Application of high frequency material series, bending and molding technology in PCB, soft and hard bonding board
Flexible circuits are designed to be bendable, but curved rigid PCBs are a bit unusual. However, many applications that do not use flexible circuit technology also require bending and shaping circuits. Some high frequency circuit materials are used in these applications to shape circuits that improve the antenna's functionality. Another application is to wind the circuit around the structure, which sometimes also acts as an antenna.
Bending and forming a circuit with dynamic bending action will require some basic principles to be understood, regardless of the circuit material used. Of course, the circuit materials used can make a huge difference in the success of the molding circuit without causing the conductor or material to break. Circuit materials used for bending, forming, and bending cannot have glass fiber reinforced materials. Therefore, it is not recommended to use a typical FR-4 material with woven glass. Several materials used in high frequency rigid plates do not have glass reinforcement and have been successfully applied.
The LCP circuit materials from the supplier ROGERS are ideal for bending applications and they also have very good high frequency electrical performance. These materials are made into relatively thin laminates, typically less than 5 mils. This thinness contributes to the successful bending of the circuit. RO4534 and RO4535 have lower conductor losses while increasing heat treatment capability, resulting in lower insertion loss and better signal consistency. The low dielectric loss makes the RO4534 and RO4535 suitable for high frequency applications where many conventional circuit boards do not meet the requirements.
However, Rogers' other high-frequency materials have been on the market for many years and are used in forming applications: PTFE-based laminates, which do not contain glass reinforcement. These materials typically use a filler on the PTFE substrate to help reduce the high CTE of the PTFE without compromising the bending ability of the material. RT/duroid5880 can be applied to PTFE ceramic laminates, compatible with automated placement, meeting lead-free process capabilities, increasing the hardness in the multilayer board structure, while reducing the cost of materials and processing, and helping Reduce the circuit size.
The basic idea of a curved circuit is based on the theory of mechanical beam composites. For example, a double-sided circuit is considered to be a microstrip transmission line with a signal conductor that is located on the top conductor layer and ground plane on the PCB, the circuit being made of different layers of material. One property that is important to understand bending is modulus; modulus is a measure of how hard a circuit is. The high modulus is rigid and the low modulus is soft. When bending a circuit, a softer material creates less stress in the circuit, and when the stress is small, the different layers are less likely to break.
The bend radius is another very important parameter. If it is necessary to bend a 1/8" thick metal plate without breaking the metal, it would be advantageous to have a large bend radius. Of course, a small, tight bend radius is more likely to cause metal fracturing. Small bend radii will More internal stress is generated on the metal and is easily broken.
Another concept to consider is the neutral axis of the composite beam (or circuit). The neutral axis is the plane with no stress in the circuit. Consider bending 1/8" stress in different thicknesses in the metal plate. First, the layer containing metal and metal flakes on the inside of the bend radius will attempt to compress and thus stress due to compression. The outer layer of the metal sheet will be stressed by tension. Somewhere in the bend zone, the metal will have a transition, and the small transition zone without stress is called the neutral axis. Ideally, when the circuit is formed, if there is a conductor on the neutral axis, even if bending is considered A circuit with a very small radius will not break because there is no stress in the conductor.
Neutral axes are often considered when modeling bends, forming and bending circuits; our idea is to make the critical copper layer as close as possible to the neutral axis. In the case of the microstrip circuit shown in Figure 1, the neutral axis will be somewhere between the ground plane and the signal plane. This means that there is stress compression at the ground level and stress is generated at the signal plane. If a different structure is considered, the neutral axis can be very close to the signal conductor layer. The top and bottom have stresses as tensile and compressive, respectively, but the signal layer in the geometric center of the cross section may have very little or no stress. Therefore, the strip line circuit can be efficiently molded without damaging the internal signal conductor.
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