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52 The PCB Design Magazine • February 2016 The following is a list of thermoplastic non- reinforced bonding materials commonly used in multilayer high-frequency PCBs, with their melt and re-melt temperatures: Rogers 3001 (melt=425°F and re-melt=350°F), CuClad 6700 (melt=425°F and re-melt=350°F), and DuPont Teflon FEP (melt=565°F and re-melt=520°F) bonding films. The re-melt temperature is lower than the initial melt temperature because of delamina- tion concerns. At the re-melt temperature, the material is soft enough to delaminate. At the initial melt temperature during lamination the material is at its lowest viscosity which allows the material to wet-out and flow for good adhe- sion in the multilayer while held under pressure in the lamination process. From the tempera- tures of the different materials, it can be seen that using the 3001 or CuClad 6700 bonding materials would be appropriate for a multilayer which is not exposed to elevated temperatures, such as soldering. The DuPont Teflon FEP ma- terial can be used for a multilayer that will be subjected to soldering, assuming the soldering temperature is controlled to below the re-melt temperature. However, some fabricators do not have the capability to reach the initial melt temperature. There is an exception to the thermoplas- tic non-reinforced bonding materials, and that is Rogers' 2929 bondply, which is non-rein- forced, but it is not a thermoplastic. It is a ther- moset material. The thermoset material does not have a melt and re-melt temperature, but it has a cure temperature (during lamination) and a decomposition temperature which is to be avoided due to delamination concerns. In the case of the 2929 bondply, the lamination temperature is 475°F and the decomposition temperature is well beyond lead-free soldering temperatures so it is robust to most elevated temperature processing after the multilayer is bonded. Each of these non-reinforced bonding ma - terials has different electrical properties. The electrical properties for these bonding materials are as follows: Rogers 3001 (Dk=2.3, Df=0.003), CuClad 6700 (Dk=2.3, Df=0.003), DuPont Tef- lon FEP (Dk=2.1, Df=0.001) and 2929 (Dk=2.9, Df=0.003). The term Dk refers to dielectric con- stant or ε r and Df refers to dissipation factor or tan-delta. There are also woven glass reinforced bond- ing materials. These are typically a combina- tion of woven glass fiber cloth, resin and some filler. Depending on the formulation, the PCB fabrication parameters for lamination can vary significantly. As a general statement, the pre- preg material which is highly loaded with filler will typically have much less lateral flow dur- ing lamination. The lack of flow can be good and bad. If the prepreg will be used to build a multilayer with cavities where the prepreg needs to be cut back and not flow into the cav- ity, then these highly filled prepregs may be a good choice. However, if the inner layers that the prepreg is intended to bond have thicker copper, it is sometimes difficult to get a good lamination with these lower flowing prepregs. Two woven glass reinforced prepregs which are commonly used in high-frequency fabrica- tion are the RO4450B and RO4450F prepregs (Dk=3.5, Df=0.004). These materials have pro- cessing parameters which are relatively similar to FR-4; however, they offer the benefit of very good electrical properties at high frequencies. These materials are highly loaded and will have low lateral flow during lamination. They are a high Tg thermoset materials and very robust to lead free soldering or other elevated processes. When designing a multilayer PCB for high frequency applications, there are a variety of tradeoffs. The fabrication aspects must be con- sidered along with the electrical performance considerations. It is always highly recommend- ed to contact your material supplier when de- signing a new multilayer PCB for high-frequen- cy applications so these different tradeoffs can be adequately discussed. PCBDESIGN John Coonrod is a senior market development engineer for Rogers Corporation. To read past columns, or to reach Coonrod, click here. Multilayer pcB Bonding Materials for high-frequency applications