Examination of a Medical Optical Scanner: Addressing Electromagnetic Interference (EMI) and Printed Circuit Board (PCB) Routing Problems

Examination of a Medical Optical Scanner: Addressing Electromagnetic Interference (EMI) and Printed Circuit Board (PCB) Routing Problems

In the realm of medical technology, the design of PCBs (Printed Circuit Boards) for advanced devices like optical scanners requires a meticulous approach to handle Electromagnetic Interference (EMI) and routing issues. A recent project involving the design of an optical scanner PCB for ophthalmology applications demonstrates this approach.

## EMI Mitigation Strategies

To reduce EMI, several strategies were employed. Guard traces and via fencing were implemented around high-noise components to create a Faraday cage effect, minimising emissions and susceptibility to external interference. Shielding was used to enclose noisy components, such as stepper motor drivers, preventing EMI propagation. Proper grounding paths were designed to keep noise currents away from sensitive areas, while differential grounding helped in reducing EMI.

## Routing Solutions

High-density interconnect (HDI) techniques were utilised to increase component density while maintaining signal integrity. The PCB stackup was carefully planned to ensure that high-current traces were isolated from sensitive signals, with ground planes used adjacent to signal layers to reduce noise. Thorough signal integrity analysis was performed to ensure high-speed signals were properly matched and terminated. Isolation techniques such as differential pairs and matched impedance lines were used to protect high-speed signals from noise generated by stepper motor drivers.

## Considerations for Complex Components

For complex components like stepper motor drivers and LQFP microcontrollers, isolation with separate power supplies and strategic component placement were key. Close placement of decoupling capacitors was ensured to reduce noise, and proper soldering techniques were used for LQFP packages.

## Designing the Optical Scanner PCB

The optical scanner PCB was designed with a stack-up configuration of 6 layers, a board thickness of 62 ± 10% mil, and a dimension of 127 mm x 88.9 mm. It contains electronic and electromechanical components such as a stepper motor connector and driver, encoder connector, motion controller, microcontroller, limit switches connector, joystick connector, USB connector, power input connector, LDO, and DC-DC booster.

Polygons pours were used instead of traces to route high-current paths, and the microcontroller was broken out using layers 3 and 6, with signals routed beneath the package. High-current traces between the stepper motor connector and the driver were routed using polygons to ensure ample current flow.

To manage EMI and routing challenges, shielding vias were placed around the inductor circuit to isolate it from sensitive components. The dielectric material used is Ventec VT-47, and the surface finish is ENIG. The board is built to IPF-6012 class 3 standards and has a finish copper thickness of 1 oz. on all layers.

Sierra Circuits fabricates and assembles medical-grade PCBs, meeting ISO 13485:2016 standards. This project showcases the importance of careful design strategies in creating reliable and safe PCBs for medical applications.

1. In the stack-up configuration of the optical scanner PCB, the design includes a controlled impedance to protect high-speed signals from noise generated by stepper motor drivers.
2. The stackup designer meticulously planned the PCB stackup to ensure that data and cloud computing, such as the motion controller, were located away from potential magnetic interference caused by fitness-and-exercise components like the stepper motor.
3. Science plays a crucial role in the design process of the optical scanner PCB, as it incorporates principles like the Faraday cage effect to manage Electromagnetic Interference (EMI).
4. To adhere to the health-and-wellness requirements of medical-conditions, the PCB is built to IPF-6012 class 3 standards, ensuring its quality and reliability matches the demands of the ophthalmology application.

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