How Endoscope Mainboard Design Breaks the Space Curse in Distal Tip Engineering

In endoscopy, there is an eternal battle for space within the distal tip of the scope. To create a thinner and more capable device, the design and layout of the circuit boards must be reconsidered. The conventional approach of rigid printed circuit boards is challenged by the desire to make the medical endoscope camera more compact and flexible. The solution is to be found in a rigid-flex printed circuit board that combines the advantages of rigid and flexible PCBs.

 

The Space Curse

A typical scenario involves the use of a medical endoscope camera: the scope is inserted into the patient's body and navigated to the target location using imaging and illumination provided by the camera module located at the distal end. To satisfy the requirement of a large field of view, conventional rigid printed circuit boards are used to mount the image sensor, light-emitting diodes, and other components. These boards are rigid and cannot be folded to save space inside the narrow tip. On the other hand, flexible circuits would introduce solder joint stresses due to bending.

 

Flexible circuit boards are typically used to provide the necessary amount of space inside the distal tip of the endoscope. A flexible PCB can be placed along the longitudinal axis of the scope, occupying a small cross-section. However, when assembling an endoscope camera, the flexible circuit is subjected to bending forces during both manufacture and operation. Solder joints connecting components to the circuit board may be damaged by this type of stress. In many cases, the damage occurs during a single procedure and results in image loss.

 

The rigid-flex circuit board allows for the best combination of rigidity and flexibility. Several rigid board segments are placed along the longitudinal direction of the flexible PCB core. It allows mounting the image sensor, LEDs, and other components at specific intervals. The advantage of this approach is that solder joints connecting components to the rigid board are not affected by bending, thus eliminating the possibility of cracking or delamination. The flexibility of the PCB is used to place the board inside the distal tip of the endoscope.

 

The Third Dimension

The rigid-flex design allows for utilizing the third dimension inside the distal tip of the endoscope. When designing the circuit board, the components are placed on rigid segments at various levels and angles. For example, a flexible circuit board can be folded several times at different heights to hold a camera module, side cameras, and LEDs. The result is a three-level rigid-flex circuit board, where the front camera and side cameras are placed at different angles relative to the central axis of the scope.

 

A rigid frame is required to hold the folded rigid-flex circuit board. Such a frame is usually made of metal and serves as a heatsink for the LEDs and image sensor. When illuminating the target area, the LED array emits light and heat. The dissipated energy must be removed from the camera module to prevent temperature effects on its performance. This becomes especially important when the endoscope is used in a retrograde direction, as most of the illumination is reflected from the internal tissues.

The signal integrity of the three-dimensional rigid-flex PCB is ensured by a special arrangement of power distribution grounds and signal layer. When arranging the electrical connections, particular attention is paid to the transition from rigid to flexible PCBs. The circuit board uses a multi-layer design on both rigid and flexible segments. The outer layers of the rigid core provide a ground plane that isolates the signal traces from electromagnetic interference. The flexible segment has a controlled characteristic impedance to reduce signal losses and crosstalk. Special measures are taken when designing the transition area to prevent signal reflections, which would distort the image.

 

To ensure signal integrity of the 4K video signal, a rigid-flex PCB requires expert-level manufacturing. A qualified manufacturer will perform precision impedance control and use the right materials to minimize signal losses. For rigid segments, manufacturers will use advanced multi-layer constructions with controlled ground planes. For flexible segments, manufacturers will perform precision laser drilling to ensure accurate via placement. Signal integrity tests can be performed on all prototypes to achieve compliance with the signal integrity requirements. The medical endoscope camera will be able to deliver a reliable image even after thousands of bending cycles when subjected to normal use.

 

Materials

The selection of materials for the manufacture of rigid-flex circuit boards is also quite important. Medical devices are often subjected to rigorous sterilization procedures, including autoclaving at 134 degrees Celsius. Flexible printed circuit boards are often made of polyimide film coated with copper foil. Such materials are subjected to high temperatures during the delamination process. When selecting materials, particular attention should be paid to the coefficients of dielectric permittivity and dissipation factor to reduce signal losses.

 

The use of materials such as polyimide Kapton, manufactured using adhesiveless copper foil technology, reduces the possibility of delamination caused by temperature changes during autoclaving. At the same time, the use of such materials eliminates the emission of toxic substances during exposure to high temperatures. One of the most important features in the selection of materials for rigid-flex PCBs is their compliance with the ISO 10993 standard.

The manufacturer must have extensive experience in rigid-flex PCB fabrication to choose the right materials. When selecting materials for flexible segments, the manufacturer must consider their bending radius. This property is especially important when manufacturing a rigid-flex PCB with a complex three-dimensional shape. The choice of such materials as adhesiveless copper foil also depends on the mechanical strength required for the circuit board.

 

Production

Rigid-flex PCB manufacturing is a complex and challenging process. To produce a medical endoscope camera, a PCB manufacturer must have extensive expertise in rigid-flex PCB fabrication. The production of rigid-flex circuit boards typically includes more than 80 steps, from laminating the flexible core to impedance testing of the finished product. Individual rigid segments of the circuit board are manufactured separately, then bonded together along with the flexible segment. Particular attention should be paid to the accuracy of positioning the rigid segments relative to the flexible segment.

 

When manufacturing rigid-flex PCBs, choosing the right supplier is crucial. Unlike consumer electronics, medical devices and equipment imply a high degree of responsibility. Therefore, manufacturers of such devices must carefully select a supplier who has extensive expertise in the field of rigid-flex PCB manufacturing. The interaction with the supplier should be built on a high level of trust and confidence in the capabilities of the chosen partner. The main objective is to develop a working prototype of a medical endoscope camera that meets all technical specifications.

 

The rigid-flex design allows for developing new types of medical endoscopes that are smaller in size, more precise, and more comfortable for patients. At the same time, it is essential to consider all aspects of the design of rigid-flex PCBs when creating medical endoscopes. This implies analyzing in detail the requirements for the characteristics of flexible and rigid segments and determining their optimal arrangement within the distal tip of the endoscope. The signal integrity of the three-dimensional rigid-flex PCB should also be considered when designing the endoscope camera to ensure reliable video transfer. The materials used to manufacture the rigid-flex PCB are also of great importance, as they must meet the strict requirements of the medical industry.