Controlled impedance requirements. The HDMI video clip input had numerous different requirements for controlled impedance. The length of the trace made use of was extremely little, and the signals were relatively sluggish compared with DDR, so trace size had not been an issue. Nevertheless, the HDMI required 100Ω differential pairs, while the memory ran at 80Ω. For that reason, it was an intriguing challenge to guarantee controlled impedance, and it was hard ahead up with a rigid flex PCB stackup that would leave ample office for 80Ω, along with 100Ω, without coming to be too thin and challenging for PCB manufacturers making.
HDI PCB and blind and buried vias PCB were put on break out traces from the HDMI and FPGA controller. The HDMI controller pattern likewise used via-in-pads. It worked to do some “what-if” circumstances and see how the HDI stackup could potentially appear. We experienced a few iterations on that particular with a couple of various kinds of materials, looking at the impedance control preparation.
At first, FR-4 material was under consideration, yet after some screening, we decided to opt for a material that had a reduced dielectric constant, achieving reduced loss, signal honesty and preferable line-space ratio for impedance controlled traces.
Utilizing HDI PCB lowered the number of layers required in the board in general, and after stabilizing the cost with the advantages of HDI PCB modern technology, it was chosen this was the appropriate instructions.
There were 3 rigid sections in the final design:.
1) A main section with the FPGA, DDR3 ICs, and power supply systems;.
2) an area with slower, analog-type components and even more power products;.
3) an area that included a very little HDMI receiver with several possible alignments to accommodate the incoming input wire.
The rigid flex PCB remedy. As the design moved on and it became clear that area was a worry, it was determined to attach the rigid boards with a flexible bow to prevent using basic physical ports that needed more area.
The last rigid flex PCB stackup was 10 layers. The rigid boards used 8 layers and carried all the impedance-controlled and high-speed traces. The other two layers were the flex PCB signal layer, which was also made use of as the VCC layer in the main rigid part of the board. There was some collaboration between layers of the rigid and flex sections, but for the most component these were treated independently.
Rigid Flex PCB permitted the board to fit into the little housing on the helmet-mounted display system. The flex ribbon could bend a variety of ways, accommodate different angles, and be rolled up and totally taken in within the volume of the container, offering alternatives on how the boards would certainly become part of the system and twist around the optical parts.
Anything that called for impedance control was taken care of entirely within one of the rigid structures. Breaking it down right into those areas permitted us to stay clear of any type of need for impedance control on the flex, which was a big win insofar as expense goes.