Microwave Tech - Analysis of Adhesive Materials for High Frequency Multilayer PCB

In the application of high-frequency multilayer PCB, the use of different bonding materials has different effects on the electrical properties of the material, and the material formula used to bond the high-frequency multilayer PCB film may also be very different. Many bonding materials are glass fiber reinforced, and there are several commonly used bonding materials that are not woven glass fiber reinforced. Non-reinforced bonding materials are usually thermoplastic polymer films, while woven glass fiber reinforced bonding materials are usually thermoset, and special fillers are often used to improve high-frequency performance.

During lamination, the thermoplastic bonding material needs to reach the melting temperature to achieve the bonding between the high-frequency multilayer PCB circuit layers. These high-frequency multilayer PCB materials can also be remelted after multilayer bonding, but remelting will lead to delamination, which is why it is usually necessary to avoid remelting. The lamination melting temperature and the remelting temperature that need attention vary with the type of thermoplastic bonding material. The remelting temperature is usually a process that requires attention after lamination, such as soldering and other processes that expose the circuit to high temperatures.

Rogers has introduced thermoplastic non-reinforced adhesive materials commonly used in high-frequency multilayer PCB, such as Rogers 3001 (melted at 425°F, remelted at 350°F), CuClad 6700 (melted at 425°F, remelted at 350°F) and DuPont Teflon FEP (melted at 565°F, remelted at 520°F) adhesive film. Due to delamination, the remelting temperature is generally lower than the initial melting temperature, and at the remelting temperature, the high-frequency multilayer PCB material is soft enough to be layered. At the initial melting temperature during lamination, the material is at its lowest viscosity, which allows the material to wet and flow between multiple layers during the lamination process to obtain good adhesion. It can be seen from the temperature of different materials that the bonding material of 3001 and CuClad 6700 is suitable for multilayers that are not exposed to high temperatures (such as welding). Assuming that the welding temperature is controlled below the remelting temperature, DuPont Teflon FEP material can be used for multiple layers to be welded. However, some manufacturers do not have the ability to reach the initial melting temperature.

However, there is an exception in the thermoplastic non-reinforced bonding material, that is Rogers' 2929 high-frequency multilayer PCB bonding sheet, which is non-reinforced, but it is not a thermoplastic material, but a thermoset material. Thermosetting materials do not have melting and remelting temperatures, but they have solidification temperatures (during lamination) and decomposition temperatures that should be avoided due to delamination considerations. The lamination temperature of the 2929 bonding sheet is 475°F, and the decomposition temperature is far beyond the lead-free soldering temperature, so for most high-temperature conditions, it is stable after multilayer bonding.

The electrical properties of these high-frequency multilayer PCB bonding materials are as follows: Rogers 3001 (Dk=2.3, Df=0.003), CuClad 6700 (Dk=2.3, Df=0.003), DuPont Teflon PCB FEP (Dk=2.1, Df=0.001) And 2929 (Dk=2.9, Df=0.003).

Another high-frequency multilayer PCB bonding material is glass fiber reinforced bonding material, usually a combination of woven glass fiber cloth, resin and some fillers. Laminated PCB manufacturing parameters will vary greatly depending on the composition of the bonding material. Generally speaking, prepregs that are highly filled with fillers usually have much less lateral flow during lamination. If prepregs are to be used to build multiple layers with cavities, these highly filled prepregs may be a good choice; but if they are The inner layer to be bonded to the prepreg has thicker copper, and it may be difficult to laminate with this low-flow prepreg.

There are two types of glass fiber reinforced prepregs commonly used in the manufacture of high-frequency multilayer PCB, namely RO4450B and RO4450F prepregs (Dk=3.5, Df=0.004). The processing parameters of these materials are similar to FR-4, but they have very good electrical properties at high frequencies. These materials have a high load and low lateral flow during lamination. They are high Tg thermoset materials and are very stable for lead-free soldering or other advanced processes.

All in all, when designing high-frequency multilayer PCBs for high-frequency applications, there are various trade-offs, and manufacturing aspects must be considered together with electrical performance.