Low transmission loss flexible substrates using low Dk/Df polyimide adhesives

T. Tasaki
Arakawa Chemical Industries, Ltd.,
Japan

Keywords: transmission loss, insertion loss, flexible printed circuit boards (FPCBs), lowDk/Df polyimides, liquid crystal polymers (LCPs)

Summary:

To meet the ever-increasing requirements of transmission data with a growth in the market for information technology gadgets such as mobile phones, the transmission frequency of circuits is increased. However, the integrity of high-frequency signals can be damaged by transmission loss. In high-frequency circuits, two factors are responsible for the transmission loss: conductor (circuits) loss and dielectric (insulating materials) loss. The general method for reducing the conductor loss is to make the surface of circuits smooth. However, smoothing the surface of copper circuits tends to weaken the adhesion between them and the insulating materials. The dielectric loss of dielectric materials depends on their current frequency, dielectric constant (Dk), and dielectric tangent (Df). Consequently, the dielectric loss increases with an increase in the current frequency. The general method for reducing this loss is to use low Dk/Df materials. Hence, low Dk/Df materials having good adhesion to the smooth copper surfaces are required for the fabrication of high-frequency printed circuit boards (PCBs). In PCBs, thermosetting insulating materials are used. These materials are mainly composed of cross-linking agents, fillers, and polymers to provide flexibility and good adhesion. In general, the polymers used in PCBs should possess heat resistance, laminating workability, good adhesion properties, and flexibility. As already discussed, low Dk/Df materials are required for PCBs. Hence, the polymers used in PCBs should also have low Dk/Df. However, it is difficult for polymers to achieve a good balance between low dielectric properties and the other properties mentioned above. To meet these requirements, we focused on the solvent-soluble polyimides that many researchers have developed. And much research has been done on decreasing the dielectric constant of polyimides. According to the Clausius-Mossotti equation, which gives the relation between the dielectric constant and molecular polarization, the dielectric constant of a material is proportional to the polarization rate and is inversely proportional to the molar free volume content. Based on the above discussion, we developed solvent-soluble polyimides with good heat resistance and low Dk/Df characteristics by optimizing the composition ratio of the aliphatic, cycloaliphatic, and aromatic groups present in the polyimide backbone. We found that the adhesives prepared using our polyimides showed good adhesion to polyimide film, good heat resistance, and low Dk/Df properties. Furthermore, we developed a flexible substrate (FCCL) with our PI adhesives, low-profile copper foils, and normal PI films. Currently, in high frequency flexible substrates, liquid crystal polymers (LCPs) substrates are mainly used. But LCPs substrates have problems with processability and productivity. On the other hand, our substrate can be prepared by using normal PI films and normal low temperature processes. In addition, our substrate showed a transmission loss similar to that of LCPs substrates at frequencies less than 20 GHz. This result indicates that by using our polyimide adhesives, lower cost substrates with good processability and productivity can be prepared, which can then be used as high-frequency substrates instead of LCPs substrates. In this presentation, we will report on the details of properties of the substrates using our polyimides.