosetting prepregs (close to pure PTFE) • Pure isotropic material Cons • Not a good choice for sequential lamination • Lamination temperature unsuitable for many thermosetting prepregs • Poor drilling performance with potential for smearing • Cannot be desmeared or etched back