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What are Optical Printed Circuit Boards?

Optical Integration on PCBs: A Look at the Advancements

As copper reaches its speed limit, engineers look at optics to replace copper for very high-speed signals. Engineers also envisage replacing copper links between servers, routers and switches with active optical cables. Already silicon chips are available with some optical components inside. The next phase is for optics inside printed circuit boards (PCBs).

Why Optical Systems in PCBs

Electro-optical printed circuit boards combine optical and copper paths on the same board. While the copper paths distribute power and low-speed data, the optical paths handle the high-speed signals. This segregation has several advantages. At high frequencies, signal integrity suffers due to skin effect, crosstalk, and skew when passing through copper systems. Optical systems do not have those issues, while also presenting greater channel density than copper does. Moreover, as optical signals do not need signal conditioning and equalization, optical systems consume lower power than do electrical signals. Additionally, optical systems can reduce the surface area of a PCB by 20% and the number of layers on the board by 50%.

Optical Technology for PCBs

Designers and manufacturers are migrating optical technology to the backplane and connectors. Although optical technology has been around in the form of SFP and QSFP interfaces for some time now, engineers are now developing optical backplane connectors and optical backplanes. These also include optical transceivers at their connecting edges. Now, it is increasingly possible to have optics appear within a board, rather than limit its presence at the edges. Therefore, optics is now moving closer to the electrical signal source. That means the processor, fiber optic patch cords, and waveguides can now be found on the PCB.

Manufacturers have been successful in developing optical backplane connectors and included a technique to align small waveguides to onboard transceivers. The future challenge is to develop onboard waveguides so that performance is guaranteed even if there are tight bends in the board.

Manufacturing Optical PCBs

Engineers use photolithography and film processing techniques to fabricate flexible optical waveguides that will be able to move light around components onboard. According to technical information available, waveguides in the build will need walls at least 100 µm thick, and a bend radius less than 5 mm. These dimensions would allow designers to place the waveguide within connectors. This will also let light travel between a line-card and a backplane, without the necessity to convert it to an electrical signal.

PCB Manufacturers usually follow two different techniques when constructing the waveguides—non-contact mask lithography and direct laser writing. In non-contact mask lithography, spin coating applies the material to the substrate. However, as this process is more applicable to semiconductor manufacturing, lithography is better suited for small areas, and cannot be scaled up to handle large areas. Engineers use a process of draw-down coating for large areas, along with a doctor blade.

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However, engineers faced two problems with the above process. One, the waveguide material would curl up, requiring 170 g of force to flatten. Second, there was the difficulty of the waveguide adhering to the substrate. Adhesion to the substrate is important so the waveguide would not crack during mechanical processes such as cutting the wafer or the substrate board.

It is important to have waveguides that do not attenuate the light too much as it travels through. Optical power measurements made with laser diodes as a source and a photodetector as the receiver indicate onboard waveguides introduce optical losses ranging from 0.046-0.050 dB/cm, even when the waveguides were bent to form two or three loops. Some signal loss is customary from wall roughness within the waveguide as well.

Optical Interconnects on PCBs

Onboard optical interconnects on PCBs can handle very high data rates and offer larger numbers of data channels than other electrical interconnections do. Moreover, as optical signal transmission is impervious to electromagnetic interference or EMI, it is suitable for mixed-signal systems such as data acquisition and signal processing where sensor applications need high accuracy of analog electronics.

Optical waveguides on PCBs require not only low attenuation but also a reliable manufacturing process for the optical layer. In an optical PCB, the fabrication steps and material properties of the waveguides need to be compatible with the manufacturing and assembly techniques prevalent in the PCB industry.

Apart from the optical path in an optical interconnection system, there must be coupling elements that can couple optical signals into and out of the waveguides. Moreover, common pick-and-place machines must be capable of suitably and automatically mounting these coupling elements without any active alignment between the optical waveguide and the coupling element. The use of structured polymer foils helps in this integration.

The main issues of using polymers are their thermal and mechanical stability against the process conditions during PCB fabrication. Additionally, with close coupling tolerances and imperfect positioning of waveguides within the PCB, mounting coupling elements often require active alignment. Engineers circumvent such problems in an optical PCB by using standard multimode glass fibers integrated within the layer stack. As glass fibers are highly stable both thermally as well as mechanically, PCB manufacturers can easily follow their proven processing steps for embedding the fibers into multilayer PCBs.

Moreover, the geometrical accuracy of glass fibers, apart from offering very low optical attenuation, is also very important for coupling methods. Engineers can passively align active optoelectronic components at the stubs of the fiber—the PCB has cutouts to make them accessible. A specific micromechanical alignment structure makes this passive alignment possible when combined with the optoelectronic chips—making mirrors and lenses unnecessary for coupling to the waveguides.

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Optical Coupling Elements

For using coupling elements on the PCB, they must be compatible with the assembly and soldering processes manufacturers use. Primarily, the alignment structure should be able to withstand the temperatures involved. Precision molding in silicon molds can achieve this. Manufacturers typically use a temperature of 180°C and a duration of 90 minutes under a pressure of up to 15 bar for the lamination process when manufacturing multilayer boards. Soldering processes expose the board to temperatures exceeding 250°C. Optical waveguide polymers often show discoloring or decomposition at such temperatures. Engineers find glass fibers to be a suitable substance.

Glass fibers remain optically stable without any damage at the above temperatures. Additionally, being mechanically strong, glass fibers offer very low attenuation and exhibit very tight tolerances for their diameter. Rather than fixing the fibers on top of a readily processed conventional PCB, engineers embed them completely into the layer stack of optical printed circuit boards, between the top and bottom layers of the PCB using standard material such as FR4.

Summary

As against waveguides made from polymer foils, embedded glass fibers allow engineers to automatically align the optoelectronic transmitter and receiver components due to the accuracy of their contours. That makes it easy to develop optoelectronic coupling elements onboard, as they can align positively on the fiber using an advanced microstructure and achieve low coupling losses without requiring active position optimization.

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The most common PCB component selection mistakes to avoid

The most common PCB component selection mistakes to avoid

Just like any other electronic components, PCB components are rapidly undergoing fast technological advancements in both size and performance. This is attributed to the invention of the Integrated Circuits, microcontrollers, and chips in the market such that the flexibility of PCB components increases significantly. On the other hand, one might think that the process of component selection for your PCB is very simple and obvious. However, there are many considerations and technical properties that one should take keen notice of. This is because an error made in the selection of components can lead to serious damages hence the ineffectiveness of the PCB. As experts in PCB design, assembly and manufacturing, we highly recommend that you seek the advice of a component engineer who should act as a liaison between supply chain, design and manufacturing sides. Please join us as we discuss the most common mistakes you should avoid while selecting your PCB components. Please note that you can learn more about PCB design, assembly and manufacturing from our blog https://www.rushpcb.co.uk/blog/. We will also offer the best advice for your PCB component selection.

  1. Neglecting the pros and cons of FTH and SMT components for each design

SMT (Surface-Mount Technology) components are generally less expensive and readily available compared to FTH (Fixed-Through-Hole) components. However, this never qualifies them as the best PCB components to be used in each design.

Several other factors like power dissipation, part density, the size of the PCB and the ease of access of the soldering pads for debugging should all be considered. It is crucial to remember that Surface Mount Technology components are best suited for smaller PCBS although soldering them by hand is really hectic compared to SMT components. One should, therefore, take into consideration the above factors and decide on the best technology to apply before carrying out PCB component selection.

  1. Inadequate ground planes and bypass capacitors

Just like a resistor, a capacitor is a passive PCB component which depends on external power sources to operate. On the other hand, a ground plane is an electrically conductive surface which is connected to an electrical ground. When selecting the components for your PCB needs, it is crucial to ensure that it has enough bypass capacitors for use in power regulation and ground planes for use in appropriate IC pins. Failure to do this leads to a PCB circuit with non-optimized electromagnetic compliance and inefficient performance. Moreover, a number of decoupling capacitors should be placed near the supply to a ground location when Integrated Circuits are used.The size of a capacitor depends on technology, size and frequencies involved thus its crucial to consider the application and the actual design of your PCB when carrying out PCB component selection.

  1. Failing to check for virtual part footprints

In the schematic drawing phase, you should consider having land patterns and footprints that have to be made in the PCB layout phase. Careful PCB component selection is crucial during this stage. Some of the major considerations one should take care of are:

  • Always remember that footprints are comprised of mechanical dimensions (X, Y and Z) of the part and electrical pad connections. This includes the body outline and the pins attached to the PCB. When selecting PCB components, it is crucial to consider packing restrictions on both the bottom and the top sides of the PCB. As an example, some components like polarized capacitors might demand a significant height clearance consideration hence a quick virtual rendering of the PCB should be visualized to give a relatively accurate representation of the components needed and their positioning.
  • The choice of components might change during PCB design thus selection of the parts to undergo PTH and those to undergo SMT in the initial design stages is crucial. Here, factors like availability, parts costs and part area density should be considered.
  • If a footprint doesn’t exist in your database, then you can always create a custom footprint from your tool.
  1. Forgetting to review the BOM

Reviewing the Bill Of Materials report is necessary for your PCB design since the manufacturer can put the PCB manufacture on hold until some parts are provided or specified. The BOM should be a clear representation of the total cost of your PCB design and all components to be used should be shown in the BOM and their respective prices. To learn more about BOM and the reasons why your manufacturer might hold your PCB design and manufacture, visit our blog https://www.rushpcb.co.uk/blog/.

  1. Failing to ensure that components fit in the packaging after PCB assembly

Selecting PCB components might be hectic but this process should be carried out professionally so that your PCB functions as expected. However, one should not forget to consider how the components fit in their packaging so that a functional and a non-compromised design are achieved. The height clearance height of the components should be considered before design. This can be best achieved by drawing a basic board shape outline in the early design but taking care to place the larger components on the best suited location on the board to allow for quick visualization of the position of the components and the best PCB components to be selected.

  1. Not using reference designators

Failure to number reference designators might complicate the process of reviewing the BOM and at the same time prevent accurate sorting. This might result in wrong PCB component selection.

  1. Not verifying spare gates

All spare gates should have inputs connected to a signal to prevent the floating of the inputs. One should always ensure that they review the spare gates and the unconnected inputs. This is because the process of debugging is more complicated when compared to simply reviewing the gates. It is vital to recall that if the inputs are left to float, the system might not operate effectively. Components should, therefore, be selected in a way that there is a complete connection between all the inputs and the outputs of the PCB.

Careful selection of PCB components is paramount to aid in averting challenges like non-functional end PCBs. So how can I acquire the services of a component engineer or high quality PCB components? Please worry no more; we at RUSH PCB Ltd have the best experts who can help you select your components, design and assemble the PCB until the final product is achieved. Please visit us at https://www.rushpcb.co.uk/ and enjoy our explicitly unique services.