The aviation industry demands the highest level of reliability and precision to ensure safe and efficient flight operations. Printed circuit boards (PCBs) used in avionic applications are custom designed and made to meet the rigorous requirements of the industry, such as withstanding extreme temperatures, vibrations and harsh environmental conditions. To ensure compliance, there are stringent testing and quality control processes. Part of these processes is the proper cleaning of PCBs; contamination poses a significant risk to PCB performance and functionality – even the smallest speck of dust or mark can result in intermittent or complete failure, leading to operational disruptions or, worse, accidents.
It’s near impossible to avoid PCBs’ exposure to contaminants during their fabrication, transport and storage processes. Yet, dust, oils, flux residue and uncured solder pastes can lead to poor solder joints, bridging, dendrite growth, incomplete conformal coating adhesion and other issues, resulting in PCB failure.
With increased complexity and miniaturisation, PCBs have become more difficult to access and clean properly, and cleaning PCBs in avionics is an even more challenging task due to their intricate designs and often tightly-packed components. Hence, developing effective cleaning procedures that reliably remove contaminants without damaging delicate components is crucial.
One cleaning method that has emerged as highly efficient for avionic PCBs is vapour degreasing. This process involves immersing the PCBs in a cleaning fluid within a vapour degreaser machine. The fluid is continuously filtered and distilled inside the vapour degreaser to dissolve or lift contaminants from the PCB’s surface. In some instances, ultrasonic agitation is added for additional cleaning power.
After being lifted from within the cleaning fluid, PCBs undergo vapour rinse and drying. The cleaning fluid condenses and drips back into the vapour degreaser for re-use. The vapour degreaser recycles and re-uses the cleaning fluid hundreds of times before it needs to be refreshed or replaced. This helps reduce the cost of hazardous waste removal.
After a typical cleaning cycle of up to 20 minutes, the PCBs come out clean, rinsed, dried and ready for conformal coating, assembly and packaging.
Vapour degreasing is fast, efficient, offers high throughput, and engineers can easily integrate it into standard production lines. However, its effectiveness relies heavily on the choice of appropriate cleaning fluid. Most modern cleaning fluids are a mixture of compounds that can include hydrocarbons like mineral spirits, isopropanol and ethanol. The vapour degreaser can use just one type of cleaning fluid, or fluid suppliers can mix and blend custom formulas to remove a specific soil from a specific substrate, maximising cleaning effectiveness.
There are several important characteristics to consider when choosing the right vapour degreasing cleaning fluid. Non-flammable cleaning fluids are highly recommended, as they ensure safer handling and storage and don’t need fire or explosion-proof equipment. Most vapour degreasers are designed to work seamlessly with non-flammable cleaning fluids.
Opting for low-boiling cleaning fluids is advantageous in terms of energy efficiency and safety. Cleaning fluids with a boiling point below 100°C can still clean PCBs effectively whilst enabling the vapour degreaser to be more energy efficient. Moreover, using lower boiling temperatures allows for the safe cleaning of delicate PCB components without risking damage.
The cleaning fluid’s surface tension and viscosity are other critical characteristics. Low surface tension and viscosity ensure the entire PCB surface is entirely wetted and can penetrate all tight spaces, including under low-mounted components.
High density cleaning fluids are particularly beneficial when dealing with particulates like dust, dirt or cloth fibres that can’t be dissolved. High density fluids are typically 20-30% heavier than water and 50% heavier than alcohol and can easily move particulates off the PCB surface.
The solvency or strength of the cleaning fluid is also a crucial consideration. Cleaning fluid strengths are often indicated by a Kb value, which ranges from mild to strong. Kb (Kari-Butanol) refers to a standardised ASTM test that measures the relative strength of a non-aqueous cleaning fluid. It is important to choose a cleaning fluid with a high enough Kb value and solubility parameter, to effectively dissolve and remove contaminants without causing damage to the PCB substrate.
Maintenance-free cleaning fluids are also desirable. Many modern-day vapour degreasing fluids remain stable, eliminating the need for acid acceptance testing, lab analysis or other maintenance. This ensures a hassle-free cleaning process and reduces the time and effort required for maintenance tasks.
Lastly, environmental stability should also be a key consideration. Newer formulations of vapour degreasing fluids offer excellent performance whilst addressing environmental concerns. These fluids have low ‘global warming potential’, reducing the impact on greenhouse gas emissions, and low ‘ozone depleting potential’ content to comply with strict regional air quality regulations. Choosing environmentally-sustainable cleaning fluids contributes to maintaining a cleaner and safer environment.
By Elizabeth Norwood, Senior Chemist, MicroCare