Selection of Bonding Materials for HDI PCBs and Performance Analysis of Different Materials
In HDI PCB applications, the use of different bonding materials has different effects on the electrical properties of the material, and the material formulation used to bond the high-frequency multi-layer film may also vary widely. Many bonding materials are glass reinforced, and several commonly used bonding materials are not woven glass reinforced. Non-reinforced bonding materials are typically thermoplastic polymer films, while woven glass fiber reinforced bonding materials are typically thermoset and often use special fillers to enhance high frequency performance.
When laminating, the thermoplastic bonding material needs to reach the melting temperature to achieve bonding between the multilayer circuit layers. These materials can also be remelted after multiple layers of bonding, however remelting can lead to delamination, which is why it is often desirable to avoid remelting. The lamination melting temperature and the remelting temperature to be noted vary with the type of thermoplastic bonding material, which is typically a concern after lamination, such as soldering, which exposes the circuit to elevated temperatures.
Rogers has introduced thermoplastic non-reinforced bonding materials commonly used in multilayer high frequency PCBs such as Rogers 3001 (425 °F melting, 350 °F remelting), CuClad 6700 (425 °F melting, 350 °F remelting) and DuPont Teflon FEP (melting at 565 °F, remelting at 520 °F) bond film. Since stratification is considered, the remelting temperature is generally lower than the initial melting temperature, while at the remelting temperature, the material is sufficiently soft to delaminate. At the initial melting temperature during lamination, the material is at its lowest viscosity, which allows the material to wet and flow between the layers during lamination to achieve good adhesion. As can be seen from the temperatures of the different materials, the 3001 and CuClad 6700 bond materials are suitable for multiple layers that are not exposed to high temperatures (eg, soldering). Assuming that the soldering temperature is controlled below the remelting temperature, DuPont Teflon FEP material can be used for multiple layers to be soldered. However, some manufacturers do not have the ability to reach the initial melting temperature.
One exception to this in thermoplastic non-reinforced bonding materials is Rogers' 2929 bond sheet, which is non-reinforced, but it is not a thermoplastic but a thermoset. Thermoset materials do not have melting and remelting temperatures, but they have a cure temperature (during lamination) and a decomposition temperature that should be avoided due to delamination considerations. The 2929 bond sheet has a lamination temperature of 475 °F and a decomposition temperature well above the lead-free soldering temperature, so it is stable after multiple layers of bonding for most high temperature conditions.
The electrical properties of the adhesive material are as follows: Rogers 3001 (Dk = 2.3, Df = 0.003), CuClad 6700 (Dk = 2.3, Df = 0.003), DuPont Teflon FEP (Dk = 2.1, Df = 0.001) and 2929 (Dk = 2.9, Df = 0.003).
Yet another type of bonding material is a glass fiber reinforced bonding material, typically a combination of woven fiberglass cloth, resin, and some filler. Laminated PCB manufacturing parameters can vary greatly depending on the composition of the bonding material. In general, highly filled filler prepregs typically have much less lateral flow during lamination, and if prepregs are to be used to build multiple layers with cavities, these highly filled prepregs may be a good choice; The inner layer to be bonded to the prepreg has a thicker copper which may be difficult to laminate with such a low flow prepreg.
There are two types of glass fiber reinforced prepregs commonly used for high frequency manufacturing, namely RO4450B and RO4450F prepregs (Dk=3.5, Df=0.004). The processing parameters of these materials are similar to those of FR-4, however they have very good electrical properties at high frequencies. These materials are highly loaded and have low lateral flow during lamination. They are high TG Thermos materials and are very stable for lead-free soldering or other advanced processes.
In summary, there are various trade-offs when designing muti-layer PCBs for high frequency applications, and manufacturing aspects must be considered together with electrical performance.