High Density Flex‐rigid Circuits — A Solution for Board‐to‐board Transmission Line Interconnects

Abstract

Flex‐rigid circuits have been used for many years, primarily by the military, as a method to reduce the size and increase the reliability of electronic systems. However, in today's emerging designs where high speed ASICs are often the dominant components, flex‐rigid circuit assemblies are now an attractive solution for providing high density transmission line interconnects from board to board. Much of today's circuitry is being committed to ASIC designs to increase both circuit density and speed. Following this path, designers are faced with the task of interconnecting high lead count SMT packages often with as many as 300 to 500 leads per device, each dissipating several watts. At these power densities conductive cooling through the circuit board is often a necessity, dictating the use of either metal cores or heat exchangers. To make efficient use of the core and minimise weight, designs generally require SMT packages to be mounted on both sides of the core with electrical communication from side to side. However, as more exotic core materials (carbon fibre matrix, beryllium, etc.) and liquid cooled heat exchangers are used, electrical communication through the core has become difficult, if not impossible, in some cases. Instead, high density flex‐rigid assemblies are used to partition the circuit, allowing the board to ‘fold’ over the core. This results in hundreds of signal lines that must cross the flex, obeying the electrical design rules dictated by the rigid sections to maintain impedance values and crosstalk margins. This paper focuses on recent work at AIT, producing high density flex‐rigid circuits using embedded discrete wiring technology to meet the above requirements. Using 0.0025 in. diameter polyimide insulated wire, as many as 100 lines per linear inch can pass over the flex region on a single layer. This generally results in a single flex layer where all wires can be referenced to a continuous ground plane from board to board. Controlled impedance is easily maintained due to the uniform wire geometry, and high frequency attenuation is significantly lower than on equivalent etch circuit designs due to the smooth surface finish on the wire. In addition, the high interconnection density offered by this technique reduces the overall thickness of the rigid sections, thereby minimising the thermal resistance to the core.