Clean connections

Liam Taylor, European business manager, fibre optics at MicroCare UK examines the importance of keeping aerospace fibre optics on PCBs clean for optimal reliability.

The demand for faster and more reliable electronics is consistently growing in the rapidly advancing world of aerospace technology. Critical aerospace applications necessitate cutting-edge solutions, and printed circuit board (PCB) fabricators are increasingly turning to fibre optics to meet these demands. Fibre optics integrated into PCB designs offer numerous advantages, including higher bandwidth connectivity, greater durability, and a smaller footprint.

In the aerospace sector, the utilisation of fibre optics on PCBs has become integral for many reasons. Their high data transmission capacity, lightweight characteristics, and immunity to electromagnetic interference have made them invaluable in various aerospace applications, including avionics systems like flight control computers, communication and navigation systems, in-flight entertainment systems, and fibre optic gyroscopes. Fibre optics are pivotal in facilitating faster data transfer, improved communication, and enhanced system reliability, making them indispensable in aerospace.

In aerospace, the reliability of PCBs is critical. Any disruption or degradation of data transmission can have serious consequences. For this reason, ensuring the cleanliness of fibre optic connections is crucial. Contamination on fibre optic end faces can lead to problems such as intermittent operation, reduced data speeds, or even complete network failure. The significance of cleanliness for these connections is vital for optimal PCB performance and should be an integral part of manufacturing processes.

Specialised care

The handling and cleaning of fibre optic components require particular attention. The same cleaning products used for the PCB are insufficient for effectively cleaning fibre connectors and interfaces. To maintain reliability in aerospace applications, adopting specialised cleaning procedures, including using the right tools and cleaning fluids is essential.

Much like contaminants on PCBs, fibre optic contamination is often invisible to the naked eye. Specialised digital inspection scopes are commonly used to identify microscopic dust particles, salt residues, or fingerprint oils. Following industry ‘best-practice’ these scopes inspect end faces post-cleaning to ensure compliance with standards like the International Electrotechnical Commission (IEC). The IEC standard 61300-3-35 outlines the process of identifying contaminants on end faces and the best practices for removing them. It recommends the three-step 'Inspect, Clean, Inspect' (ICI) process for end faces.

The Inspect-Clean-Inspect process:

The ICI process provides a standardised procedure to verify and achieve cleanliness of fibre optic end faces. By following these three simple steps, technicians can ensure aerospace fibre optic PCBs meet exacting standards:

1. Initial inspection: Using a digital inspection scope, fibre optic connector end faces are thoroughly inspected to identify contaminants. Any foreign matter's size, location, and coverage area are noted.
2. Cleaning: If contamination is found, specialised optical cleaning tools and fluids effectively remove it from the end face surface.
3. Final inspection: After cleaning, the end faces undergo repeat inspection to confirm the complete removal of contaminants. The cleaning process is repeated if any residue remains until the end face achieves certification cleanliness levels.

Regular performance of the systematic ICI technique ensures aerospace fibre optic connectors consistently attain the pristine cleanliness required for optimal transmission and identifies damage or wear to prevent degraded performance.

Wet to dry cleaning

Wet-to-dry cleaning is the most effective method for thoroughly removing all types of debris, oils, and residues from fibre optic connections. This technique also eliminates the electrostatic charge build-up that can attract more dust particles and lead to recontamination.

Wet-to-dry cleaning involves carefully applying a small amount of high-purity cleaning fluid to the corner of a dry, lint-free wipe. The end face is gently wiped from the wet section toward the dry section of the wipe. For those using cleaning sticks, the tip of the stick is moistened with cleaning fluid from a dispenser cap or a cleaning wipe. It is then inserted into the end face for cleaning.

When using optical cleaning wipes or sticks, it is critical to avoid any contact with fingers, clothing, or surfaces. Even minute amounts of skin oil or particles can cause contamination, rendering it ineffective for properly cleaning fibre optic end faces. If accidental contact occurs or a wipe/stick is dropped, immediately discard it, and start again with a new one to prevent cross-contamination.

Right tools for the job

Choosing the appropriate fibre cleaning tools is crucial for optimal cleaning. Only high-purity, non-flammable optical grade fluids should be used for wet-to-dry cleaning. These fluids are engineered to be fast-drying and static-dissipative. They are also supplied in hermetically sealed cans to maintain cleanliness.

Avoid slow-drying fluids like water-based cleaning solutions or Isopropyl alcohol (IPA), which has a slower evaporation rate than modern fluids and is also hygroscopic, attracting contaminated water molecules from the atmosphere and redepositing them onto the fibre end face.

Be cautious of pre-saturated wipes and cleaning sticks, as they can contain microscopic plasticiser contaminants from their packaging that may transfer to the end faces during cleaning. Instead, use optical-grade wipes designed explicitly for fibre cleaning. They are highly absorbent to effectively remove the contamination without generating lint. They are also designed to be soft enough not to scratch the end face.

Fibre cleaning sticks are engineered to match the internal configuration and size of female end-faces. When using them, ensure the stick is rotated in one direction to prevent re-deposition of contaminants. Typically, six rotations are sufficient to remove contamination. Use only one stick per end face to avoid cross-contamination and keep the sticks in their re-sealable container until ready for use.

Click-to-clean tools are best for end-face connectors with light to medium contamination levels. They are quick and convenient to use and clean connectors with variable geometries.

It is important to use high-quality click-to-clean tools correctly. A static charge can come from clicking a mechanical cleaning tool too many times in quick succession. This results in static charge building up with no place to dissipate, resulting in dust particulate sticking to the end-face. Applying a small amount of cleaning fluid to a wipe first is recommended, then touching the tool to the dampened area. Then, insert the tool into the port and push the handle until fully engaged. It is important not to spray the cleaning fluid directly onto the end face or the tool itself to prevent over-application of fluid.

An essential practice

In aerospace applications, the stakes are high, and system reliability is non-negotiable. Ensuring the cleanliness of fibre optic connections on PCBs is a critical aspect of maintaining the robustness of these systems. Any interruption or degradation of data transmission can have far-reaching consequences. By adhering to specialised cleaning procedures and using the right tools and cleaning fluids, aerospace manufacturers can uphold the reliability and performance of fibre optic connections in critical aerospace applications.

www.microcare.com